Hear ye, hear ye… calling all insulation professionals… and those not so professional.
The insulation you use does not have a higher “effective R-value” than its tested R-value, stop saying that it does. R-value is R-value; it is derived and documented based on a tested value, K-factor, and does not magically have a higher resistance than the value listed on its TDS just because you think it is better.
Let me explain…
K-factor is measured using ASTM C-518 and is important when it comes to optimizing a building’s thermal performance and meet energy code. K-factor is a material’s thermal conductivity; measured as the amount of heat that passes through a thickness of material per unit area in one hour, if the temperature difference between the hot and cold side of the material is one degree. So the lower the k-factor, the better the thermal performance. To put units to it, it is the number of BTUs that would pass through 1 inch of a spray foam material per square foot, in one hour, if there was a 1 degree Fahrenheit difference between the hot side and the cold side of the foam sample.
While K-factor is the measured value, most of the construction industry works with and refers to R-value. R-value is thermal resistance and is the inverse, or opposite, of thermal conductivity (K-factor). R-value is the amount of heat that does NOT pass through a thickness of material per unit area in one hour, if the temperature difference between the hot and cold side of the material is one degree. This is thermal resistance, so the higher the R-value, the better the thermal performance.
When it comes to the building industry, one hitch to all of this thermal performance business is that K-factor and R-value only measure one form of thermal transmission, conduction; these values do not address convection (heat transfer by air movement) or radiation (heat waves).
R-value alone does not tell the whole story.
Tune in next week as we continue this discussion and learn why two insulations with the same tested R-value might perform differently.
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