R-value of Insulation

An R-value is one of the most popular building metrics – but you may find, as you’re thinking about building or finding your next home, that you don’t quite understand what it is and how it can be useful.

To measure heat transfer in insulation, an R-value is used. There are two tests that are essential to determine an R-value.

ASTM C518 Test

This test is relevant to single materials – a sample of the insulation material is placed inside a heat flow meter apparatus between a cold plate and a hot plate. As heat flows from the hot to cold plate through the insulation, the testing device measures precisely how much heat is flowing.


This test uses a guarded hot plate to measure heat flux through materials that have a low thermal conductivity. It is usually used for flat materials.

Interestingly enough, testing for R-values measures every aspect of heat transfer: convection, conduction, and radiation. If you were testing a rigid foam board, heat would be moving through the foam via conduction, and through the air bubbles within the foam via radiation. There wouldn’t be convection through such an airtight material, but convective loops forming within the air bubbles would speed the transfer of heat through the material and would affect the heat transfer measurements.

By capturing all three methods of heat transfer through materials, R-value gives us a great way to compare insulation products. In fact, the R-value measurement was created and popularized because it easily communicates relative insulation values.

Make sense? What other questions do you have as you move forward with building your home? Shoot us an email at info@saratogaconstruction.net


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