Insulating Your Old House, Part 1: How Heat Moves In and Out of Your House

Is your old house drafty in winter, swampy in summer? Almost impossible to heat and cool effectively?

That's because when your house was built a half-century or more ago, no one thought much about insulation. Energy was abundant and cheap. Half of the world's oil was produced in the U.S. Conserving energy was just not very important. Experts believed that the 4" of "dead" air space captured inside the stud cavities of your walls, combined with a vapor barrier, was enough to keep heat inside your home.

We now know the experts were wrong.

In theory, air is a good insulator, if it can be kept from moving. Dry, absolutely still air has an R-value of 3.6 per inch of air — as good as most insulation materials.

But, the air inside your walls is never still. It's moving constantly, and with that movement creates a convection flow that results in significant heat transfer out of your house in winter, and into your house in summer.

R-value, U-value... What Do They Mean?

Each of these values is a measure of heat transfer through a material.

The U-Value (or U-factor) is a measure of thermal conductivity — how well heat flows between the warm side of a material and its cold side. The lower the U-value, the more slowly heat is transferred.

The R-value is a measure of thermal resistance to conductivity. The higher the R-value, the more resistance to heat transfer the material has.

The two rating systems are opposites. The more a material resists heat transfer (high R-value), the more slowly heat is transferred (low U-value). A material that does not resist heat transfer (low R-value), conducts heat very well (high U-value). In fact, the U-value of a material is what mathematicians call a reciprocal of its R-value, and vice-versa.

Converting from U-value to R-Value, and Back Again

The conversion formula from R-value to U-value is: U-value=1/R-value. So if the material resistance to thermal transfer of R-2.2, its conductivity rating or U-value is 1 divided by 10 (1/10) or U-0.45. This is a typical U-value for a double-pane thermal window or a single-pane window with a storm window.

U-value is typically used in rating windows. The U-value of a window is an average of measurements taken at several points on the window. Converting these into the more easily understood R-value is basically the same process as converting from R-value to U-value. R-value = 1/U-value. So a window glass rated at U-0.45 has an R-value of 1/.45 or R-2.2. Compare this to the R-13 required for house walls by the Nebraska Energy Code, and you can see that the window is a significant hole in your wall insulation.

American vs. European (Metric) U-values

Just to make things even more confusing, there are actually two widely used U-value ratings, the English/American rating and the European or metric rating also called the K-value or K-factor. When looking at U-values, you need to know whether it is the English/American U-value, or the European rating. Generally, a U. S. rating will be written on window labels in the form "U-value (U.S./I-P)" which distinguishes it from the metric factor.

R-value is used primarily in the U.S. and Canada. The rest of the world uses the European U-value except Britain, which uses the English U-value. The European U-rating (based on meters and degrees Kelvin) is not a reciprocal of a material's American R-value (based on feet and degrees Fahrenheit). To arrive at the metric U-value of a material, divide 1 by its R-value then multiply the result by 5.682. To convert a metric U-value to an American U-value, multiply the R-value by 0.176 then divide 1 by the result.

Will the Real R-Value Please Stand Up?

To enormously complicate decisions about insulation materials, there is not just one R-value but several. Each of these conveys useful information but confusion can result unless you know what R-value is being reported.

Center of Cavity R-Value

The reported R-value rating for an insulation material rates only the insulation material. A 4" batt rated at R-13 states only the resistance of the batt material itself. It does not rate the entire wall in which the batt is installed. This rating is commonly referred to as the "Center-of-Cavity" rating. When you see R-13 printed on the back of a fiberglass batt, this is its center of cavity rating and is likely to be higher than its actual performance in your wall after it is installed. Manufacturers of insulation materials are required to calculate and conspicuously display this R-value on their materials by federal law.

Clear Wall R-value

A more accurate way of measuring thermal loss is to install the material in a wall and then measure the thermal resistance of the wall including its necessary framing members (but not windows, corners, or joints at roofs, foundation, and floors). This is the "Clear-Wall" R-value and it is almost always lower than the center-of-cavity rating because it includes such things as wood framing members in the measurements, and wood framing members are usually not as insulating as dedicated insulation materials such as fiberglass or cellulose. (See the chart in the main article).

Whole Wall R-value

In a recent study of wall insulation ratings, the Oak Ridge National Laboratory (ORNL) has developed a more accurate rating: the "Whole Wall" rating. According to the study, measures of "Clear-Wall" and "Center-of-Cavity" thermal resistance are misleading because they do not take into account all of the possible framing material "thermal shorts" or "bridges" through the insulation. A short or bridge is simply a place in the wall where the insulation is interrupted by other materials. A stud in a conventional wall is a short, as is the gap left for an electrical box.

Oak Ridge proposes an R-value rating for the entire opaque wall (not including windows and doors) to measure the thermal performance of not only the insulation and structural elements but also the effects of their installation and typical wall interface details such as intersections with other walls, floors, foundations, and windows. The standard also considers previously ignored factors such as moisture resistance (the insulation value of some materials when wet may degrade considerably), thermal mass, and air transfer resistance (heat moves with air) of insulation materials.

The results were surprising and even scary. The Laboratory studies found large differences between the reported ratings of insulation and its actual thermal performance in a wall. Materials can lose up to half of their rated R-value when installed in a typical wall. The best performers were insulated concrete forms and structural insulated panels(SIP). A 4" SIP wall was found to be more effective at blocking heat transfer than a 6" conventional stud-framed wall and with 15 times less air infiltration. The worst performers were batt materials, especially fiberglass batts. Even very careful installation of these materials leaves small gaps and voids through which heat escapes, dramatically lowering the material's effective R-value.

To read a summary of the study report, go to the ORNL Building Envelope Research website. To calculate the R-value of the insulation in your home, use the ORNL Whole Wall Thermal Performance Calculator. The results will probably surprise you.

Anyway, now we have been introduced to U-values and R-values and understand better why insulation contractors spend a significant portion of the year in therapy.

There is nothing we can do to stop air from moving and heat from moving with it. All we can do is slow it down. We do this by creating a barrier between the hot and cold objects so that transfer takes longer. This barrier is insulation.

The Building Envelope

No matter the shape or size of your house, it is, from an environmental scientist's perspective, merely a box composed of a roof, floor, and walls. This box separates us from the outside environment. It keeps the wind, rain, bugs, and critters out. It also is our main line of defense against being too hot or too cold. Environmental engineers refer to the box as the "building envelope".

Most people are most comfortable when the temperature of the air around them is about 70° F and humidity is around 40%. To maintain this environment in our homes, we add heat (and sometimes humidity) to the box in winter and extract heat and humidity through air conditioning in summer.

When we do this we create a heat imbalance. Adding heat to our house in winter means the inside of the building envelope is warmer than the outside. Nature abhors a heat imbalance and begins searching for ways to restore the balance. The heat inside fights hard to get outside where there is cold air to warm up. To get outside, it has to pass through the building envelope. This is where we try to block it.

It's a contest we cannot win. Heat always finds a way out — eventually. The best we can do is a guerrilla-like delaying action. We can make it so hard to get out that it takes a long time. And, that's the objective of insulation and other weatherization measures — not keeping heat from moving through the building envelope but making it take longer.

The longer we can hold heat inside the building envelope, the less often we have to add heat. The less often we have to add heat, the more money we save, and the less pollution we generate. Without insulation, our homes can lose all their heat up to seven times each hour in winter. With adequate insulation and weatherization, we can reduce that to as little as once every three hours. This is a very substantial difference, saving you a lot of money, and reducing your impact on global warming.

How Heat Moves

Heat can transfer through your building envelope in three ways: convection, conduction, and (to a much lesser extent in our area) radiation.

Convection is the star player. It has a role in nearly all heat movement in and out of your house. Convection currents move air in and out of your house through gaps in your walls and roof and around windows and doors. Heat and cold piggy-back on the moving air. Hot air leaking out of your house carries heat out of your house, and cold air leaking into your house has to be heated up.

Convection also carries heat through the walls and roof that make up your building envelope. Conduction and radiation also play a part but convection is the prime mover. If convection can be slowed, your heat loss will be dramatically reduced, and the primary aim of most insulation is to reduce convection.

Atmospheric Convection: Air Leaks and Heat Transfer

Heat and cold piggy-back on flowing air. If you open your door in winter, hot air flows out at the top of the door, and cold air flows in at the bottom. A heat transfer has taken place — an infiltration of cold air, and an exfiltration of warm air. The same effect occurs where your walls have air leaks in them. Air moves through even very small voids in the wall coverings and through the gaps that may be left around the window and doors or where unlike materials meet.

Different materials expand and contract at different rates in response to changes in temperature and humidity. The joint where two different materials meet is always a weatherization problem. Even if the joint was well sealed to begin with, after a few years of expansion and contraction, a gap has probably opened. It may be a very small gap but every small gap hurts. Heat moves with airflow through the gaps in your building envelope.

Some air movement through your building envelope is necessary. You need to exhaust stale interior air from your home and bring in fresh outdoor air. At least eight complete air exchanges every 24 hours is the minimum recommended by the American Society of Heating, Refrigerating and Air-con­di­tioning Engineers. Dwellings that are so tightly sealed that less than that minimum exchange occurs need to use some form of mechanical ventilation system to augment passive air transfer.

As the owner of an old house, you don't have that particular problem. You have the opposite problem. You have too much airflow through your walls and roof. Old houses may allow as many as 168 complete air exchanges every day.

Each air exchange means that all of the heated air in the house has leaked out and cold outside air has entered your house to replace it — all of which has to be heated or cooled depending on the time of year. As much as 40% of your heat loss is through air transfer.

Controlling and minimizing this transfer is the job of "weatherization" — the process of sealing cracks, gaps, and holes (especially around doors, windows, pipes, and wiring) with caulk and weatherstripping and replacing drafty doors and windows or weatherproofing them.

But, insulation also plays a role. Certain types of insulation, notably the foams and cellulose are good at seeking out and sealing small voids and cracks. These types of insulation permit very little airflow, and while they do not replace a good program of weatherization, they contribute significantly to helping it succeed.

Convection Inside Your Walls: The Heat Conveyor

Air infiltration and exfiltration are not, however, the only way convection transfers heat in and out of your house. Most of the heat transfer through an uninsulated wall is by air convection, which creates, a conveyor belt of air inside your wall that is very effective at moving heat from the warm side of the wall to the cold side. Here's how it works:

Let's say it's winter. You are pouring heat into your house to stay warm. It's a toasty 75° inside your house. The interior drywall or plaster of the stud cavity is, therefore, nice and warm. Outside, it's 35°. The exterior siding and sheathing enclosing the wall cavity are very cold.

Your Old Windows

The largest thermal break in your walls is your windows.

From an insulation engineer's point of view, windows are holes in your wall through which a lot of heat gets out no matter how airtight and well-insulated the rest of the wall might be. Unfortunately, there is not much that can be done about it.

The culprit is glass. Glass is a terrible insulator. And, windows are mostly glass. A single pane of glass has an insulation value of a little less than R-1. Adding a storm window improves this to R-2.2.

Triple pane thermal windows, using all the latest technologies including argon or krypton gas fill, low-Emissivity (low-E) coatings, and careful sealing can reach at best R-7.5. But, many of these technologies are just temporary. (low-E coatings break down over time, losing their effectiveness and, gas fills leak away eventually.) And, R-7.5 is a still far cry from the minimum R-19 that should be in your walls.

Are we ever going to get R-19 in windows? Probably. There are almost science fiction, space-age technologies in development but they are not ready for prime time just yet.

To learn more about insulation and your old windows, see Your Old Windows.

The air next to the interior wall draws a little heat from the warm interior drywall and, like all warm air, starts to rise. As it rises, it continues to draw heat from the warm side of the wall. When it gets to the top of the stud cavity it can no longer rise. But, there is more warm air below continuing to rise, pushing up on our little packet of air, crowding it against that frosty exterior side of the wall. As soon as it touches the exterior wall, it starts giving up heat, growing colder.

Cold air is heavier than warm air, so it starts to fall. As it falls it loses still more heat to the cold outside surface of the wall cavity, growing colder and colder. At the bottom of the stud cavity, it stops and would be delighted to stay there forever but above it is a heavy column of cold air pressing down on it until it is eventually pushed against the warm interior side of the wall. It starts drawing in heat again and rises once more. And, the cycle starts over.

This is the heat conveyor. It occurs inside every uninsulated wall cavity. The greater the temperature difference between the warm side of the wall and the cold side, the faster the air circulates. This circulation is a heat exchanger — and, unfortunately, a very effective heat exchanger. It draws heat from the interior side of the wall and conveys it to the exterior side, which in turn conducts it to the outside air.

The conveyor is continuous, every minute of every day all year round, and unstoppable. In the summer it merely reverses, transferring heat from the warm exterior side of the wall to the air-conditioned interior side. From 50% to 70% of the winter heat loss in your walls is through this conveyor process.

Heat Conduction and Thermal Bridging

Heat can also be transferred through conduction — the movement of heat on a microscopic level from molecule to molecule within a material. When an atom is heated, its electrons move faster, which tends to excite the electrons of adjacent atoms so they move faster. These, in turn, excite even more electrons, and the process spreads. This is how heat moves from one atom to the next. Some materials, like most metals, are good heat conductors. Heat one end of a metal bar with a propane torch, and very quickly the other end gets hot.

Most gases, including air, are poor conductors. The air in your wall cavity is a lousy conductor of heat. And, when it is replaced by suitable insulation material, so convection is slowed, the wall cavity is an effective barrier to heat transfer. But, air is not the only material in your walls. There is also the wall's wood framework. The wood framing penetrates through the wall from outside to inside, creating what is called a "thermal bridge" along which heat can pass through conduction.

Wood (which is denser than air, and contains water — a very good heat conductor) conducts heat better than air, and much better than most insulation materials. The R-value of the pine, fir, and spruce used in wall framing is about 1.25 per inch. Compare this to 3.6 per inch for dry, still air, 3.85 per inch for dense-pack cellulose, and 6.25 per inch for closed-cell foam. Other materials are even less resistant to heat transfer. Steel studs, for example, are very good heat conductors. Fortunately, they are rarely used in residential construction in exterior walls.

To minimize thermal bridging, we have to reduce framing members to as few as possible. There is a limit to this, of course. If you reduce framing too far, your house might fall down. But, many things can be done. For example, spreading out studs from 16" on center to 24" on center provides walls that are just as strong but contain fewer thermal bridges. Using less lumber in framing is also good for the environment since it requires the destruction of fewer trees. It also uses less labor, so it costs a bit less than traditional framing.

The framing techniques used to reduce lumber use have been developed by construction engineers over the past 20 years under the sponsorship of the Department of Energy and Housing and Urban Development (HUD). They are collectively called Optimum Value Engineering or OVE.

OVE uses engineering principles to minimize lumber use while meeting model building code structural performance requirements. (1) It reduces the number of framing members (2) eliminates pockets in framing, particularly at corners and wall intersections that cannot be effectively insulated, and (3) reduces the number of framing members that penetrate completely through the wall.

In new construction or when building additions, if we don't use structural insulated panel (SIP) construction (which we prefer), then we use OVE techniques in all of our wall and roof framing.

In your old house, framed the traditional, lumber-intensive way, the techniques are of less use. But, the studies that led to OVE standards have told us a lot about where insulation problems are likely to occur in traditional walls.

For example, we pay special attention to corners and to places where interior walls meet exterior walls and all corners. These are special problem areas for installing effective insulation.


Heat, like light, can move as electromagnetic waves. These are waves in the infrared spectrum. We can't see them but we can feel them as heatwaves. Like light, they need no material to move through. They easily move through a vacuum, which is how we get heat and light from the sun. And, like light rays, heat radiation can be blocked or reflected.

Radiation in winter is good. In carefully designed passive solar systems it can add a lot of free heat to the house. But, in summer, radiation can significantly contribute to your cooling load.

The sun heats the outside of your house walls and roof. Any warm material radiates heat. The hot wall radiates heat into your wall cavity where it is picked up by the convection and conduction processes and transferred to the interior side of your wall where the air-con­di­tioning has to deal with it. The hotter your outside wall or roof gets, the more problem it causes.

On your west wall, in summer, steel siding can easily get hot enough to cook on. Vinyl and cement siding materials stay a little cooler, and wood is by far the best performer.

Rev. 10/10/20