R-Value Cleveland
What Is R-Value?
R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat flow per unit area,) passing through it. The bigger the number, the better the building insulation's effectiveness.
There are many factors that come into play when using R-values to compute heat loss for a particular surface. Manufacturer R value ratings only apply to properly installed insulation. Squashing two layers of batting into the thickness intended for one layer will increase, but not double the R-value.
Parallel Heat Conduction
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.
The R-value is a measure of insulation's heat loss under specified test conditions. The primary mode of heat transfer slowed by insulation is convection but it also prevents heat loss by all three heat transfer modes: conduction, convection, and radiation. Typically the first means of heat loss is across an uninsulated air-filled space (attic and walls) is convection, which occurs because of changes in air density with temperature. Insulation greatly slows natural convection.
Radiation Transfer
Most insulation trap air so that convective heat loss is minimized thus leaving only conduction and radiation transfer parameters. The primary role of insulation is to reduce the thermal conductivity of trapped, stagnant air. Glass wool, or foam insulation that is needed to prevent convection, increases heat conduction compared to still air.
Radiative heat transfer is minimized by having many surfaces interrupting a "clear view" between the inner and outer surfaces of the insulation. As such multiple surface areas are abundant in batting and porous foam. Radiation is also minimized by low emissivity (highly reflective) surfaces.