A measure of the resistance of building materials and structures to the flow of heat; the higher the R-value the more effective the substance is as thermal insulation.
where the temperature difference is in degrees Fahrenheit, the area is in square feet, the time in hours, and the heat loss in Btus. If you know the R-value of a partition, you can use this formula to find the heat loss.
The reciprocal of R-value (1/R) is known as the U-value. The higher the U-value, the better the conduction of heat.
In Europe it is customary to use U-values instead of R-values. There, U-values are defined¹ by the equation:
This is not the reciprocal of the American R-value (kelvin instead of degrees Fahrenheit, meters instead of feet, etc.) To convert an American R-value into a European U-value, divide 1 by the R-value, then multiply the result by 5.682. To convert a European U-value to an American R-value, multiply by 0.176, then divide 1 by the result.
1. ISO-DP10292 draft standard and CEN-ISO 9050. In the German standard DIN 4701, this quantity is called “K”.
The R-value of a structure made of layers of different materials can be estimated by adding the R-values of the layers. The R-value of a layer can be estimated by multiplying its thickness in inches by the R-value per inch. These techniques don't give strictly accurate results (among other factors, a layer of air stuck on the surfaces between layers is itself an insulator), but they come close.
Handy online calculators for estimating the r-value of a multi-layer wall, floor or roof are at
|Type of insulation||R-value
per inch of thickness
|Vermiculite, loose fill||2.08|
|Perlite, loose fill||2.7|
|Fiberglass, blankets and batts||3.33|
|Fiberglass, blown loose fill||2.2|
|Rock wool, batts||3.66|
|Rock wool, blown loose fill||2.93|
|Cellulose, blown loose fill||3.6|
|Wood bevel siding, ½″ × 8″ lapped||0.81|
|Wood siding shingles, 16″ × 7½″ exposure||0.87|
|Stucco, per inch||0.20|
|½-inch nail-base insulation board sheathing||1.14|
|½-inch insulation board sheathing, regular density||1.32|
|insulation board sheathing, regular density||2.04|
|3/8 inch plywood||0.47|
|5/8 inch plywood||0.78|
|Softwood, per inch||1.25|
|Softwood board ¾-inch thick||0.94|
three oval cores
|cinder aggregate, 4 inches thick||1.11|
|cinder aggregate, 8 inches thick||1.72|
|cinder aggregate, 12 inches thick||1.89|
|sand and gravel aggregate, 8 inches thick||1.11|
(expanded clay, shale, slag,
8 inches thick
two rectangular cores
|Sand and gravel aggregate, 8 inches||1.04|
|class="l"Lightweight aggregate, 8 inches thick||2.18|
|Common brick, per inch||0.20|
|Face brick, per inch||0.11|
|Sand-and-gravel concrete, per inch||0.08|
|½-inch gypsum board||0.45|
|5/8 inch gypsum board||0.56|
|½ inch lightweight aggregate gypsum plaster||0.32|
|Hardwood finish flooring||0.68|
|Asphalt, linoleum, vinyl, or rubber floor tile||0.05|
|Carpet with a fibrous pad||2.08|
|Carpet with a foam rubber pad||1.23|
|Asphalt roof shingles||0.44|
|Wood roof shingles||0.94|
|3/8 inch build-up roof||0.33|
|Solid wood 1 inch thick||1.56|
|Solid wood 1 inch thick with wood storm door||3.3|
|Solid wood 1½ inches thick||2.04|
|Solid wood 1½ inches thick with wood storm door||3.7|
|Solid wood 2 inches thick||2.33|
|Solid wood 2 inches thick with wood storm door||4.17|
|Heat flow up||0.87|
|Heat flow up, one surface reflective||2.23|
Building codes specify a minimum level of insulation. The optimum amount of insulation depends on guessing what weather and the cost and availability of heating fuel (or electricity for cooling) will be over the life of the house, and comparing that with the cost of insulation. Oak Ridge National Laboratory provides an applet that gives an estimate which takes many factors into account.
The Oak Ridge applet requires Java. The web contains many much simpler suggestions for optimal insulation values, most based on Dept. of Energy guidelines. For example:
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Last revised: 19 October 2015.