Insulation and the R-Value

by Tim Gould

Believe it or not the common adage that heat rises is not true. In fact, heat moves toward cold-regardless of direction. Given the large area to which walls, floors, ceilings, and roofs add up, it should be no surprise that insulation in, on, under, or behind these parts of your house plays a major role when it comes to heat loss in the winter.

Commonly used insulation types are fiberglass batts, loose-fill fiberglass, loose-fill cellulose, open cell foam, and closed cell foam. Knowing which type to use for a given application and set of conditions isn't always easy. There are many factors that determine which insulation to use. Some of these are the type of cavity to be insulated (closed such as an interior wall or open such as an attic), level of air sealing needed, and obviously price.

The R-value listed on insulation packaging is the level of thermal resistance of that particular product if installed to the manufacturer's specifications. Many homes have inadequate insulation-whether it is too lean a layer, not carefully installed, or just missing in hard to reach areas. When fiberglass insulation is not installed properly (i.e., there are gaps between batts and/or gaps between batts and framing), as much as a 30% drop in the R-value can occur. One of the best methods for determining the adequacy of your home's insulation is with infrared imaging. The infrared image (right) showing fiberglass batts in an attic highlights the gaps (in bright yellow) where heat is escaping.

Another way to determine at least whether insulation is present in a closed cavity wall space and the type of insulation is by drilling one-quarter inch holes approximately four inches apart and shining a light in one while looking in the other. In most instances, you will be able to see what type of insulation is in the wall.

Attics are often the best place to insulate for the most return on your heating dollar and the most added comfort to your home; also, they are usually the easiest in terms of access. A minimum R-38 is typically recommended.

Anecdotally, brick is not considered insulation, but if you have brick-only walls, you may want to insulate and drywall a portion of it because it takes a 5-foot thick wall of brick to equal an R-19 wall. Wood has an R-value of about 1 unit of thermal resistance per inch, which is much lower than insulation and therefore suggests that framing should be minimized (as long as it is built to code) in order to maximize the insulated area.