From Academic Kids

R-value is the measure of thermal resistance. The SI definition of thermal resistance is R = K·m²/W. This is the definition used for e.g. thermal insulation materials in buildings.

Confusingly, there are two quite different definitions; some countries use a non-SI definition: R = ft2 h/Btu.

The conversion between the two is is 1 ft² F° h / Btu = approx. 0.1761 K·m²/W.

R-Values can be calculated from thermal conductivity, 'k', and the thickness of the material, 'd'. R = d/k. Thus, for 100 mm thickness, it is possible to calculate that Fiberglass blanket has a value of 2, whereas Aerogel has a value of 5.9.

In terms of lay-comprehension the formula of K.m² per Watt is not friendly. Contrasting R with U, thermal conductance provides an easier counterpoint. U is the inverse of R (1/R), and R is the inverse of U. Thus, the formula for U is Watts per k.m². Simply put, it is the number of Watts that will be lost per square metre, at a given temperature difference (K). For a practical example, if the interior of your home is at 20C, and the roof cavity is at 10C, that gives a temperature difference of 10. Then, assuming a ceiling insulated to R2, 5 Watts of energy will be lost for every square metre of ceiling. This is of course a theoretical example, and therefore fails to take into account a myriad of other factors.

Foam insulations with R-values higher than that of still air have been blown with some type of gas, typically containing chlorofluorocarbon or hydrochlorofluorocarbons (CFCs). 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.

Non-SI Information

Aerogel has the 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.

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