Main Page | See live article | Alphabetical index The R-value is a measure of thermal resistance used in housing insulation. It is defined as R = ft2 F° h / Btu. For comparison, the SI unit of thermal resistance is R = K.m²/W, and 1 ft² F° h / Btu = approx. 0.1761 K.m²/W. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.