Decker's Blog

Understanding R-Value

“R-value” is a measure of a material’s resistance to heat transfer. The higher the R-value, the better the insulation factor. R-values include all three heat-transfer mechanisms; conductive, convention and radiation. Many builders install fiberglass batts that allow heat to flow from the hot side of the batt to the cold side. Wherever individual fibers of insulation touch each other, heat is transferred from fiber to fiber by conduction. Where fibers are separated by air, the heat is transferred from a hot fiber to a cooler fiber by radiation and by conduction through the air.

Finally, the effects of any convective loops within the insulation are also captured by the test procedure.

R-Value Matters, But So Does Air Leakage

Although the R-value is important, the measure the amount of air leakage through a wall assembly once the insulation is installed needs to be performed. To obtain the best performance from fiberglass insulation, the Energy Star Homes program requires fiberglass-insulated framing cavities be air sealed by caulking and sealing all wall penetrations, cracks and areas around doors and windows. The house is tested for air tightest and must pass minimum requirements to rated as Energy Star. Basic principles of air leakage and moisture movement must be understood and practiced and builders and insulation experts must study and understand building science principles.