Buildings

Super Insulation

Q: One builder I talked with said he builds "super insulated" houses, based on the fact that he blows 13 inches of insulation into the attic. Is all that insulation really worth it?

A: Thirteen inches" of attic insulation really doesn't tell you much. What you really need to know is the R-value of the material. R-value measures Resistance to heat flow. The higher the R-value, the higher the resistance will be. If he's blowing fiberglass, that thirteen inches may be R-30, the minimum allowed by code. If, on the other hand, he's using cellulose, that thirteen inches may be as much as R-45. Each material, and often each brand of the material, has a different R-value. Ask the builder what the R-value is. If it's not more than code, it certainly isn't "super insulated!"

There are some other things to look for that may be even more important than how much insulation is there. In the typical attic, the trusses or rafters meet the top of the outside wall in such a way as to allow only a few inches of insulation over the wall itself. This leaves a band, often a foot or more wide, where the insulation is less than full thickness.

In other words, you have the least amount of insulation (well under code) along the perimeter, where you're in direct contact with outside air. Many builders — and building inspectors — simply accept this as unavoidable.

Energy-efficient builders have created a number of ways to solve this problem. By slightly altering the roof framing, such as with oversized trusses or "raised heel" trusses, you can place the full thickness of insulation all the way out to the edge of the wall plate. While this does cost a few dollars more, it can pay big dividends. In some areas of North Carolina, Habitat for Humanity does this on all their houses.

There are two other major problem areas we see in terms of attic insulation in new homes. The first is around attic access hatches and pull-down stairways. Typically, we find insulation sloped away from these areas so that it doesn't fall on you when you open them up. Simply boxing around the opening with plywood would allow full-thickness insulation to come right up to the edge.

Some builders also build an insulated box hinged over the opening, so that area is fully insulated, too. Squashing a piece of insulation under the ladder is simply pretending to insulate it. Tight weather-stripping around the opening is also important.

The other major problem area in ceilings is recessed light fixtures. The best way to avoid this problem is simply to not use them. If you are really attached to the notion of putting all those holes in your ceiling, there are several things you can do. First, make sure they are rated "IC" (Insulation Contact) so that the attic insulation can be put right over them. Otherwise, each one creates a totally uninsulated area directly above it and three inches all the way around it.

Second, use fixtures that are rated as "low air leakage," tested to have leakage of less than 2 cfm at 1.57 psf. And finally, make sure the space where the fixture meets the ceiling is well sealed or caulked. This will avoid pulling in 130o attic air when your air conditioner turns on.

To get an effective attic insulation job, you just have to ask two key questions: what's the R-value of the insulation and am I getting that R-value uniformly across the attic, or are there places where it's less or missing altogether?