Does Temperature Effect R-Value?


This question came up during a webinar we conducted recently on Low Slope Roof Insulation but it’s a question that we often get when we are talking about insulations in general.  The R-value of a product is a measure of its resistance to heat flow: the higher the R-Value, the greater the insulating power. However, there is a relationship between a materials stated R-Value and the temperature at which you test the material.

317347_prod409x237I’d prefer to not go into the science at a level of minute detail so instead point out that the
Federal Trade Commission (FTC) has something called the R- Value Rule which very strictly governs the temperatures at which you test materials and publish the results for given R-Values.  Those temperatures are meant to give you an accurate representation of the insulation’s performance across the United States climates.

It’s a rather complex issue with very complex science behind it and as a result, some have taken what is a kernel of truth and planted a whole field of confusion with it. You can rest assured that the FTC has stepped in to create a set of rules to ensure that the R-Values stated by manufacturers are truthful. Yes, temperature does have an effect but perhaps not as much an effect as one would.  Is it real? Yes, but it’s not really important.


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Lucas Hamilton is Manager, Building Science Applications for CertainTeed Corporation

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