R-value (insulation)
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The R-value is a measure of thermal resistance. The SI units for thermal resistance are K·m²/W. R-values are commonly used to characterize thermal insulation materials in buildings. In this context, the unit is often written as RSI (for R-value Système International), and a specific value such as 5.53 may be indicated as RSI–5.53, but may also simply be written as R5.53. One tenth of an RSI is called a tog.
Some countries use a non-SI definition: R = ft²·°F·h/Btu. Values using this definition are often written as R–31.4 (corresponding to RSI–5.53 above), although this form is also used in countries where SI measures are more universally accepted (e.g., New Zealand).
The conversion between the two is 1 ft²·°F·h/Btu ≈ 0.1761 K·m²/W, or 1 K·m²/W ≈ 5.67446 ft²·°F·h/Btu.
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 a 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/W is not friendly. Contrasting the R-value with thermal conductance, U, provides an easier counterpoint. U is the inverse of R, that is U = 1/R. Thus, the SI units for U are W/(K·m²). Simply put, it is the number of watts that will be lost per square metre, at a given temperature difference in kelvins. For a simple example, if the interior of your home is at 20 °C, and the roof cavity is at 10 °C, that gives a temperature difference of 10 K. Assuming a ceiling insulated to R–2, energy will be lost at a rate of 5 watts for every square metre of ceiling.
Foam insulations with R-values higher than that of still air have been blown with some type of gas, typically containing chlorofluorocarbons 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. This also applies to 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.
[edit] Example values
Note that these examples use the non-SI definition and are per inch. Vacuum insulated panel has the highest R-value of (approximately 45 in English units) for flat, Aerogel has the next highest R-value 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. Loose cellulose, fiberglass both blown and in batts, and rock wool both blown and in batts all possess an R-value of roughly 3. Straw bales perform at about R–1.45. Snow is roughly R–1.
Absolutely still air has an R-value of about 5 but this has little practical use: Spaces of one centimeter or greater will allow air to circulate, conveying heat and greatly reducing the insulating value to roughly R–1.
Also see typical R-values per inch.
