Insulation, Wilson explained, slows the passage of heat through your walls and roof and this in turn reduces the amount of energy needed for heating and cooling. In winter the insulation helps to keep the heat from seeping out; in summer it does the opposite and helps to keep the heat from seeping in. The rate at which an insulation material slows down the heat, or its “resistance,” is stated in terms of its “R value.” The higher the R, the better.
Most home builders will list an R value for their wall and roof insulation in their sales literature. But, Wilson said, the actual R value may be lower by as much as 15 to 20 percent because the wood framing that holds the insulation in place reduces its effectiveness. Wood conducts heat faster than insulation, a phenomenon known as “thermal bridging.” Offering an example, Wilson said the fiberglass sheets or “batt” in a 2-by-6-inch wall can be rated R-19 by its manufacturer, but the conductive effects of the wood framing would reduce this to about R-15.
When judging insulation, Wilson looks at the R value per inch of thickness. Using this metric, the R value for the two most common insulation materials in residential construction, cellulose and fiberglass batt, is similar (R-3.7 for cellulose and R-3.3 for fiberglass batt) because they employ the same principle—they work by trapping billions of tiny air pockets between their fibers.
Of the two materials, Wilson prefers blown-in cellulose because of its high recycled content (about 80 percent is recycled and most of that is newspapers), and because relatively little energy is used in its manufacture. In “Insulation,” Wilson’s recently published and highly readable book (Building Green, 2012, $129), Wilson lists cellulose as his top choice for conventional wood-frame construction.
Fiberglass batt is more widely used than cellulose to insulate walls, but Wilson finds it problematic. Though it has more than 50 percent recycled glass content (an important source is the bottles put out for curbside pickup), fiberglass batt is time-consuming to install properly. When installation isn’t done right, as is often the case, the insulation is seriously compromised. The issue: fiberglass batts must be carefully cut to fit tightly around plumbing pipes, wiring, boxes for outlets and light switches, and any other irregularity in the outside wall. When there are gaps, heat passes easily through the wall.
For walls, Wilson prefers a less common type of fiberglass called “spray applied” because it completely fills the space between the wood studs, leaving no gaps. It contains about 25 percent recycled material and costs significantly less than batt insulation.
Because heat rises, you’ll have more winter heat loss through your roof than through your walls, and you’ll need more insulation in your attic area under the roof than in the exterior walls below. The extra insulation will also cover summer heat gain, which is highest in the attic because the sun is beating down on it all day (it is not uncommon to have summer daytime temperatures in an attic cavity that exceed 140 to 150 degrees F). Because it’s faster to install blown-in type insulation into an attic cavity, most home builders use that, and Wilson prefers blown-in cellulose to the more commonly used blown-in fiberglass. If a builder insisted on fiberglass, Wilson would use the spray-applied type.
Wilson ended our conversation by emphasizing that insulation is part of a building system that includes the framing and the windows; to get the best performance you need to take these all into account.
Katherine Salant has an architecture degree from Harvard. A native Washingtonian, she grew up in Fairfax County and now lives in Ann Arbor, Mich. If you have questions or would like ot suggest topics for coverage, contact her by e-mail at firstname.lastname@example.org