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The Case of the Disappearing R-Value The latest word on urethane and polyisocyanurate: The older they get, the worse they look by Paul Hanke Most engineers, architects and builders probably know that rigid foam insulation boards (such as polyurethane and polyisocyanurate) have relatively high R-values per inch of thickness at the time of manufacture, but that these values decline as the products age. For this reason, the products have been marketed on the basis of the so-called "aged R-value, " which is determined by standard testing procedures. But now these aged R-values are coming under fire by the Midwest Roofing Contractors Association (MRCA). Citing MRCA field tests of urethane and polyisocyanurate materials, a group of panelists at the association's convention