Your Old Windows

Where an old wood win­dow is re­stored and eq­uipped with a good storm window, it has been re­peat­ed­ly shown in field stu­dies to per­form at least as well as a ty­pi­cal ther­mal re­place­ment win­dow — and at a low­er cost.
 
If the craftsmanship and charm of your old windows is quickly being eroded by cold drafts and frost on the panes, it may be time to consider doing something about them.

Can This Window Be Saved?
Old Windot It may look 'way beyond fixing, but this old window can be restored and made as energy efficient as any standard thermal replace­ment window. We replace a lot of windows. We probably replace nine old windows for every old window we restore and save. The fact is that most windows made in the past 60 years are not good windows. The post-war push to build a lot of new housing quickly virtually eliminated the traditional wood window. It took too much time to build and install the window, and a high level of craftsmanship that just was not available. Builders opted for steel and aluminum windows, and factory Photo: Andersen Windows. Andersen Woodwright Window

Nearly all major window manufacturers market windows designed to look like heritage windows. This Andersen window looks like an original Craftsman window. But, most replacement windows last at best 40 years while original windows were built to last generations with proper care. This window starts at about $600.00 — not including installation. Restoring the original window is about half that and since the window is already installed, no installation is required. made, self-contained wood window units that did not need a lot of site preparation and wall modification. Just put them in the opening and nail them up. It was faster, it was cheaper, and builders then, as now, were for anything faster and cheaper. (For more on the post-war housing boom, see Postwar Styles: Cape Cod, Colonial and Ranch.)

But, the sad consequence is that most of the windows installed since the World War are not worth saving even if they could be saved, and most cannot be. Manufacturers that are not out of business don't make those windows any longer, so parts just are not available. The only option, then, to fixing your window problems is to replace the windows.

But, most pre-War housing, and some better housing built since the World War have shop-crafted wood windows. These most often can be saved, and as for parts — if you have a glass company, hardware store and lumber yard in your town, then you have all the parts you are likely to need.

Photo: Old House Journal.
Click to enlarge
Click to Enlarge
Jonathan Poore's classic "Anatomy of a Double Hung Window" published in Old House Journal in 1932. This is how windows were made for hundreds of years. Every one of these parts is a single piece of wood that can be removed and replaced without replacing the whole window. OK, so old windows can be saved, but can they be made as energy efficient as modern windows?

The answer is "yes".

While old wood windows cannot be upgraded to rival the finest (and incredibly expensive) superwindows, they can be restored to rival the performance of a more typical dual-pane thermal replacement window, and usually at a fraction of the cost. Plus, there are other, nearly as important, advantages of restoring rather than replacing9. You not only save on your own heating and cooling costs, which reduces waste and your carbon footprint on the planet, but you also save the resources and energy cost required to manufacture new windows — which considering what new windows are made out of, is not an inconsiderable savings.

Reader Comments
Finally, an article on replacement windows that makes sense.... I've known for years that modern replacement windows were not a good fit for period homes, but I could never find information about repairing old windows that I could share with customers. How do I get reprints?

Carlos L., Denver, Colorado Restoration Remodeler

There is so much GREAT information in this article!!! I would not change a thing, .... it is the BEST article I've seen on making the case for window restoration and what I've been preaching to clients for years!

Jack A., Albany, New York Architect You also preserve, not just wonderful old-time workmanship, but the superb old growth wood from which your windows were made. We can't build windows like that any more. It's not that our craftsmen do not have the skill and experience. Any of our master carpenters or cabinetmakers could build a traditional window. But, we can't get that dense, heavy old growth wood, and the new wood is ... well, we're pretty sure it's wood, but it's not very good window wood.

If you own a home with old wood windows, then you are one of those fortunate beings that live in one of the 20 million American houses that do. You are a lucky soul. You have a choice of what to do about your windows. You can restore them or you can replace them with a modern factory-made window. Those who live in houses with steel, aluminum or vinyl windows don't have the choice. These windows cannot be restored. They can only be replaced.

This article takes a look at the case for restoring rather than replacing heritage wood windows. We would like you to consider, just consider, the possibility that your old wood windows can be saved and restored to as good as new and even better than new condition for less money and at a much lower cost to the environment than replacing them. There are advantages to doing so, but also some disadvantages. We look at both and leave it up to you to make up your mind.

Are Replacement Windows a Good Investment?
Before 1996, primarily as a consequence of the pervasive and unceasing marketing of replacement windows after the energy "crisis" of the 1970's, it was nearly universally thought that replacement windows were vastly superior energy performers.

Why does the Army care about windows?

The Pentagon is the biggest landlord on earth, public or private, owning over 300,000 housing units all over the world, in every climate you can imagine from the Arctic Circle to the Azores.

This does not include barracks, office buildings, guard shacks, hangers, or storage structures, all of which have to be repaired and restored from time to time.

The average military housing unit contains 14 windows. The cost to replace a window would be about $600. The cost to restore a window, about $200. This amounts to a savings of $1.6 billion — which is not small change even by Pentagon standards. But, then the State of Vermont and the U.S. Army Cold Regions Research and Engineering Laboratory joined together to actually test the performance of restored heritage wood windows, not by using computer simulations and a laboratory setting, but with a field test during actual winter weather conditions1. ....If it is your intent to pay for your re­place­ment win­dows out of energy cost sav­ings, think again. It won't hap­pen in your lifetime....

They repaired and restored 150 old wood windows all over Vermont, then tested them against replacement windows in similar homes. What they discovered was completely unexpected. They found that Well, this was not the result anyone expected. Everyone had assumed that the thermal replacement windows would be more efficient than restored windows, the only question to be answered was "how much more efficient". The last thing anyone expected to find was that restored old wood windows could be just as efficient.

The study concluded that while replacing a restored wood window/storm window combination with a low-e coated dual-glazed window did slightly improve thermal efficiency, the improvement resulted in a heating cost savings of only $4.45 per year.
Heat Cost Savings
Using a restored wood window with storm window as the base window unit, how much more savings in annual heating costs would result from these various strategies? (Numbers in red represent additional costs, not savings).
Window ConfigurationYearly Heat Cost Savings
Restored wood window with storm(reference window)
Replace window sashes with new single pane sashes, with storm($0.80)
Replace window sashes with new single pane sashes, with low-e and storm$2.57
Replace window sashes with new double pane sashes($0.37)
Replace window sashes with new double pane sashes, with low-e$3.13
Replace the whole window with a new double-pane window, with low-e$4.45


The low-e coating seemed to be the key to the slight improvement in thermal efficiency of the double-pane replacement window over the restored wood window with storm. The restored windows did not have low-e coatings. But what if low-e coatings were added to either the restored window or a new storm window?

This was one of the issues addressed in a study by the Lawrence Berkeley National Laboratory (LBNL) under contract with the Department of Energy to field test the thermal performance of single-pane wood windows with storms and low-e coating against dual pane vinyl windows with low-e coating2.

This study is interesting for the way it was done. LBNL has developed an "Accurate Window Thermal Test Facility" that is a mobile laboratory unit that can be moved into various climate situations. For this study it was moved to Reno, NV and faced north. The test windows were installed using normal installation and weather sealing practices, then left for the winter. An instrument called a calorimeter was used to measure temperature, wind speed and direction, solar intensity, and so on — all the factors needed to accurately determine thermal performance in the field. Measurements were taken every ten minutes.

What Others Say
Tom Silva "... [i]f your windows are single-paned, look into the cost of adding storm windows for a fraction of the cost of new windows."

Tom Silva
General Contractor
This Old House


Renovating Old Houses Cover "...While the thermal perfor­mance of a refurbished single-glazed window fitted with a tight storm can never quite equal that of the best fac­tory-made dou­ble-glazed win­dows, the dif­ference is not so great as to merit the re­place­ment of old win­dows solely for rea­sons of improved energy ef­fi­ciency..."

George Nash
Renovating Old Houses


Save American's Windows cover "...Homeowners tell me they know something is wrong with ripping out all their old windows and throwing them away, but they don't quite know what the alternative might be. They cannot find trades people to do the work.... If you are not a do-it-yourselfer don't worry. More and more savvy trades people are recognizing this new marked for traditional window maintenance and repair services.

John Leeke
Save America's Windows
 The study found that: Given that any improvement in thermal performance obtained by replacing your old windows with a new double-panel wood window would be very slight, if any. Is the energy saving worth the cost?

This is precisely the question researcher, architect and engineer Keith Haberern set out to answer in his study of heating costs in New Jersey. He developed a chart of energy savings based on various window improvement strategies that appeared in an article published in the Old House Journal in October, 20073. The article caused quite a stir at the time, and its results are quoted widely even today.

Mr. Haberern concluded that the difference in energy savings between a modern double-pane replacement thermal window and a restored wood window are insignificant. In fact, an old wood window with a new storm window outperformed a new double pane window, and was a lot cheaper with a short 4.5 year payback period (for the cost of installing the storm window). If low-e coating was added to the double pane window, then it outperformed an old window/storm window combination, but not by very much. In fact, if you already had a wood window/storm combination, replacing it with a double pane window with low-e resulted in very little energy savings. So little, in fact, that the payback period was 240 years. Ouch!
How Long to Pay Back Your Re­place­ment Window Investment?
According to Keith Haberern, the most cost-effective option, with a payback in 4.5 years, was simply adding a good storm window to the existing single pane window. The worst option is replacing a single-pane window that already has a storm window with a new double-pane thermal window. This option had a payback of 240 years. The most interesting finding was that a single pane/storm window combination with a U-value of 0.50 (R-Value=2) slightly outperformed the basic double-pane thermal window with a U-value of 0.58 (R-value = 1.72).
ActionResulting
U-value/
R-value		Cost*Annual Energy Savings (BTU)Annual Savings per WindowPayback Period (Years)
Add a storm window to a single-pane window0.50
2.00		$50.00722,218$13.504.5
Replace a single-pane window with double-pane thermal windowU-0.58
R-1.72		$450.00625,922$11.0740.5
Replace a single-pane window with double-pane thermal window, low-e glassU-0.35
R-2.86		$550.00902,772$15.1034.0
Replace single-pane window/storm window combination with double-pane thermal window, low-e glassU-0.35
R-2.86		$550.00132,407$2.29240.00

* Costs include installation and are based on costs in New Jersey, in­clud­ing heat­ing costs, at the time of the stu­dy.


These results are supported by research done by energy consultant Michael Blasnik. The methodology of his study could hardly have been simpler, or more elegant. He simply checked building permits in his upstate New York community to see which houses had installed replacement theremal windows, then examined utility bills both be­fore and after re­place­ment to see if any actual savings had resulted. He found the ac­tual aver­age an­nual sav­ings per house­hold was just $40.00, not per win­dow — but per house.

Based on these find­ings, Blasnik calculated that it would, on average, take 250 years for the cost of the replacement windows to be repaid from energy cost savings alone4. Again, ouch!

1930s Windows Advertisement

If you think the drumbeat of almost incessant advertising for replacement windows is something new, take a gander as this 1930's advertisement for Alcoa aluminum windows. Portrayed as absolutely essential for the up-to-date modern household; easy opening and low maintenance, too. Aluminum was the must-have up-scale replacement window of the period. Replacement windows have always been a large part of window sales. Today, replacement windows account for about half of the $30 billion annual window market. A 1990 study by William Hill5 at Indiana State University found much the same result. Looking at actual window per­for­mance in the field rather than the laboratory, Hill concluded that merely replacing old windows, without any other energy improvements in the home, results in an annual savings in energy costs of just 1.4% per year. He did not bother to translate this into dollars, but we did. In our town of Lincoln, Nebraska, this would amount to $33.12 for an average home. This is about what you would save replacing 10 incandescent light bulbs with compact fluorescent bulbs.

The Department of Energy's Energy Star estimates that in the upper midwest replacing 15 old single pane windows with Energy Star qualified thermal windows saves about $290.00 per year in a 2,000 sq foot house, or $19.33 per window6 per year. An Energy Star qualified window must meet certain minimum standards that make it generally superior to an average thermal window, but also more costly.

All of these studies have something in common. They were done by researchers who had no connection to the window industry, and have, therefore, no interest in selling you any replacement windows. And, not a single one of these third-party studies found evidence of substantial energy savings from replacing old wood windows, especially restored wood windows, with modern thermal windows.

Lies, Damn Lies and Computer Models
Window Advertisement So, where is the 35% savings on my heating bill everybody keeps talking about? We've all seen the ads in magazines, and on television: "Save 35%, 40%, even more on your heating bill. Replace your old, tired wood windows with our new Magnifico plastic windows!" How can replacement window manufacturers claim such huge energy savings? Are they just lying?

Not really (well, maybe just a little). First, in defense of window manufacturers, no reputable manufacturer makes such claims, and those companies that do are usually out of business pretty quickly. Second, window manufacturers don't do actual field testing of their windows. They rely almost entirely on computer models or simulations and laboratory testing. Computer programs are used to build mathematical models of the various window components and then calculate the window's resistance to heat transfer. Then, a physical test in a device called a “hotbox” is carried out to confirm the model. The problem is that the laboratory testing environment, the "hotbox", is very artificial. This artificiality skews the computer models, making them inaccurate. Heat is not actually lost the way the computer simulations assume it is.

Thermodynamics 101
As we indicated in our article on old house insulation (Insulating Your Old House), heat moves in three ways, radiation, conduction and convection. It tends to move from warm to cold. Heat in a something warm will move to a nearby cooler thing until the temperature of the two is equal, Then heat will stop moving. This is "thermal equilibrium", and although it can be achieved in a laboratory setting, it does not happen in the real world. Photo: U.S. Department of Energy Infrared photograph of heat loss. Yellow and red show places where heat is escaping. Even after replacing old windows with new thermal windows, about 25% of the heat loss in the house is through the windows. But, a significant amount is also being lost through the wood framing. Wood is a better conductor of heat than the surrounding insulation, and like the windows, is a a thermal break through which heat can more readily travel.

Radiation is the movement of heat in electromatic waves. It is how the sun's heat gets to Earth. Anything warm, even air, radiates some heat. IF it is very warm, it radiates a lot of heat. Put your hand in front of a radiant heater, and you can feel the radiated heat. Put you hand in front of your wall and you won't feel any heat. You wall is, in fact, radiating heat, just not enough to feel.

Thermal Conductivity of Common Materials
Conductivity of a material is determined by measuring how long it takes heat to move through a specified thickness of the material. The resulting is usually referred to as the material's Thermal Transfer Coefficient. The higher the number, the more conductive the material is.

Here are the Thermal Transfer Coefficients of common materials.
MaterialThermal Transfer Coefficient
Air (gas) 0.004
Aluminum 44.028
Argon (gas) 0.003
Copper 70.620
Fiberglass insulation 0.008
Glass (window)0.169
Gypsum drywall0.030
Hardwood (oak, maple)0.028
Krypton (gas) 0.001
Mineral wool (insulation)0.007
Plaster (wood lath)0.049
Silica aerogel0.001
Softwoods (fir, pine)0.021
Styrofoam0.006
Vinyl (PVC) 0.034
Conduction is the movement of heat on a microscopic level from molecule to molecule within a material. Heat one end of a steel bar with a propane torch and soon the other end is too hot to touch. Heat moved through the metal until it reached the other end of the bar. To reduce heat loss by conduction, materials that do not conduct heat very well are used. Most insulation, for example, is made of low-conductivity materials. Unfortunately for window engineers glass is not a low conductive material. If it were, then the simple solution to window insulation would be to install thicker glass to slow down heat movement. But, unfortunately, while glass is not as good a conductor as metal, it's good enough to move heat right along.

Convection is the movement of molecules within fluids (i.e. liquids and gases). Convection does not take place in solids because solids do not flow, nor in a vacuum, because there are no molecules in a vacuum to move around. But, in gases such as air, convection is one of the prime movers of heat, and one of the hardest to slow. It moves heat to your window, within your window, and then away from your window.

The Amazing and Complex Journey of Heat Through Your Window
All of these processes are involved in moving heat through your windows, and they all operate at the same time. Heat is at the same time conducting, convecting and radiating through your windows. But while all the processes operate all the time, each step in moving heat through your window involves a primary process.

Radiation, for example, operates all the time, but is not a prime mover of heat through your windows in winter. Radiation is, however, a prime source of heat gain in the summer. In winter the only things radiating out are your room walls, furnishing and the room air — and this is not much radiation. But, there is some, and window engineers combat this form of heat loss using low-e (for "low-emissivity") coatings. Low-e coatings are metal or metal oxides thinly deposited on the surface of the window glass. They reflect thermal radiation, but allow light waves to pass through, so you can still see out. Properly used, they reflect most of the long waves of heat radiation that originate in your house, but do not reflect as mucn of the short UV waves that come from the sun. This allows sun rays in to warm up your room, but does not allow radiation originating in your house to escape, thus helping retain the heat in your house.

Aerogel: Almost Science Fiction
Nicknamed “frozen smoke,” aerogel is a solid material so lightweight that it is just slightly heavier than air. Invented in 1931 by chemical engineer Samual Kistler to win a bar bet, it is, despite its name, not a gel, Aerogel Block A block of aerogel, almost, but not quite transparent. It's so light that a small puff of air would blow it across the room. but a dry solid that feels more or less like Styrofoam®. Most of aerogel is millions of tiny cells filled with air. This makes aerogel an excellent insulator. One inch of aerogel has an R-value of about R-30 — 10-15 times greater than fiberglass insulation.

It would be a great insulator in windows if it were transparent. It's not. It's translucent, but not transparent. And, while silicon aerogel has been used in skylights, it is not yet suitable where clear vision is needed.

It's also very expensive. Its production is slow and costly, although recent advances in production methods will probably bring the cost down soon.

There is little question that eventually researchers will come up with a transparent aergel — they're pretty close now. In fact, a company called Aspen Aerogel has already produced a modified aerogel that in thickness of 1/2" or less is as transparent as ordinary window glass, but it's still in the largely experimental stage.

Sandwich a thin slice of transparent aerogel between two pieces of glass, and you have super-insulated window glass. A mere 1/4th inch of aerogel could yield a window pane with an R-value of 12.5, which is approaching with the R-value of a 4" wall. This could make windows net heat sources in winter. Most house windows would take in more heat from solar radiation than they would lose. Large window walls could be a building's primary source of heat.

Initially aregel windows will be very expensive, but, like early LCD television screens, the price will come down fairly quickly as new production methods come on line and competition grows. But while radiation can make a major contribution to your heat loss, it is more likely that most of the heat loss through your windows is through a combination of conduction and convection. Loss through conduction is fairly straightforward. Heat moves through the glass and framing materials molecule by molecule until it reaches the outside. How quickly it moves depends on how resistant to heat transfer the material is. Glass, unfortunately, is a good conductor of heat, and glass makes up most of your windows.

Convection is a lot more complex. Heat moves with air, so air infiltration through leaks in and around your windows moves heat out of your house. Room air convection currents are the force that moves room air to your windows. Atmospheric air convection is responsible for whisking heat away from your windows. And, finally, air convection between panes of glass in a window are the primary mover of heat from warm inner pane to cooler outer pane.

Air Infiltration can be a major cause of heat loss, and not just through your windows. Warm air escapes from your house and cold air gets into your house through tiny cracks and gaps in walls and roof, and through gaps in and alongside your windows. A lot of homeowners are motivated to replace their old windows because they are "drafty". The proper cure for drafts is weatherization, not window replacement. Weatherization is simple and costs little — certainly much less than replacing your windows, which is akin to buying a new car because the old one has a flat tire.

Heat flow through a window. Convection and conduction are the primary forces through most windows. A poorly sealed window, however, may lose a lot of heat through air infiltration. Radiation is a relatively minor player in winter, and easily blocked. Room Air Convection: To get through your windows heat must first be moved to your windows. Heat moves to your windows piggy-backed on moving air. Cool window glass attracts air convection currents which do their best move all the heat in the room to your window glass. Some heat is lost through the walls and ceilings, of course, but in a well insulated house as much as 25% of your total heat loss is through your windows according to Department of Emergy estimates7. A 2000 sq/foot house has about 3440 square feet of walls and ceilings, and about 200 square feet of windows. Windows comprise just 6% of the building envelope. If six percent of your building envelope loses 25% of your heat, you can see how weak the thermal protection of a window really is. To heat, your windows are just very weak spots in your wall's insulation. And, if the windows are the easiest way out of your house, that's the path heat will take.

Your room air molecules give up heat to the glass in your windows by bumping up against the glass molecules. Then the heat moves, molecule by molecule, by conduction, through the glass where it is again in contact with air. The journey does not take very long. Window glass is only about 1/8" to 3/16" thick. Glass is a good conductor of heat. Not as good as metals, but plenty good enough. How to slow heat transfer through glass is one of the perplexing problems confronting window engineers. Insulating window glass may not be far away (See "Aerogel: Almost Science Fiction", this page), but it's not here yet. The current solution is to add another (or two more) panes of glass, trapping air between the panes.

Window Air Convection: The air trapped between two panes of glass if often described as "dead air", but it is anything but dead. It is, in fact, a very lively convection current that draws heat from the inside pane of glass and conveys it to the outside pane. The air touching the warm inside pane picks up some heat and starts to rise — warm air, as you know, rises. It soon reaches the top of the window sash where it cannot rise any further. Eventually it gets crowded against the colder outside pane and gives up some heat. Now colder and heaver, it falls, and soon there is a constant convection current moving inside the glass, conveying heat from the warm inside glass to the cold outside glass. The process is, unfortunately, very efficient and ruthlessly unstoppable.

Fill Gas is Guaranteed by Window Manufac­turers to Leak Out
No manufacturer war­ran­ties in-fill gas from leak­ing, be­cause they know full well it will leak out over time. This war­ranty lan­guage from Mil­gard Win­dows is ty­pi­cal: ....The gradual dissipation of the [fill] gas may occur natur­ally over time and is not a defect.....

"For Milgard Products with argon or krypton gas-filled insulating glass, Milgard injects the gas at the time of manufacture. The gradual dissipation of the gas may occur naturally over time and is not a defect. Other than gas loss due to seal failures, this warranty does not cover the gradual dissipation of inert gas or the amount of inert gas remaining in the Milgard Products at any time after manufacture."
But while convection cannot be stopped, it can be slowed. So, this is an area where window engineers try to retard heat flow. One solution that has been tried is to eliminate most of the air between the panes, creating a partial vacuum. We know that vacuum is an excellent insulator — in fact, probably the best insulator rated at R-45 per inch. Neither convection nor conduction work in a vacuum. The problem is that so far no one has been able to create a reliable glazing system that will hold up over time. The extremes of climate and the brutal environment in which windows exist always defeat the vacuum seal, usually in fairly short order. Every year there is a new technique with promise of solving the vacuum problem. And, every year it turns out it does not work in the long term.

Fill Gases So, the solution most often adopted by window manufacturers at present is to replace the air between the glass panes with a heavier gas such as Argon or Krypton (which has nothing to do with Superman® — that's Kryptonite, not Krypton). Heavy gases are more viscous and, thus, flow more slowly. They are also less conductive. Heavy gases can greatly slow down convection currents and reduce conduction, slowing heat transfer as much as 50%. Unfortunately, however, the gases are not permanent. They leak out. Not because of seal failure, but because the gas molecules simply work they way through the seal material, while air molecules, in turn, their way in. Seals have certainly improved since gas-filled IGUs were introduced over 40 years ago. But, they still permit gas transfer. Window companies say they have it under control, and that leakage is down to as little as 1% per year, in the lab8.

That's what they say, but how confident are they in that say? Their confidence can be judged by the fact that not one single manufacturer guarantees its windows against gas leakage, not one. In real life, you can expect that in 10 or so years the effectiveness of your in-fill gases will be all but gone. So that extra $5,000 you spent for a house full of Krypton-filled windows just went bye-bye.

Eventually, fill gas or no no fill gas, heat will reach the outer glass pane. Again, conduction takes over as the promary transfer process to move it through the glass, and now (unless this is a triple pane window, (in which case you need to go back a few paragraphs and start over) the heat is outside.

Atmospheric Convection: The heat jumps from the window glass to the outside air by again bumping molecules. If the heated air molecules stayed pressed against the window glass, it would act as pretty good insulation. But the molecules are immediatly whisked away by atmospheric convection currents. Outside air is constantly moving. It may be just a mild breeze, or it may be a tornado. Even if you can't feel it on those hot, muggy summery days, it's still moving, if only a little. These air currents keep moving cold air molecules against your warmer window glass where they keep drawing heat out of your window.

Rating Windows
So, as we have seen, your windows exist in an environment filled with air convection. Room air convection moves heat to your windows. Air convection between glass panes helps pass the heat through your windows and atmospheric air convection whisks heat away from your windows into the great beyond to add your tiny contribution to the problem of global warming.

What is U-Value?
NFRC Logo Windows are not rated using R-value like every other insulation product. Windows use a measure called a U-value or U-factor, based on a testing and rating protocol established by the National Fenestration Research Council, an industry-sponsored association.

U-value is not a measure of how well a material insulates. It is, in fact the opposite. It is a measure of how well a material transmits heat. A material with a high U-value permits a lot of heat flow. A 1" steel block, for example, has a relative high U-26.2. A material with a low U-value transmits little heat. A 1" block of Styrofoam® has a U-value of just 0.15.

Originally all insulation materials were rated using U-values. Then insulation makers realized that showing resistance to heat flow rather than heat flow itself would better help the public understand insulation effectiveness. So they created the R-value rating in use today in North America. R-value is nothing more than the inverse or reciprocal of a U-value rating.

U-value rating of a window is more understandable once it is translated into the corresponding R-value. The calculation is easy, just divide 1 by the U-value. For example, a double-pane thermal window typically scores at about U-0.45, which converts to an R-value of 1÷0.45 or R-2.2.

Why do window companies use U-Value rather than the more familiar R-Value? Simple, U-values are more confusing and less well understood, so they can be made to sound impressive. One young and enthusiastic window salesman told us that his company's windows were rated U-0.45 which meant that they "allowed only 4.5% of heat to pass through." We had to explain to him what the U-value really meant.

U-values when translated to R-values are anything but dazzling. As far as heat is concerned, a window with U-0.45 or R-2.2 is just a large thermal hole in your otherwise well-insulated R-19 wall.

For more information on R-values and U-values, see Will the Real R-Value Please Stand Up?. NFCR Window Testing Equipment Does this look like your house? In this "hotbox" testing environment approved by the National Fenestration Research Council, air convection is almost eliminated. The heat source is a blazing radiant heater rather than warm room air and minor radiation from walls, ceilings and furnishings. This very artificial testing environment is a far cry from the real world environment occupied by windows in your home. This is not, however, the environment in which windows are tested and rated for thermal insulation. To test for thermal performance, a window is placed between a hot plate and a cold plate inside a tightly sealed, environmentally controlled chamber, colloquially known as a "hotbox". Heat flow between the two plates is measured with a device called a heat flux sensor. The window's thermal performance is then estimated based on how long it takes to heat to transfer from the hot to cold plates, and then stated as a U-value.

What's missing from this test environment is the air that supplies convection currents. While it may seem illogical to ....The unfortunate con­se­quence of this ar­ti­fi­cial test­ing envi­ronment is that win­dow man­ufacturers tend to build win­dows that score high in the test envi­ronment, but do not necessarily per­form best in the real-world environment...." eliminate a process known to contribute substantially to heat transfer through windows, there is a reason. Convection is much too hard to control. It's even very hard to model. Climate scientists have been trying for decades to develop an adequate model of atmospheric convection. It takes a long time and a super-computer to produce even a rudimentary model.

A successful testing process must be relatively simple, inexpensive, and easily replicated not only from window to window, but from lab to lab. Air convection is far too complex. It is almost impossible to duplicate air convection currents reliably from one test environment to another. So the solution adopted by the testing protocol is to eliminate them as much as possible.

A Direct Measure of Heat Loss Through Windows
The current window thermal resistance testing process results in a U-value rating of a window. What a U-value tells us is how readily heat flows through a window, but in an abstract and arcane language that most of us don't understand and cannot interpret. What I really want to know is how much heat will be lost through my window. And the window's U-value does not tell me that, at least not in a straightforward way.

Testing how much heat a window loses is actually much less complex than the convoluted combination of hot box tests, computer simulations, and estimates now needed to come up with the U-value. It's a simple and straightforward measurement of heat loss. If we know how much heat a window loses, we know how much heat we have to add to make up for the loss.

Here's the test.

Build a a big well-insulated, air-tight box. This will be the testing chamber. Maintain a constant 0°/F outside the box. Add heat to bring the air temperature inside the box to 70°/F and maintain that temperature for 24 hours. For heating and cooling purposes, heat is usually measured in North America in British thermal units or BTUs. Any number of highly accurate devices called calorimeters can measure the number of BTUs added to the testing chamber. A British Thermal Un­it is the amount of heat en­ergy needed to raise the tem­pera­ture of one pound of wa­ter one degree Fah­ren­heit (1°/F) at a con­stant pres­sure of one at­mos­phere. It is equal to 1055 joules, which is the metric mea­sure of heat energy, and the one most used in scien­tific cir­cles. But in the U.S. and Canada the humble and large­ly out­dated BTU it is the tra­di­tional mea­sure of heat energy in the heat­ing and air con­dition­ing in­dus­tries. The number of BTUs of heat added to the box to keep the air temperature at 70°/F is always equal to the number of BTUs lost. If 2,000 BTUs must be added to keep the temperature constant, it's because the test chamber lost 2,000 BTUs through the floor, ceiling and walls over the 24 hour period. The number of BTUs lost is the Base Heat Loss.

Now cut a hole in the wall and install a window to be tested (in accor­dance with the manu­fac­turer's in­stal­lation in­struc­tions). Repeat the test with the window installed. The additional BTUs required to heat the box is the Additional Heat Loss (AHL) due to the test window. If the Base Heat Loss is 2,000 BTUs, but with the window installed, the chamber lost 6,500 BTUs, then 6,500 - 2,000 = 4,500 is the AHL of the window.

Let's say Window "A" has an Additional Heat Loss of 4,500 BTUs over a 24 hour period, while window "B" had an Additional Heat Loss of just 2,200 BTUs, do you have any trouble telling which is the better thermal window?

Isn't this all we really want to know about the window? Do we care which window has the lower U-value or higher R-value? Do we need to know what features of Window "B" made it the better window? Maybe it leaks less air, perhaps it is less conductive, or its low-e coating is more effective. The point is we don't know or nned to care why it is a better thermal performer. We just know that Window "B" is a better thermal window because it retains heat better than Window "A".

This test has the advantages of nearly exactly duplicating an actual home environment, being easy to understand, and a more direct and forthright test of window thermal performance. So, if it is easier to understand and a more direct and more honest test, why don't window manufacturer's use it? Well, it's easier to understand and a more direct and more honest test — it would be very hard to spin. Another simplification used in laboratory testing is the heat source. In your home, the heat source is complex and multi-faceted: Most of the heat lost through your windows comes from warm room air. But your inside walls, floor, ceiling, and furniture contribute a little radiant heat (not much, but some) and a radiator inside the room could contribute quite a bit.

In the laboratory the sole and only heat source is a bank of radiant heaters aimed at the window. Radiant heat can be precisely controlled and greatly simplifies measurement, so it is the standard for laboratory testing. But it produces just the one heat transfer process, radiation.

As a consequence laboratory test results are skewed. Lab tests have almost uniformly concluded that about 70% of the heat loss through your windows is by radiation. Conduction and convection account for only 30% or so. This is true, keep in mind, in an environment in which almost all air has been eliminated and radiant heaters are aimed directly at the inside of your windows. How close is this to the actual environment of your house?


"[We recommend] the retention and repair of original windows whenever possible...."

National Park Service
Technical Preservation Services The unfortunate consequence of this artificial testing environment is that window manufacturers tend to build windows that score high in the test environment, but do not necessarily perform best in the real-world. For example, most window manufacturers place a lot of emphasis in their low-e coatings to block radiation. Yet, outside the laboratory setting, radiation plays only a small role in heat loss in net heating climates like Nebraska's. Field studies have shown that low-e coatings have very minimal effect in winter, and just a small effect in keeping our houses cool in summer. But, in lab tests, windows with better low-e coatings score well, so window companies emphasize low-e coatings rather than working on measures to reduce convection and conduction.

So, while U-value comparisons between windows may help you decide which of two windows is the better thermal performer in the abstract, it does not tell you much about how either window will perform in your house. For the most part this necessary research has not been done, althougn it would be fairly simple to do (See: "A Direct Measure of Heat Loss Through Windows", above). Window manufacturers are not the least bit interested in showing that their windows do not perform as well as advertised, and no government agency seems to have been aroused enough to do a comprehensive formal field study — even though going green is now officially the government's policy.

From the limited field studies that have been done, however, we know that actual thermal window performance is well below that predicted by U-value ratings. There is plenty of evidence that properly restored old wood windows with storms perform at least as well as new thermal windows, and in the long run, as seals start to leak and the fills and coatings that temporarily boost new window thermal performance start to degrade, restored old windows may perform better.

Modern Window Design and Materials
Back in the day, windows were designed for longevity. Now, they are engineered for energy efficiency, with a long service life being a lesser consideration. This focus has drastically changed how windows are designed and manufactured, and how long they last. Most modern windows have an expected lifespan of less than 30 years — some of the best may reach 40 years — but 10-30 years is more likely. The problem is not that the windows are poorly made. Today's windows are usually very well made. The problem is the engineering and the materials used. They just don't last like old wood windows.

Complex Spring Balances A comparison of old and new sash balances, for example, illustrates the difference. If you raise the sash of an old Some window manufacturers still make windows the old way. This Heritage Series double hung window with traditional weight and pulley balance from Kolbe and Kolbe Millwork Company, Inc. is available in a number of wood species, including oak, pine and cherry. If we did not make our own windows, this is the one we would buy. double hung window, it stays in place at whatever position you leave it. This is possible because the weight of the window sash is counter-balanced by two iron weights attached to the sash by ropes that ride in pockets built into the wall alongside the window. The mechanism is simple, and works by gravity. There is nothing to break but the ropes, which can be easily replaced and once replaced last between 50 and 100 years, or even 200 years if bronze chain is used in place of rope.

Modern, self-contained, replacement windows cannot use sash weight pockets built into the wall, so different balancing mechanisms had to be developed. These are all some form of metal spring. The tension in a spring is what holds the sash in place when the window is open. Spring ballasts, however, unlike simple iron weights, are complex mechanical Complex Spring Balances
One type of modern tension spring sash balance. Compare to a simple iron weight hanging from a cord, and it is easy to see how many more pieces there are to malfunction in this mechanism. devices prone to breaking. Metal springs themselves are subject to metal fatigue which can cause the spring to lose tension over time or even fail completely. We have replaced some worn out spring balances less than four years old.

Vinyl (PVC) Frames Vinyl is another culprit. Vinyl expands and contracts nearly twice as much as wood with changes in temperature, and seven times more than glass. This amount of expansion and contraction makes it hard to keep gaps from developing between vinyl components of the windows. The Canadian Natural Resources Consumer Guide from 2005 states “The disadvantages of vinyl framing material is that vinyl expands and contracts with temperature, opening up cracks for air leakage.” The Canadian Center for Mineral and Energy Technology, study of long term performance of operating windows10 concluded that “Air Leakage in vinyl windows increases 136%, significantly more than aluminum or wood. Un-reinforced PVC profiles have a lack of rigidity and a high coefficient of expansion; PVC profiles are subject to distortion.” In their studies, they found that vinyl windows have a tendency to bend, distort, and even crack, especially in "regions of Canada that experience cold winter months". (What regions of Canada don't experience cold winter months?)

Vinyl deteriorates when exposed to ultra violet (UV) rays. Those of us of a certain age remember well how vinyl dash boards in the ol' Chevy used to crack and split after only a few years exposure to sunlight. The vinyl is better these days. UV inhibitors retard deterioration, but nothing can stop it entirely. Vinyl window parts, especially the thin, flexible vinyl used in balance mechanisms, will deteriorate over time, especially with repeated use. After a few years they become brittle, break and need to be replaced.
Where to Find Window Parts
Some of these sources are not used to dealing with consumers, and expect you to know exactly what you want. Others, like Blain Hardware, even have technical support departments that can help you decide what you need.

AA Window Parts & Hardware, 800-804-0147
http://www.aawindow.com/index.htm

All About Doors & Windows, 816-221-8543
http://www.allaboutdoors.com/

Blaine Window Hardware, 800-678-1919
http://www.blainewindow.com/

Phelps Company, 802-257-4314
http://www.phelpscompany.com/

Pickens Window Parts, 513-931-4432
http://www.pickenswindowparts.com

Strybuc Industries, 800-352-0800
http://www.strybuc.com/

Swisco, 856-283-0390
http://swisco.com/  
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Please report broken links.
Vinyl softens at temperatures greater than 165°/F, and its not that uncommon to see such temperatures in the unventilated space between window panes on a bright, sunny day. Excess heat can also cause vinyl to warp and twist. While vinyl window manufacturers say they have that problem under control, few manufacturers actually warrant vinyl windows against warping, twisting or cracking, and those that do limit the warranty to just a few years.

Links & Resources
from The National Trust for Historic Preservation
Repair or Replace Repair or Replace: A Visual Guide
Download PDF.
Field Guide Window Types:
A Residential Field Guide
Wood Windows Tip Sheet Historic Wood Windows Tip Sheet
Download PDF
Contractor Map Window Know-How:
A Guide to Old-Building-Friendly Contractors
General Directory of Weatherization
Links & Resources
Windows Directory of Window
Links & Resources
Are any of these links broken? Please report broken links.
For more information in vinyl window problems, see vinyl-windows.org for a round-up of government and other third party studies of vinyl windows.

Leaking Spacers Double- and triple-pane glass in thermal windows are manufactured in what are called Insulated Glass Units (IGUs). Spacers between the glass panes not only separate the panes, but seal in the gas between the panes. Air between panes of glass must be very dry. If the spacer holding that two panes of glass together springs a leak, moist room air will get between the panes and condensation will form, which may lead to mold, mildew and other nasties inside your IGU where you cannot get at it. There is no cure for this problem. It cannot be repaired. The entire unit has to be replaced. Seals are much better now than they were when first IGUs were first marketed in the 1930s as Thermopane®. Still, no one has yet invented a seal that does not leak. Some leak sooner, some later, but they will all leak someday. The environment that spacers have to survive is brutal. Temperatures can be as hot as 180° in summer, plummeting to -30° in winter, or even worse. It's very hard to come up with an adhesive that works for a long time in that kind of environment. It is common to find spacer leaks in even very good windows within 10 years of installation and in some poorly made windows within one or two years.

Window Sashes that Cannot Be Repaired Like today's Integrated Glass Units, modern window sashes are often single units that cannot be taken apart for repair. They can only be replaced using a new sash supplied by the original manufacturer. You have to hope the window manufacturer (1) is still in business and (2) still makes the part. Plus, you can expect the replacement parts to cost nearly as much as a new window. The manufacturers have a monopoly on replacement parts for their own windows and are not at all bashful about charging what the market will bear.

Old Windows: Built to be Re-Built and Re-Built and Re-Built....
What Others Say
National Trust for Historic Preservation
Click to Read the Report
Click to Read the Report.
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"...Given that an average house has between 24 and 30 windows, and the typical replacement window unit costs between $500-1,000 each, does an investment of $12,0000 or more make sense? On the flip side, the cost to restore an existing window and add storm windows (where appropriate) will generally be much less....

Many window replacement manufacturers claim greater savings than actually occur. Since windows account for at most 25% of heat loss, the payback and time to recoup your investment in terms of energy savings could take between 40 and as much as 200 years, based on various studies. A study from Vermont show the saving gained from replacement windows as opposed to a restored wooden window with a storm is only $.60. The added problem is most replacement windows will not last as long as 40 years, much less over a hundred years. And, some are being replaced only after 10 year of service."

National Park Service
Click to Read the Report
Click to Read the Report.
   Is this link broken? Please report broken links.

"[We recommend] the retention and repair of original windows whenever possible. We believe that the repair and weatherization of existing wooden windows is more practical than most people realize, and that many windows are unfortunately replaced because of a lack of awareness of techniques for evaluation, repair, and weatherization. Wooden windows which are repaired and properly maintained will have greatly extended service lives while contributing to the historic character of the building. Thus, an important element of a building's significance will have been preserved for the future." (Emphasis supplied). The 300 Year Old Window
Photo: Annapolis Heritage Society
Oldest Window Built between 1690 and 1720, this over 300-year-old window in the Sinclair Inn Museum in Annapolis Royal, Nova Scotia is possibly the oldest still-working wood window in North America. If you know of one older, please let us know. The Sinclair Inn Museum in Annapolis Royal, Nova Scotia can claim what is possibly the oldest double-hung wood window in North America. Built sometime between 1690 and 1710, it was probably recycled from a still older building.

After 300 years, it still works.

Unlike modern windows, old wood windows were made to last for many generations, so they were built to be repaired, over and over again, if necessary. The old-time craftsmen knew that their windows would last a good long time, but not forever. So they built windows that could be easily fixed when something finally did give way.

An old wood window is put together so it can be easily taken apart and any of the individual parts repaired or replaced. There are no exotic materials in an old window, just wood, glass, iron, rope and a little bronze or brass for the hardware. No vinyl, no unpronounceable chemical compounds, just basic stuff available almost anywhere that has Photo: Phelps Company,
Restored Weight Pulleys Restored sash weight pulleys with new weight chains in place of pulley ropes. This is a 200 year repair. a lumber yard and a hardware store. There is no chance the parts won't be available even 200 years from now, unless we simply run out of trees — which, despite the the hysterical alarms of the more radical elements of the environmental movement, is not all that likely. Anyone with some basic carpentry tools, a little understanding of how windows work, and decent eye-hand coordination can restore an old window.

Restoring Window Function Unlike modern windows, old windows rarely break. They may stop working, but the culprit is seldom the window itself. It is accumulated layers of paint that have glued the sashes to the frame, and broken pulley ropes. These are easy fixes that usually take just a few minutes. The sash ropes we use today are not the ropes of 100 years ago. The cotton ropes used then had a life expectancy of just 25 years. The new nylon/cotton ropes last up to 100 years. Freed from decades of paint, with new pulley ropes, and with a little scraping and sanding, most old windows work like new. If you want the repair to last nearly forever, use bronze sash chain instead of rope. Some people don't like chain because it's noisy, but considering that a properly installed sash chain never has to be replaced again, the trade-off is, in our humble opinion, a no-brainer.

Photo: Gorell Windows & Doors. Correll Storm Windows Adding a good storm window to a restored wood window increases its energy performance to the same or better than a standard replacement window. Repairing Deterioration The next step is to check for rot and deterioration. Water is a window's worst enemy. Although poor design, sloppy installation, wood-loving insects and baseball-loving kids can contribute to a window's demise, the usual culprit is wood rot caused by standing water.

We find this primarily on the sashes and stool or sill of the window. It's not hard to fix. If the problem is minor, and exterior (which is where it usually is), then a little outdoor spackling and some new paint solve it. Otherwise there are special two-part epoxy fillers that are — or so their manufacturers' say — even stronger that the wood being replaced. If the problem is even more serious we may splice in some wood or even remove and replace the deteriorated part with a new, matching, part made in our cabinet shop. If necessary, we can build and entirely new sash that duplicates the old one exactly. In 40 years we have had to do this exactly twice.

Photo: SpencerWorks SpenserWorks™ Storm Window Wood combination storm windows from SpencerWorks combine traditional appearance with up-to-date functionality and very efficient thermal performance. Weatherproofing Now we need to look at weatherproofing. Over the years the wood in your window has dried out and shrunk a little. This is the reason your sashes may be loose in their frames and sometimes rattle in the wind. Since the sash is now smaller, air can creep around the sash. The weatherstripping may also need attention. A lot of old window makers used bronze for weatherstripping, and it may be intact, but often it has come loose because the nails used in those days to attach the weatherstripping have worked themselves out — or the bronze may have been removed by some old painter too lazy to mask it off when painting. We use new spring bronze slipped around the sides of the sashes to eliminate air leaks, tighten them up in frame and provide a nice slick surface to ride on. Horsehair felt made specifically for windows, or silicon bulb weatherstripping (but not rubber or plastic which do not last) can be used where the sashes meet the frame at top and bottom and at the meeting rail to bar air infiltration.

Spring Bronze Weatherstripping Spring bronze in the sash channel not only seals air leaks but provides a slick surface for the sash to slide up and down on. Insulating Around Windows Once the weatherstripping is done, we look at the insulation where the window meets the wall and in the sash weight pockets. Even if you have had your old house insulated, the insulators usually miss that small 1/2 inch or smaller gap between the window frame and the wall stud. We seal this area with low-expansion foam.

The pocket on each side of the window that holds the sash weights is also likely to be empty of any kind of insulation. Obviously we can't fill it with foam because the sash weights need to move freely so the window will work properly. But, the sash weights do not take up all of the space in the pocket, and the space they don't take up can be insulated using a high R-value rigid board like polyisocyanurate (we don't try to pronounce it either — its "poly-EYE-so" to us) which has a rating of R-7.2 per inch. We use 2-1/2 to 3" of it in total together with expanding foam to arrive at a total R-value of R-18 to R-22 in the sash pocket — which is probably more than the R-13 to R-19 you have in your walls.

Replacing Single-Pane Glass What we never do, and don't recommend, is replace single pane glass with dual pane glass. We used to try this, but it never worked very well. Old sashes are just not milled for the extra thickness of dual pane glass, and the sash pulley/weigh system is not designed for the increased poundage of dual pane glass. There are also technical difficulties in installing the glass. New Integrated Glass Units are generally not installed in the same manner as the traditional single-pane glass. And, in the end, the gain in energy performance, if any, is negligible. A much better choice is to install a good storm window over restored sashes. Unless its cracked or broken, leave the glass alone.

Lead Paint and Putty Before April, 2010 no special precautions were required, other than common sense, in dealing with old lead-based paint and glazing compound. Now, elaborate measures are mandated by the EPA, most of which are still, hoever, just good common sense. Never grind off lead paint so that the paint particles discharge into the air. The best course is to use removal methods that do not produce dust, such as heat or steam removal. But if you must grind, make sure any dust is captured in a HEPA (Not HEPA-like, but actual HEPA) vacuum. Cover the ground or workroom floor in heavy plastic, and drape any doorway into the rest of the house in plastic. Cover all air intake vents in the room. Use reasonable care to ensure that lead particles are not tracked into other rooms on boots or shoes — usually a brush left by the doorway for dusting off shoes is all that is needed. We hang ours on a string. Wear a N-95 dust mask. Vacuum thoroughly at the end of each day, and at the end of the project, fold the plastic edge to center to trap dust particles, put the plastic in a heavy plastic bag, and tie the neck of the bag using a "gooseneck" tie.

How to Restore an Old Wood Window
Learn the basics of restoring old wood windows from this video by Preservation North Carolina. It won't make you a window preservation expert, but it will get you started safely. If you have never restored a window before, you might check out our article Can I Do It Myself before you begin.

Adding Storm Windows If this seems like a lot of work, it is. But, restoring your old window is about half the price of replacing it with a new thermal window, and wastes nothing. Old growth hardwood is saved from the landfill, and a lot of good old-time craftsmanship is preserved. A typical window can be restored for between $200 and $300. Of course it is not yet as energy efficient as a new window. For that we are going to have to add a storm window.

A good quality white aluminum storm window installed will run about $80. An upscale wood combination storm window from a company like SpencerWorks will cost a bit more. If you already have storm widows, then you are just that much ahead. But, assuming you don't, your cost to repair your old wood windows and add a good storm window is about $325.00 vs. $500 and more to replace them. This is a savings of $5,500.00 in a 20-window house. For your investment you get a window that should be good for another 100 years, while a replacement window is doing very well to last 30 years. Your window performance is just as good if not slightly better and you saved 45% of the cost of installing replacement windows.

If you really want to save energy costs, assuming your attic and walls are already insulated at least to code, your heating and cooling system is already very high efficiency and all your doors are weather-stripped, go buy a high efficiency water heater ($1,000), put $4,400 in the bank against a rainy day, and treat yourself to a really lavish steak and lobster dinner ($100.00), for being "energy smart". Just the water heater alone will save many times more energy dollars than a whole houseful of replacement windows.

The Energy Cost of Making New Windows
Abby Did someone say "pizza"? Electricity from Coal
The average thermal energy content of a ton of coal is 6,150 kilowatt hours (usually written as 6,150 kWh/ton). But, not all of that energy is turned into electricity in the power plant. Most goes out as heat. Only 40% on average becomes electricity in a very efficient coal-fired plant — or about 2,460 kWh/ton.

A 500 megawatt coal fired power plant produces 3.5 billion kWh per year, burning 1.43 million tons of coal to do so. It also produces a lot of pollution, including... And, it produces small amounts of just about every other element in the periodic table, including radioactive elements. In fact, a typical coal-fired generating plant emits more radiation into the atmosphere than a nuclear power plant.

So don't pay much attention to the notion of "clean" coal. Its just PR hype. One of the fastest ways to get tossed out of our decidedly un-pretty, over-the-warehouse offices is to start talking about how environmentally friendly your company's replacement windows are. (Its not the fastest way. The fastest way is if Abby, our old warehouse dog, decides she doesn't like you. She is a remarkable judge of character.)

Replacing old wood windows is not green. It sounds like it ought to be green, and certain window salespersons will imply that it's green. But, it's not even close to being green.

Replacing a window means that the old window is discarded, usually in the landfill. What is thrown away with an old window? A lot of irreplaceable old growth lumber, hours of careful craftsmanship, and a little American history. It took work and energy to build the window in the first place — to fell the tree, mill the lumber, build the window and install it. That is all now in the trash.

And, it takes a lot more energy to build the replacement window. Energy consultant Keith Haberern estimates that building a new replacement window requires 4.3 million BTUs. of energy. If this sounds like a lot, keep in mind that it's not just window manufacturing that uses energy. It's producing all "....Replacement win­dows can­not pos­si­bly save enough ener­gy dur­ing their life­span to pay back even the energy in­vest­ed in their man­ufac­ture....." the components and materials that are go into the manufacturing process. Making the glass, felling the trees and sawing the lumber, producing the paint, plastics, metal fasteners, even the paper labels all involve energy expenditure. Not to mention all the shipping involved — some of it from half-way around the world. By the time the window leaves the factory the total energy investment in the window is very substantial.

How much energy is 4.3 million BTUs? It takes about 13,000 kilowatt hours of electricity to produce 4.3 million BTUs. And, it takes 13.65 tons of coal to generate that much electricity (according to Department of Energy estimates). In an entire year, an average home in Nebraska uses 18,000 kilowatt hours. So one replacement window embodies almost 9 months of household power usage. That's for just the one window. If you have the average 20 windows in your house, just building your windows will use nearly 15 years worth of household energy. So even if your replacement windows saved 25% of your electric bill (and there are no replacement windows that come even close to that), it would take 60 years to pay back just the energy used to build your new windows. The math just does not work out. The life expectancy of the windows is less than the payback period.

Replacement windows cannot possibly save enough energy during their lifespan to pay back even the energy invested in their manufacture.

Which is why, if you are a window salesperson and start talking about your green replacement windows, you are going to get the bum's rush. We hate to be impolite, but there are absolute limits to civility.  
1. B. James, A. Shapiro, S. Landers, D. Hamenway, "Testing Energy Performance of Wood Windows in Cold Climates", (1996.) University of Vermont, U.S. Army Cold Regions Research and Engineering Laboratory. (Download (PDF):

2. Joseph H. Klems, "Measured Winter Performance of Storm Windows", Lawrence Berkeley National Laboratory, (2002) (Download (PDF).

3. Keith Haberern quoted in Noelle Lord, "Embracing Energy Efficiency"The Old House Journal pp 40-45, Oct, 2007.

4. Michael Blasnik quoted in R. Yagid, "Should Your Old Wood Windows be Saved", Fine Homebuilding, Issue 210, March 11, 2010. Download PDF

5. William W. Hill, "Replacement Windows and Furnaces in the Heartland; Indiana's Energy Conservation Financial Assistance Program", Center for Energy Research, Ball State University, Indiana, 1990. Download PDF

6. U.S. Department of Energy (2005). www.energystar.gov/windows.

7. U.S. Department of Energy Energy Savers.

8. The study most frequently quoted to support the claim that gas leakage is down to as little as 1% is J. Plavecsky, "Leaking Out the Facts, Door and Window Maker, Sept-Nov, 2000, which found that the amount of leaking was determined by quality control in the factory, the method of inserting the gas, the type of seal used, and the care with which the seal was sealed. He tested seals in a laboratory environment using a process of "accelerated ageing....said to represent five years of normal field exposure". The results were average leaking as high as 18% and as low as 0.9% over the simulated five years. Quite a range, and there is no way for a consumer to judge whether his replacement window is at the high or low end of this leakage rate.

9. W. Sedovic, J. H. Gotthelf, "What Replacement Windows Can’t Replace: The Real Cost of Removing Historic Windows" Journal of Preservation Technology, Vol. 36, Number 4, pp25-29, 2005.Download PDF

10. Canadian Centre for Mineral and Energy Technology. A Study of the Long Term Performance of Operating and Fixed Windows Subjected to Pressure Cycling. Republished by CANMET Efficiency and Alternative Energy Technology Branch. Publication # M91-7/214-1993E. 1993.

 
More Good Reading
Understanding Energy-Efficient Windows. Fine Homebuilding, The Taunton Press.

Energy Savers: Tips on Saving Energy and Money at Home. U.s. Department of Energy. (PDF)

Do-It-Yourself Home Energy Assessments. U.S. Department of Energy.

Selecting Windows for Energy Efficiency. U.S. Department of Energy.

Alvarez, Kimberly K. & John D. Alvarez II, AIA, "Restoring Our Appreciation of Historic Wood Windows: Making a Case for Restoration Versus Replacement", The Local Landmarket, Issue 11, March 2009. New York State Office of Parks, Recreation and Historic Preservation, Field Services Bureau, Division for Historic Preservation

How to Restore Windows
John Leeke's Historic Homeworks. Has a number of helpful videos and articles on restoring old windows, and a discussion forum where you can ask questions and get helpful answers. If you are serious about restoring your windows, you will want to invest in Leeke's Save America's Windows book which is pretty much the old window bible, and has, among other useful information, a list of window restoration experts organized by region.

Beth Goulart, "How To Restore Sash Windows" Old House Journal.

Thomas Baker, "How to Repair Sash Windows" This Old House.

William T. Cox Jr., "Sash Window Clinic: Maintaining the Mechanics of Double-Hung Windows" Old House Journal.

John H. Myers, "Preservation Briefs 9: The Repair of Historic Wooden Windows". Technical Preservation Services, National Park Service., U.S. Department of the Interior.

John Michael Davis, "New Life for Old Double-Hung Windows" Fine Homebuilding
Are any of these links broken? Please let us know.