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Beyond R-Value: The Truth About Insulation Performance in the Real World of Buildings

When the "Energy Crisis" of the 1970's sent home heating and cooling costs soaring, demand for building insulation rose right along with the prices of oil, gas,and electricity. 

As often (unfortunately) happens when demand for a product increases suddenly and dramatically the promise of fast and easy profits occasionally led to fast and easy sales and marketing practices by some insulation sellers. Eventually the claims and confusion in the insulation market became so extreme the Federal Trade Commission instituted "The R-Value Rule." This regulation placed clear limitations on the claims and statements manufacturers and marketers can make about insulation products and the energy savings they may produce.

The regulation is called "The R-Value Rule," because it is based on a mathematical term known to engineers as "The R-Factor."

R-Factor is a measure of the ability of insulation material to resist heat transfer. It's determined by placing carefully prepared test specimens between two plates in a laboratory apparatus and measuring heat flow through the insulation. "R-Value" is the R-Factor of the insulation multiplied by the amount of the material.

If insulation has an R-Factor of 3.8, and there are 3.5 inches of insulation between the warm side and the cool side of the assembly, the R- Value of the insulation in the system is 13.3.

The R-Value Rule is a good regulation that has significantly reduced misrepresentation and outright fraud in the insulation industry. It's one of the most important and successful consumer protection regulations ever enacted by the U.S. Federal Government.

Consumers often assume The R-Value Rule will automatically lead them to the insulation product with the greatest energy savings when it's installed in a building. Unfortunately, this isn't the case. In the real world of buildings things are a bit more complicated. In fact, they're a lot more complicated.

R-Value is a very accurate and reliable expression of how insulation materials perform in a laboratory apparatus. But people don't live in laboratories. They live in homes with real walls and ceilings, and in the real world of buildings R-Value is only one factor in the actual performance of insulated building assemblies.

Scientists and engineers refer to building systems that separate the interior of a structure from the ambient environment as the"building thermal envelope." Many factors affect the energy efficiency of the thermal envelope. These include:

  • Total R-Value of all system components.
  • Air Infiltration due to leakage through gaps in the system.
  • Air infiltration due to permeability of system elements.
  • Convective flows within insulated systems.
  • Thermal bridging across the building envelope.
  • Thermal mass of building components.


R-Value is important, but building scientists know that focusing on R-Value to the exclusion of all other factors can result in disappointment with thermal envelope performance. It's known, for instance, that thermal bridging can reduce the actual energy efficiency of a wall by up to 50 percent. U.S. scientists have proven that convective flows in very light density attic insulation can reduce its performance by more than 40 percent under winter conditions. Canadian researchers have reported a similar effect in walls.

Highlighting the importance of the integrity of the installation, a national fiberglass manufacturer wrote:

“Nearly 50 percent of all wall cavities in today’s homes contain some type of obstruction, such as wiring, pipes, electrical boxes, along with non-standard framing in width and height... Gaps or voids in insulation can reduce sound control and reduce the R-value resulting in heat loss"

In a letter to Home Energy magazine an experienced, prominent thermographic inspector stated:

"Of the hundreds of buildings I have inspected with infrared thermography I have yet to see even one fiber glass job that doesn't suffer some reduced thermal performance. Often the degradation is substantial, especially under windy conditions."

Other surveys of energy consumed to heat actual homes in Pennsylvania, Kansas, and the United Kingdom have shown cellulose performance superiority ranging from 20% to 33%. Extensive and expensive air sealing measures must be used for fiber glass buildings to approach the tightness of buildings insulated with cellulose. The extra expense may yield few benefits.

This real world performance difference does not mean consumers and specifiers should ignore R-Value. R-Value is important to insure that buyers receive all the insulation they contract for. It's also important in comparing prices among vendors proposing to supply the same type of insulation.

The shortcomings of R-Value as a measure of real world insulation performance are recognized by the building community. The Building Environment and Thermal Envelope Council of the National Institute of Building Science (BETEC) has received a number of research proposals to develop practical methods for measuring the energy-saving performance of total building thermal envelope systems and the relative performance of different insulation materials.

Until this research is completed buyers and specifiers must remember that "R" for "R" all insulation is not created equal. In the real world of buildings it's necessary to install more "Rs" of fiberglass insulation or use expensive air sealing techniques to achieve the energy-conservation performance of cellulose and other insulation materials less susceptible to air infiltration, internal convection, and installation defect problems than mineral fiber products.

Conclusion: R-Value is an important aid to consumers when comparing proposals and specifications for different brands or suppliers of the same type of insulation. It is less useful when comparing different insulation materials. R-for-R, all insulation is NOT created equal! Of course, savings vary and research reports and scientific studies conclusions only apply to the specific materials tested which may or may not be different than those used in your home.
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