R-value, in terms of insulation, refers to the thermal resistance that different insulating materials have. The higher the R-value of a material, the better it insulates from heat and cold. The R-value of insulation depends on the type of material, its thickness, and its density.
The Basic Physics of R-Values
In simple terms, heat moves from warmer areas to cooler areas until there is no longer a difference in the temperatures of the two areas. In your home, heat will move directly from a warm area into a cold area (or outdoors) through any open space, like a cracked window, the gaps in a door jamb or small holes like those in a wall outlet.
Heat will also move through material like plywood, drywall, glass, concrete, and other building materials. In cold weather, heat moves from inside a building toward a cooler area, usually outdoors. In warm weather, heat moves in the opposite direction, from outside toward a cooler interior space.
Insulation is therefore critical to keeping heat where it belongs, and to lowering your heating and air-conditioning costs. While all building materials have some kind of R-value resistance to heat movement, insulation greatly increases the R-value of a wall, ceiling, floor or other building components.
As one example, drywall at 1/2" thickness has an R-value of 0.45—a fairly low value. Other typical wall materials have similarly low values: 1/2" exterior plywood has an R-value of 0.63, and exterior wood bevel siding has an R-value of just 0.80. But when you add 3-1/2" of fiberglass insulation, with an R-value of 11.0, the entire wall structure can have a thermal resistance of nearly 14—not bad, but not ideal, especially in very cold or very hot climates.
Windows and doors are generally the biggest areas of heat loss in any building since the R-values for them tend to be much lower than the R-values of a solid wall. A single pane of glass has an R-value of just 0.91—adding a storm window will bump that value up to around 2.0. Triple-insulated glass with 1/2" spaces between the panes has an R-value of 3.23 (but it's not often used because it's expensive and very heavy).
Construction Materials With High R-Value
It's important to know that R-values aren't constant across many surfaces. In typical framed walls, the vertical studs of the wall have no insulation and can transfer heat easily in a process called "bridging." For that reason, many critics charge that R-value isn't a good way to measure the insulation value of an entire wall assembly.
Structural insulated panels, or SIPs, are solid panels of OSB (oriented strand board) sheathing and a solid foam core. A typical wall made of SIP construction can have an R-value between 15 and 20, according to most sources.
ICF construction uses insulting concrete forms made from lightweight polystyrene foam. These hollow blocks are stacked to form exterior walls. When complete, the interior of the wall is filled with steel-reinforced concrete to create a strong, energy-efficient building. ICF walls typically have R-values around 20.
Payback Period for Added Insulation
Anyone considering new construction would do well to investigate the costs, benefits and payback periods of these and other new construction materials for improved energy efficiency. To improve the R-value in your existing home's walls, floors, and ceilings, the Department of Energy recommends that you check the attic, walls, and floors adjacent to any unheated space, like a garage or basement. You can then see what type of insulation you have and measure its depth or thickness.
Fortunately, adding insulation is a relatively inexpensive way to save energy and money. Depending on where you live and how much insulation you need, the payback period for adding insulation can be as short as just a few years. For example, increasing the R-value of your attic insulation from 19 to 30 by adding batt insulation with an R-value of 11 can save so much money in reduced heating bills that its payback period is just over five years.