NRCA has updated their R-value recommendation for polyisocyanurate roof insulation with the publication of the 2011 The NRCA Roofing Manual: Membrane Roof Systems.
The new recommendation for R-value is based upon testing by NRCA from 2005 to 2009 on current generation polyisocyanurate roof insulations. The new NRCA R-value recommendations are as follows:
R-value = 5.0 per inch thickness in heating conditions.
R-value = 5.6 per inch thickness in cooling conditions.