Understanding Insulation

Everybody thinks “R-value” when they think about insulation. Somehow, a myth has taken hold that more “R” will cure all ills but there is more to effective insulation than just piling on material. The key is proper air sealing.

Air sealing

Testing air leakage with a blower door in Cleveland, OH
Testing air leakage with a blower door in Cleveland, OH


you think about it, it is quite simple: if air can flow around your insulation, does it really matter how thick that insulation is? A knitted sweater won’t keep you warm on a windy winter day but add even the thinnest nylon windbreaker and all of a sudden you are warm. The same applies to a building: if it is full of holes, no quantity of insulation will make it efficient or comfortable.

At Pure Seal we have always paid attention to air sealing, so much so in fact that we made it part of our name.

We measure air tightness with a Blower Door

, which is a device that sucks air out of the building and creates a difference in pressure between the inside and the outside. By measuring the amount of air that must be moved to reach a specific pressure differential, we can calculate how leaky the building is. We routinely achieve results that are three to four times better than required by code.

What exactly is R-value?

R-value is a numerical expression of a material’s resistance to heat transfer. It is the opposite of the U-value, which tells us how much energy flows through a material (R=1/U and U=1/R). The U-value is the number of British Thermal Units (Btus) that transfer through a 1 square foot area in one hour for each 1 degree Fahrenheit temperature difference between the two sides of the structure. The smaller the U-value, the better the insulation properties of the material. We use U-values to calculate a building’s energy loss.

(An example: what is the energy loss in Btus per hour through 100 square feet of wall if its U-value is 0.05 (R-value = 20), the inside temperature is 70 degrees and the outside temperature is 30 degrees? Answer: 100 x 0.05 x (70-30) = 200 Btus per hour. Calculate the loss through the various parts of the building, add them all together and you have its total energy loss at a given temperature.)

The reason that we use two different values, U and R, is that they allow us to perform different types of calculations. U values can be used in multiplication and division problems but cannot be added or subtracted, R-values can! Windows are given an overall U-value, which takes the various components into account. Insulation materials get an R-value.
R-value is the numerical reciprocal of U-value. That is: R = 1/U and U = 1/R
For Example:
If the U-value is 0.1, then the R-value is 10 (R=1/0.1)
If the R-value is 20, then the U-value is 0.05 (U=1/20)

One Btu

is the amount of energy required to heat one pound of water from 58.5 to 59.5 degrees Fahrenheit. This is roughly the equivalent to the energy in one kitchen match.

The R-value myth

Manufacturers of traditional insulation materials have done an excellent job of marketing “R-value” as the only number to care about when choosing insulation. In fact, they have been so effective that their efforts are mirrored in building codes throughout the country. But there is much more to the efficiency of an insulation system than its claimed R-value. In fact, research has shown that up to fifty percent of the energy loss in a typical building has nothing to do with “R-value.

Heat loss (or gain) happens in three ways:

Conduction is the transfer of heat within a material. Materials must touch to transfer heat by conduction. Think of a steak on a grill – the sear marks are created by conductive heat from the steel grate that the meat is sitting on.

Convection transfers heat from one object to another by air movement and occurs within closed spaces. As air is heated, it rises so if one surface of a wall cavity is warmer than the other, a convective current will transport heat from one surface to the other until the temperatures are equalized. Convective heat transfer happens without materials touching. It is (primarily) the convective heat rising from the gas flames on the grill that heats the grate.

Radiation heat transfer happens through electro-magnetic waves as one material releases energy (heat) to warm another. If materials are the same temperature, there is no radiation. Close the grill and on a cold day you will feel the radiation heat.

Three additional factors influence how well an insulation system performs:

Air Infiltration. Most of us have experienced air infiltration first hand. Bring your hand close to an outlet on an outside wall on a windy day and chances are that you will feel the draft long before you touch the wall. Clearly, infiltration must be considered when evaluating insulation. (Ex-filtration from living spaces into attics is a major source of energy loss and attic problems).

Air Intrusion. Gaps and cracks in the sheathing of a building allow the wind to penetrate into the wall cavity. If the inside drywall is glued to the studs and is without openings, there will be no infiltration into the living area. But, the intrusion of air into the cavity creates currents that transfer heat.

Moisture. Air infiltration and air intrusion account for almost all of the moisture that penetrates into an undamaged wall system. It has been determined that during a normal heating season, as much as 30 quarts of water can be collected in a wall through a 1 square inch hole in a 4’ x 8’ area of drywall. In contrast, diffusion would generate only 1/3 of a quart of accumulation. Water is an excellent conductor of heat so the wetter insulation becomes, the less effective it is.

Up to fifty percent of the energy loss in a building is caused by factors not influenced by the R-value. Consequently, an effective insulation material must deal with all of the six factors that affect heat transfer. Spray foam does that.
• The R-value of foam is as high or higher than traditional types of insulation and it controls conductive heat loss.
• Foam insulation does not allow airflow within itself so it blocks convective currents.
• The cells in foam are tiny so there is very little temperature difference from one cell wall to the next – without temperature difference, there can be no radiant heat loss.
• Foam completely fills and seals any opening in the wall sheathing. There can be no air infiltration or intrusion.
• Air infiltration accounts for the vast majority of moisture in a wall system – without air infiltration, no moisture problem.

How much R-value is enough?

We must double the R-value to cut conductive heat loss in half. This is clearly a worthwhile thing to do when we first add insulation but there is a rapid drop-off in return on investment as insulation thickness increases. At R-20 we eliminate about 95% of the conductive energy loss compared to a wall with no insulation. To cut the remaining loss in half, we must double the R-value to 40. Could there be such a thing as too much insulation, too high an R-value?

The short answer is no. But the more thoughtful answer is that the appropriate level of insulation depends on where you are building and on your specific goals. Most of our customers are happy with our standard insulation package, which results in a very comfortable house that is cheap to heat and cool.

Other customers want a home that they can heat and cool with electricity from the sun. They must match all the components in the building to each other to maximize efficiency and minimize cost. Simply put, you stop adding insulation when the energy saved can be supplied more cost-effectively by adding another solar panel. In our climate, a Net-Zero house will be constructed with minimum R-values of 5 (windows), 10 (sub-slab insulation), 20 (foundation), 40 (walls) and 60 (attic).

This review was based on information found in the following sources:
“Principles of Heat Transfer”, NCFI Point Paper, www.ncfi.com/insulation.htm
“Insulation Basics” www.diynet.com
Joseph Lstiburek, Builder’s Guide to Cold Climates, 2004. www.buildingscience.com