Roofing Insulation: Impact of Temperature on R-Value

By: Tamara Middleton, Sales Representative

Roofing3016_Page_1Most flat roof commercial buildings today use one of three insulations, polyisocyanurate (polyiso), extruded polystyrene (XPS) or expanded polystyrene (EPS).  These insulations are used above the roof deck and protected by the roof membrane.  Polyiso is the most commonly used of the three, this is due to its higher initial R-Value; however, not only does polyiso lose R-Value over time, its R-Value varies with temperature.  In reality all three roof insulations R-Value will vary based on the mean temperature, but how they vary is rather important.

Polyiso has a published R-Value of 5.6 per inch at 750 F mean temp.  XPS is published at R-5.0 per inch and EPS Type II is R-4.2.  The 750 F mean temp is based on testing using a 500 F differential, cold side at 500 F and warm side at 1000 F.

All three insulations have been tested extensively by BSC (Building Science Corporation) and the NRCA (National Roofing Contractors Association).  These tests have shown that all three insulations have changes in R-value as the mean temperature changes.  At a mean temp of 400F polyiso’s R-value drops to R-4.9, while XPS Type IV and EPS Type II increase to R-5.4 and R-4.6 respectively. As the mean temp drops to 250F polyiso’s R-value changes significantly to R-4.0, while XPS and EPS continue to improve at R-5.6 and R-4.8 respectively.  All the mean temperatures are determined using a 500F differential, so at 250 F mean the cold side is at 00F and the warm side at 500F.  At mean temps well above 750F all three have lower R-values.  At a mean temp of 1100F (cold 800 and warm at 1300), polyiso’s R-value is 5.0, XPS Type IV, R-4.7 and EPS Type II, R-3.9.

It has already been show and proven that Polyisocyanurate loses R-value over time, now we can see that polyiso also loses R-value at the temperatures drop.  We can also see the polystyrene insulations improve in R-value with decreasing temps.  Based on these tests by BSC and NRCA, when the outside temperatures drops roofs with polyiso insulation need to have at greater thickness than that based on a 750 F mean temperature.


Professional Roofing, “Testing R-values, Polyisocyanurates R-values are found to be less than their LTTR values”, March 2015

Building Science Corporation Information Sheet 502, “Understanding the Temperature Dependence of R-values for Polyisocyanurate Roof Insulation”, September 2013

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