Understanding R-Value

The R-value is a measure of insulation's heat loss retardation under specified test conditions. The primary mode of heat transfer impeded by insulation is convection but unavoidably it also impedes heat loss by all three heat transfer modes: conduction, convection, and radiation. The main means of heat loss across an un-insulated air-filled space is natural convection, which occurs because of changes in air density with temperature. Insulation greatly restricts natural convection. Most insulations trap air so that significant convective heat loss is eliminated leaving only conduction and radiation transfer.

The chief role of such insulation is to make the thermal conductivity of the insulation that of trapped, stagnant air. However this cannot be realized fully because the glass wool or foam is needed to prevent convection and increases the heat conduction compared to still air.

Radiant heat transfer is minimized by having many surfaces interrupting a "clear view" between the inner and outer surfaces of the insulation. Such multiple surfaces are abundant in batting and porous foam. Radiation is also minimized by low emissivity (highly reflective) surfaces. Lower thermal conductivity and, therefore, high R-values can be achieved by replacing air with argon when practical such as between sealed double-glazed windows and within special closed-pore foam insulation.