On January 1st, 2014, the Long-Term Thermal Resistance values (LTTR ) or “R” value for polyisocyanurate insulation will be reduced from 6 per inch to 5.6 per inch. To make things more confusing, the “R” value per inch is not consistent. The thicker the insulation, the greater the “R” value per inch. So, 1” polyisocyanurate insulation will have an “R” value of 5.6, where an identical 1.5” sheet will have an “R” value of 5.7 per inch. The following is a table comparing the old “R” values to the new “R” values:

One layer of rigid insulation


Prior to 2014 “R” ValuesAfter 2014 “R” Values
LTTR Value“R” per inchLTTR Value“R” per inch

Two Layers of rigid insulation

“R” ValuePrior to 2014 ThicknessAfter 2014 Thickness
152 layers of: 1.3”2 layers of: 1.4”
202 layers of: 1.7”2 layers of: 1.8”
252 layers of: 2.1”2 layers of: 2.2”
302 layers of: 2.5”2 layers of: 2.6”
352 layers of: 2.9”2 layers of: 3.1”
402 layers of: 3.3”2 layers of: 3.5”

According to Hunter Panels, one of the leading manufacturers of rigid polyisocyanyurate insulation, the reason for this change is due to the new ASTM C1289-11A testing method. The easiest explanation is that the edge of every board of insulation has a facer to it, which doesn’t have any “R” value to it. That size of the edge facer remains the same, whether it is 1 inch or 1.5 inches of insulation, so the thicker the insulation, the greater “R” value per inch. For more information on the actual testing method, check out Hunter Panels LTTR change page. The manufacturing methods of rigid polyisocyanurate insulation haven’t changed at all, so a piece of 1” rigid insulation manufactured in January 2013 will be identical to a piece manufactured in January 2014. All that has changed are the testing methods and the corresponding “R” values. Prior to 2014, to achieve an “R” value of 20, the commonly accepted minimum rigid insulation required in a roof removal and reinstall application was 3.3” thick. After January 1st, 2014, for an identical piece of insulation manufactured the exact same way, it will need to be 3.5” thick to achieve the same “R” value. However, if two staggered sheets of insulation are used, the “R” values change even more, since each individual sheet of rigid insulation loses the “R” value from that edge facer. So, to achieve an “R” value of 20 using two staggered rigid insulation sheets, you now need two layers of 1.8” insulation (Total thickness of 3.6”) instead of the previous two layers of 1.7” insulation (Total thickness of 3.4”, which is actually an “R” value of 20.4) For more information, Hunter Panels has an excellent FAQ’s section about R values on their website.

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