mbba98xlt says:The R value or R-value is a measure of thermal resistance [1] used in the building and construction industry. The bigger the number, the better the building insulation's effectiveness[2]. R value is the reciprocal of U-value.
Increasing the thickness of an insulating layer increases the R value. For example, each inch of glass wool batting thickness adds approximately 3.5 (ft²·°F·h/Btu) to its R value. Heat transfer through an insulating layer is analogous to adding resistance to a series circuit with a fixed voltage. However, this only hold approximately because the effective thermal conductivity of some insulating materials depends on thickness. The addition of materials to enclose the insulation such as sheetrock and siding provides additional but typically much smaller R value.
There are many factors that come into play when using R values to compute heat loss for a particular wall. Manufacturer R values apply only to properly installed insulation. Packing two layers of R-19 into the thickness intended for one layer will increase but not double the R-value. Another important factor to consider is that studs and windows provide a parallel heat conduction path that is unaffected by the insulation's R value. The practical implication of this is that one could double the R value used to insulate a home and realize substantially less than a 50% reduction in heat loss. Even perfect wall insulation only eliminates conduction through the insulation but leaves unaffected the heat loss through such materials as glass windows and studs not to mention heat losses from air exchange.
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 unavoidablably it also retards heat loss by all three heat transfer modes: conduction, convection, and radiation. The primary means of heat loss across an uninsulated space is by natural convection, which occurs because of changes in air density with temperature. Insulation greatly retards natural convection. Most insulations trap air so that significant convective heat loss is eliminated leaving only conduction and radiation transfer. The primarily 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. Radiation 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 special closed-pore foam insulation.
