PIR is a closed-cell foam that contains a low-conductivity gas (HCFC) in the cells. Over time, the R-value of the foam drops as some of the gas escapes and air replaces it. As mentioned earlier, stabilized R-val – ue is a term used by some manufacturers to accurately represent the long-term R-value of the installed board product. The thicker the product, the longer it takes to stabilize. Impermeable facers such as aluminum foils or patented coated-paper facers used on PIR products are essential in maintaining a relatively high, aged R-value. For example, foil-faced PIR boards have R-values that range from 7.2 to 8.0 per inch. Glass-fiber-faced PIR sheathing, such as that used as an EIFS or roofing substrate, has an R-value of 5.6 per inch.5