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The R-value is a non-SI measure of thermal resistance used in housing insulation. It is defined as R = ft2 F° h/Btu.
For comparison, the SI unit of thermal resistance is R = K·m²/W, and 1 ft² F° h / Btu = approx. 0.1761 K·m²/W.
Aerogel has the highest highest R-values per inch (approx. 10), followed by isocyanurate and phenolic foam insulations with, 8.3 and 7, respectively. They are followed closely by polyurethane and polystyrene insulation at roughly R-6 and R-5 per inch. Loose cellulose, fiberglass both blown and in batts, and rock wool both blown and in batts all possess an R-value of roughly 3 per inch. Straw bales perform at about R-1.45 per inch. Snow is worth R-1 per inch.
Absolutely still air has an R-value of about 5 per inch but this has little practical use; spaces of half an inch will allow air to circulate, communicating heat and destroying whatever insulating value there was to be had.
Foam insulations with R-values higher than that of still air have been blown with some manner of exotic gas, typically something in the chlorofluorocarbon or hydrochlorofluorocarbon family. Over the years this blowing agent will inevitably escape and be replaced by air, thus reducing the effective R-value of the product. The same goes for gas filled window systems. This process is quite slow but should be kept in mind when calculating R-values over spans of a decade or more.
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