R-value (insulation) as ASHRAE 90.1 and the IECC prescribe U-values. However, R-value is widely used in practice to describe the thermal resistance of insulation products, layers, and most other parts of the building enclosure (walls, floors, roofs). Other areas of the world more commonly use U-value/U-factor for elements of the entire building enclosure including windows, doors, walls, roof, and ground slabs. The SI (metric) unit of R-value is whereas the I-P (inch-pound) unit is For R-values there is no difference between US customary units and imperial units, so the same I-P unit is used in both. Some sources use ″RSI″ when referring
R-value (insulation) R-value (insulation) In building and construction, the R-value is a measure of how well an object, per unit of its exposed area, resists conductive flow of heat: the greater the R-value, the greater the resistance, and so the better the thermal insulating properties of the object. R-values are used in describing effectiveness of insulation and in analysis of heat flow across assemblies (such as walls, roofs, and windows) under steady-state conditions. Heat flow through an object is driven by temperature difference (e.g. formula_1) between two sides of the object, and the R-value quantifies how effectively the object resists this drive: