R-value is a term predominantly used in the building industry to describe the insulation properties of certain building insulation materials . Its use is limited to situations where thermal insulation is achieved by retarding the flow of heat through the material itself rather than reflecting radiant heat away. The higher the R-value, the greater the insulation. Although they offer a useful means of comparing the performance of different products, other factors need to be considered in maintaining thermal comfort .
R-value is a measure of apparent thermal conductivity, and thus describes the rate that heat energy is transferred through a material or assembly, regardless of its original source.
Units of measurement and relationship to other parameters
The SI unit for R-value is kelvin square meters per watt . (Km²/W).
The imperial unit for R-value is degree Fahrenheit square feet hours per Btu.
The conversion factor is 1 ft²°Fh/Btu ≈ 0.1761 Km²/W, or 1 Km²/W ≈ 5.67446 ft²°Fh/Btu.
Sometimes the nomenclature RSI is used to denote the SI form of the value. In contrast, the imperial unit is often written as R–## where the ## is the R-value. To complicate matters, some countries that employ the SI system (e.g.
The relationship between U-factor or R-value and thickness is not always exactly inverse and therefore R-value cannot be precisely extrapolated for a material of different thickness, but assuming an inverse relationship may be adequate.
R-value should also not be confused with the intrinsic property of thermal resistivity and its inverse, thermal conductivity . The SI unit of thermal resistivity is Km/W. Thermal conductivity assumes that the heat transfer of the material is linearly related to its thickness.
The U-factor (or U-value) describes how well a building element conducts heat. Methodologically, it measures the rate of heat transfer through a building element over a given area under standard conditions. The usual standard is at a temperature gradient of 24oC at 50% humidity in no wind conditions.[
U is the inverse of R i.e. U = 1/R and the SI unit for U is W/(m² K).
For example, if the interior of your home is at 20 °C, and the roof cavity is at 10 °C, the temperature difference is 10 K. Assuming a ceiling insulated to R–2, energy will be lost at a rate of 10 K / 2 Km²/W = 5 watts for every square metre of ceiling.
The table below provides an overview of most of the available insulation forms, insulation materials, their installation methods, where they're applicable to install in a home, and their advantages.
Table 1. Types of Insulation | ||||
Form |
Insulation Materials |
Where Applicable |
Installation Method(s) |
Advantages |
Blanket: batts and rolls |
Fiberglass |
Unfinished walls , including foundation walls , and floors and ceilings . |
Fitted between studs, joists, and beams. |
Do-it-yourself. |
Concrete block insulation |
Foam beads or liquid foam:
Vermiculite or perlite pellets |
Unfinished walls , including foundation walls , for new construction or major renovations. |
Involves masonry skills. |
Autoclaved aerated concrete and autoclaved cellular concrete masonry units have 10 times the insulating value of conventional concrete. |
Foam board or rigid foam |
Polystyrene |
Unfinished walls , including foundation walls ; |
Interior applications: must be covered with 1/2-inch gypsum board or other building-code approved material for fire safety. |
High insulating value for relatively little thickness. |
Insulating concrete forms (ICFs) |
Foam boards or foam blocks |
Unfinished walls , including foundation walls , for new construction. |
Installed as part of the building structure. |
Insulation is literally built into the home's walls, creating high thermal resistance. |
Loose-fill |
Cellulose |
Enclosed existing wall or open new wall cavities ; |
Blown into place using special equipment; sometimes poured in. |
Good for adding insulation to existing finished areas, irregularly shaped areas, and around obstructions. |
Reflective system |
Foil-faced kraft paper, plastic film, polyethylene bubbles, or cardboard |
Unfinished walls , ceilings, and floors. |
Foils, films, or papers: fitted between wood-frame studs, joists, and beams |
Do-it-yourself. |
Rigid fibrous or fiber insulation |
Fiberglass |
Ducts in unconditioned spaces and other places requiring insulation that can withstand high temperatures. |
HVAC contractors fabricate the insulation into ducts either at their shops or at the job sites. |
Can withstand high temperatures. |
Sprayed foam and foamed-in-place |
Cementitious |
Enclosed existing wall or open new wall cavities ; |
Applied using small spray containers or in larger quantities as a pressure sprayed (foamed-in-place) product. |
Good for adding insulation to existing finished areas, irregularly shaped areas, and around obstructions. |
Structural insulated panels (SIPs) |
Foam board or liquid foam insulation core |
Unfinished walls , ceilings, floors, and roofs for new construction. |
Builders connect them together to construct a house. |
SIP-built houses provide superior and uniform insulation compared to more traditional construction methods; they also take less time to build. |
Typical R-values per inch of thickness
The U.S. Federal Trade Commission '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.
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.
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.
Values per inch
Material |
Value per inch (Min) |
Value per inch (Max) |
|
Air with no external wind |
R-1 or less (with convective currents) |
R-5 (Still) |
|
Wood chips and other loose-fill wood products |
R-1 |
|
|
Snow |
R-1 |
|
|
Straw bale |
R-1.45 |
|
|
Wood panels, such as sheathing |
R-2.5 |
|
|
Vermiculite loose-fill |
R-2.13 |
R-2.4 |
|
Perlite loose-fill |
R-2.7 |
|
|
Rock and slag wool loose-fill |
R-2.0 |
R-3.3 |
|
Rock and slag wool batts |
R-3 |
R-3.85 |
|
Fiberglass loose-fill |
R-2.2 |
R-3.7 |
|
Fiberglass rigid panel |
R-2.5 |
|
|
Fiberglass batts |
R-2 |
R-3.85 |
|
High-density fiberglass batts |
R-3.6 |
R-5 |
|
Cementitious foam |
R-2 |
R-3.9 |
|
Cellulose loose-fill |
R-3 |
R-3.8 |
|
Cellulose wet-spray |
R-3 |
R-3.8 |
|
Icynene spray |
R-3.6 |
|
|
Cotton batts (Blue Jean Insulation) |
R-3.7 |
|
|
Icynene loose-fill (pour fill) |
R-4 |
|
|
Urea-formaldehyde foam |
R-4 |
R-4.6 |
|
Urea-formaldehyde panels |
R-5 |
R-6 |
|
Polyethylene foam |
R-3 |
|
|
Phenolic spray foam |
R-4.8 |
R-7 |
|
Phenolic rigid panel |
R-4 |
R-5 |
|
Molded expanded polystyrene (EPS) |
R-3.7 (low-density) |
R-4 (high-density) |
|
Extruded expanded polystyrene (XPS) |
R-3.6 to R-4.7 (for low-density) |
R-5 to R-5.4 (for high-density) |
|
Open-cell polyurethane spray foam |
R-3.6 |
|
|
Closed-cell polyurethane spray foam |
R-5.5 |
R-6.5 |
|
Polyurethane rigid panel (Pentane expanded ) |
R-6.8 initial |
R-5.5 aged (5-10 years) |
|
Polyurethane rigid panel (CFC/HCFC expanded) |
R-7 to R-8 initial |
R-6.25 aged (5-10 years) |
|
Polyisocyanurate spray foam |
R-4.3 |
R-8.3 |
|
Foil-faced polyisocyanurate rigid panel (Pentane expanded ) |
R-6.8 initial |
R-5.5 aged (5-10 years) |
|
Silica aerogel |
R-10 |
|
|
Vacuum insulated panel |
as high as R-30 |
|
|
Cardboard |
R-3 |
R-4 |
|
Thinsulate clothing insulation |
R-5.75 |
|
|
Values for a specified unit (not per inch
Material |
Value not per inch (Min) |
Value not per inch (Max) |
|
Foil-backed bubble pack |
R-1 to R-2 (as per industry testing, despite manufacturers' claims) |
|
|
Single pane glass window |
R-1 |
|
|
Double pane glass window |
R-2 |
|
|
Double pane glass window with low emissivity coating |
R-3 |
|
|
