Insulation is rated by R-value, which is a measure of thermal resistance, or resistance to heat flow. Each type of insulation has a particular R-value for an inch of thickness. Hence a 6-inch fiberglass blanket may be valued at R-19, or about R-3 per inch, while a 6-inch sheet of polystyrene foam board is a better insulator than fiberglass, inch for inch. But that doesn't mean that foam board is always a preferable material.
We often use fiberglass and cellulose loose-fill insulation in attics, for example, because we have enough room there to install 16 to 24 inches of insulation. The lower R-value of these materials is not an issue when there is plenty of space. Fiberglass and cellulose are inexpensive, relatively non-toxic, and easy to install. When choosing insulation, we consider the R-value per inch, the overall cost, the ease of installation, and other factors.
The R-value of the insulation itself doesn't describe the overall thermal resistance of an assembly such as a wall or ceiling. The R-value for a wall accounts for both the insulated areas and the areas occupied by framing lumber. Since lumber has a relatively poor R-value per inch, the overall R-value of the assembly is lower than the R-value of the insulation itself. This overall measurement is known as the whole-wall R-value.
The whole-wall R-value of a building assembly can be considerably lower than the R-value of the insulation. In typical frame walls, for example, lumber occupies 15to 25 percent of the surface area. Because wood has an insulating value of only about R-1 per inch, these framed areas create thermal bridges in the insulated walls. The interior walls over the framing are cold in winter and hot in summer because heat conducts through wood more rapidly than through the insulated space between the framing. If metal framing is used, the reduction in whole-wall-R-value is even greater.
For a wall with a common two-by-four wood frame and R-11 3.5 inch fiberglass insulation, the whole-wall R-value will be R-9 or R-10. For a similar two-by-six wall with 5.5 inch fiberglass insulation, the whole-wall R-value will be R-11 to R-13.
Most of our homes have too little insulation. This has happened because energy prices have been low for many years, allowing our building codes to neglect energy conservation. Some of our homes have adequate attic insulation, because it is fairly inexpensive to install. But out walls, floors, and foundations are often the weakest energy detail in our homes.
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