Heat flows naturally from a warmer to a cooler space. During the warmer seasons, heat flows from outdoors to the home’s interior.  In the winter the flow is reversed when the heat flow moves directly from all heated living spaces to adjacent unheated attics, garages, and basements.  Heat may also travel indirectly through interior ceilings, walls, and floors--wherever there is a difference in temperature. To maintain comfort, the heat lost in winter must be replaced by your heating system and the heat gained in summer must be removed by your air conditioner. Insulating ceilings, walls, and floors decreases this heat flow by providing an effective resistance to the flow of heat.

Insulation is rated in terms of the thermal resistance the insulation provides, called R-value.. The higher the R-value, the greater the insulating value. The R-value of thermal insulation depends on the type of material, its thickness, and density. In calculating the R-value of a multi-layered installation, the R-values of the individual layers are added.

The effectiveness of an insulated wall or ceiling also depends on how and where the insulation is installed. Compressed insulation will not provide the full rated R-value. Also, the overall R-value of a wall or ceiling will be somewhat different from the R-value of the insulation itself because some heat flows around the insulation through the studs and joists. That is, the overall R-value of a wall with insulation between wood studs is less than the R-value of the insulation itself because the wood provides a thermal shortcut around the insulation.