The R-value Myth

The insulation value of a window is always expressed as a U-value.
Insulation materials are usually given an R-value.

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, official research has shown that up to fifty percent of the energy loss in a typical building has nothing to do with “R-value.

Let us investigate a little further.

The Six Factors of Heat Transfer

We need to consider not one but six factors.

Heat loss (or gain) happens in three ways:

Conduction (R-value)
Convection
Radiation

Three additional factors influence how well an insulation system performs:

Air infiltration
Air intrusion
Moisture

Foam insulation addresses all of these elements of concern. But for now, let us examine the six factors in a little more detail.

Conduction

Conduction is the transfer of heat within a material and R-value is a numeric expression of a given material’s resistance to this transfer. The higher the R-value, the greater the resistance to heat transfer.

To be affected by conductive heat from one to another, materials must touch. Think of a steak on a barbeque grill - the sear marks are created by conductive heat from the steel grate that the meat is sitting on.

Convection

Convection is basically currents of gasses (air) or liquids. Convection currents can transfer heat from one object to another and can occur within closed spaces. As air is heated, it rises so if one surface of a wall cavity is warmer than the other, a natural convective current will transport heat from one surface to the other until the temperatures are equalized.

Convective heat transfer happens without materials touching one another. It is (primarily) the convective heat rising from the gas flames on the grill that heats the grate.

Radiation

Radiative 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.

Air Infiltration

Most of us have experienced air infiltration first hand. Bring your hand close to an electrical receptacle box 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.

Conclusions

Up to fifty percent of the energy loss in a building is caused by factors not influenced by the level of r-value. Consequently, an effective insulation material must deal with all of the six factors that affect heat transfer. Spray foam insulation is that material:

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.

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