Since 1987, NRCA has recommended designers use an in-service R-value of 5.6 per inch thickness for polyurethane and polyisocyanurate roof insulation used in roof systems. With the January 2011 publication of The NRCA Roofing Manual: Membrane Roof Systems2011, NRCA is updating its design R-value recommendation for polyisocyanurate roof insulation.
Most roofing professionals recognize polyisocyanurate roof insulation loses some of its R-value over time. At the time polyisocyanurate insulation is manufactured, it has a relatively high R-value. As the insulation ages and is exposed to varying temperatures and field conditions, the gas producing the high R-value in the insulation's cells slowly diffuses and is replaced with air.
Considering an insulation's actual in-service R-valueinstead of its as manufactured R-value or long-term thermal resistance (LTTR) valueis important for designers because the in-service R-value more closely represents conditions in the built environment.