Building Material Efficiency Ratings

The materials from which a building is constructed can have a marked impact on the structure's efficiency. Materials that allow a lot of heat to pass through them lower the overall efficiency level of the building. Conversely, materials that resist a significant amount of heat transference can help ensure greater efficiency. R-value and U-value are the two most common measurements of building material efficiency.

Efficiency Rating

Definition

Explanation

R-value

. . . is the measurement of how effectively a material resists the transfer of heat via conduction.

The higher the R-value, the less heat transfer can take place through the material.

Some materials are more resistant to heat transfer than others, giving them higher R-values. One of the best ways to enhance the product's R-value is to increase the amount of gas (including air) inside or immediately surrounding it. For instance, the glass of a single-pane window has virtually no R-value, but the thin film of air that normally exists on either side of the glass gives the window an R-value of about 0.83. Adding a second pane of glass and sealing the space between the panes will increase the thickness of one of the insulating gas layers, thereby more than doubling the window's R-value.

Another example of how the presence of dead-air spaces affect a product's R-value can be seen with wood. Hardwoods, like oak, typically have an insulating value of R-1 per inch of thickness. However, softer woods, such as pine, might have R-values twice as high due to their greater number of air-filled pores.

Products developed especially for the purpose of impeding unwanted heat transfer are called insulation. Insulation can be made of a variety of materials, including old newspapers and wood fibers, glass fibers, and synthetic foams. It can also come in a variety of configurations, including soft blankets, rigid boards, or fluffy loose-fill. But what all of these configurations have in common is their abundance of air-filled pores or pockets.

The actual R-value of insulation products can vary greatly, depending on their composition and form. The least resistant and least common are perlite and vermiculite loose-fills, at R-2.2 to R-2.7 per inch of thickness; the most resistant are polisocyanurate rigid boards, at R-7 per inch of thickness. Fiberglass blankets and cellulose loose-fills, two of the most common residential insulations have R-values of 3.1 to 3.7 per inch.

U-value

. . . is the measurement of how much heat can be conducted through a building component (such as a wall or window). As such, it is the opposite of R-value.

The higher the U-value, the more heat the material will allow to be transferred through it. The lower a mate-rial's U-value, the higher its R-value will be. U-values are always expressed in decimals (e.g., U-0.166).

To determine the R-value of a product for which the U-value is given, you first convert the U-value to its equivalent fraction and then invert it. For instance, the equivalent fraction of U-0.166 would be 166/1000, or 1/6. This inverts to 6/1 or 6, giving you an R-value of 6.