In architecture, the R-value is a measure of the ability of a two-dimensional barrier (such as insulation, a window, or a complete wall or ceiling) to resist the heat flow of thermal conductivity. The R-value is the temperature difference per unit of heat flow required to maintain one unit of heat flow between the warm and cold surfaces of the barrier under steady state conditions. Therefore, this measure applies equally to reducing energy bills for heating in winter, cooling in summer and general comfort.

The R-value is a construction industry term for thermal resistance "per unit area". If SI units are used, they are sometimes also expressed as rsi values. The R-value can be given for a material (such as polyethylene foam) or a group of materials (such as a wall or window). In the case of materials, it is usually expressed by the value of r per meter. The R-value is additional to the material layer, and the higher the R-value, the better the performance.

The U-factor or U-value is the total heat transfer coefficient, which can be found by taking the reciprocal of the R-value. It is a description of the performance of a building element to conduct heat per unit area over a temperature gradient. These elements are often combined from multiple layers of materials, such as those that make up a building envelope. It is measured in watts per square meter Kelvin :W/(m2⋅K). The higher the u value, the lower the heat transfer resistance of the building envelope. A low u value or conversely a high r value usually indicates a high insulation level. They are useful because it is a way to predict the composite behavior of the entire building element, rather than relying on the properties of individual materials.