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) formula_1 divided by the R-value and then multiplied by the surface area of the object's side gives the "total rate of heat flow through the object" (as measured in Watts or in BTUs per hour). Moreover, as long as the materials involved are dense solids in direct mutual contact, R-values are additive; for example, the total R-value of an object composed of several layers of material is the sum of the R-values of the individual Note that the R-value is the building industry term for what is in other contexts called ″thermal resistance per unit It is sometimes denoted RSI-value