R-value (insulation)

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The R-value is a measure of thermal resistance used in heat transfer problems. 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, t: R = t/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.

1 U-value
2 Infiltration of Air
3 Example values
- 3.1 Typical R-values per inch of thickness
- 3.2 U.S. regulation
4 See also
5 External links

[edit] U-value

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 Kelvin. 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 10 K / 2 K·m²/W = 5 watts for every square metre of ceiling.

[edit] Infiltration of Air

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, convecting heat and greatly reducing the insulating value to roughly R–1.

[edit] Typical R-values per inch of thickness

The FTC's R-value Rule generally prohibits calculating R-value per inch of thickness. (16 C.F.R. 460.20.) The FTC explained the reason for this prohibition: Since the record demonstrates that R-values are not linear, advertisements, labels, and other promotional materials that express a product's thermal resistance in terms of R-value per inch deceive customers. The FTC further explained that references to the R-value for a one-inch thickness of the material will encourage consumers to think that it is appropriate to multiply this figure by the desired number of inches, as though R-value per inch were constant. (44 Fed Reg. at 50,224 (27 August 1979).)

All values are approximations, based on the average of the values listed on dozens of websites. If I saw wildly different values, then I took the lowest and highest values and expressed the R-value here as a range somewhere between them. For a more-official list, refer to one of these websites with duplicate R-value tables: [1] [2] [3]

Furthermore, comparisons per inch of thickness are mostly relevant for conductive and convective heat transfer -- not radiant heat transfer -- but some of the materials listed below are designed to prevent radiant heat transfer.

Air with no external wind = R-5 (still) to R-1 (with convective currents).
Single pane glass window = R-1.
Wood chips and other loose-fill wood products = R-1.
Snow = R-1.
Straw bales = R-1.45. [4]
Double pane glass window = R-2.
Double pane glass window with low emissivity coating = R-3.
Triple pane glass window = R-3.
Wood panels, such as sheathing = R-2.5.
Vermiculite loose-fill = R-2.13 to R-2.4.
Perlite loose-fill = R-2.7.
Rock and slag wool loose-fill = R-2.0 to R-3.3.
Rock and slag wool batts = R-3 to R-3.85.
Fiberglass loose-fill = R-2.2 to R-3.7.
Fiberglass rigid panel = R-2.5.
Fiberglass batts = R-2 to R-3.85.
High-density fiberglass batts = R-3.6 to R-5.
Cementitious foam = R-2 to R-3.9.
Cellulose loose-fill = R-3 to R-3.8.
Icynene spray = R-3.6.
Icynene loose-fill = R-4.
Urea-formaldehyde foam = R-4 to R-4.6.
Urea-formaldehyde panels = R-5 to R-6.
Phenolic spray foam = R-4.8 to R-7.
Phenolic rigid panel = R-4 to R-5.
Molded expanded polystyrene (EPS) = 3.7 for low-density, 4 for high-density.
Foiled-faced expanded polystyrene (EPS) = 5 to 6.3.
Reflective-faced expanded polystyrene (EPS) = 5.25 to 10.25.
Extruded expanded polystyrene (XPS) = 3.6 to 4.7 for low-density, 5 to 5.4 for high-density.
Polystyrene spray foam = R-5.
Open-cell polyurethane spray foam = R-3.6.
Closed-cell polyurethane spray foam = R-5.5 to R-6.5.
Polyurethane rigid panel = 6 (expanded with air), 6.25 to 9 (expanded with CFC or HCFC).
Dow "Great Stuff" = ?. [5]
Polyisocyanurate spray foam = R-4.3 to R-8.3.
Foil-faced polyisocyanurate rigid panel = R-5.8 to R-7.4.
Silica aerogel = R-10.
Foil-backed bubble pack = R-6 to R-18? NOTE: This value looks highly misleading, because this material is aimed at preventing radiant heat transfer.
Vacuum insulated panel = as high as R-30?
Cardboard = R-3 to R-4

Materials such as natural rock, dirt, sod, adobe, and concrete have poor thermal conductivity (R-value typically less than 1), but work well for thermal mass applications because of their high specific heat.

[edit] U.S. regulation

The Federal Trade Commission (FTC) governs claims about R-values to protect consumers against deceptive and misleading advertising claims. "The Commission issued the R-Value Rule to prohibit, on an industry-wide basis, specific unfair or deceptive acts or practices." (70 Fed. Reg. at 31,259 (May 31, 2005).)

The primary purpose of the Rule, therefore, is to correct the failure of the home insulation marketplace to provide this essential pre-purchase information to the consumer. The information will give consumers an opportunity to compare relative insulating efficiencies, to select the product with the greatest efficiency and potential for energy savings, to make a cost-effective purchase and to consider the main variables limiting insulation effectiveness and realization of claimed energy savings.

The Rule mandates that specific R-value information for home insulation products be disclosed in certain ads and at the point of sale. The purpose of the R-value disclosure requirement for advertising is to prevent consumers from being misled by certain claims which have a bearing on insulating value. At the point of transaction, some consumers will be able to get the requisite R-value information from the label on the insulation package. However, since the evidence shows that packages are often unavailable for inspection prior to purchase, no labeled information would be available to consumers in many instances. As a result, the Rule requires that a fact sheet be available to consumers for inspection before they make their purchase.