Q: Some customers complain that their bonus room above the garage is either too hot or too cold. We've started putting in bigger HVAC systems, but the complaints still come in. In every case, the ceiling, the knee walls, and the floor are well insulated, so how can we address this problem?

A: Bonus rooms are those spaces under the rafters, whether above a garage or in an attic, that are finished off to provide extra living space. Builders get more comfort complaints about these spaces than about any other room in the house, according to Brian Coble, quality control technician with Advanced Energy (AE) in Raleigh, N.C., a company that helps builders solve building performance problems. “We're seeing 500-square-foot bonus rooms with their own 1.5-ton HVAC system,” he notes.

But Coble says that mechanicals are rarely the problem. Instead, adding a few simple framing and insulation details that most builders never consider can make that dedicated system unnecessary. He calls these details “a belt and suspenders” system that will help prop up the insulation's effectiveness. “The added cost for these details may be $500 to $1,000, but they can save thousands of dollars in HVAC equipment,” he says. And they usually take only a few weeks for a large builder to incorporate into its schedule.

WEAK IN THE KNEES The biggest changes involve insulation for these spaces' knee walls.

Insulation performs best when totally encapsulated—that is, covered on all six sides, as it is in a standard exterior wall. Coble says that the knee walls he sees often lack a top or bottom plate, which means drafts can move freely between the insulation and the drywall. And because these walls almost never have a backing, the insulation can pull away from the dry-wall toward the attic space behind the knee wall.

It's theoretically possible to do a good insulation job on a knee wall without plates or backing, but the insulation would have to be installed perfectly. The chances of that happening are slim.

Backing, usually a cardboardlike material, does more than help hold the insulation in place—it helps provide some redundancy, which raises the wall's tolerance for an imperfect insulation job. That's because it acts as another block to airflow. “It's not something most people expect to have done in the attic, but it does illustrate how important stopping airflow is,” Coble says.

The Energy Star program considers backing so important that, starting this year, builders who want their homes certified will have to put a backing on all insulated knee walls, so that the insulation is enclosed on all sides.

An alternative to backing the knee wall is to insulate between the roof rafters and to make the roof system airtight, which will bring the attic behind the knee wall into the conditioned space. This detail can be very effective, if done correctly. (For more information on building an unvented attic, see “Eliminating Attic Vents,” January 2006.) Coble says that individual builders need to weigh the relative costs, as well as the skill of their installers. “You need to determine what is the most likely thing your installers will get right,” he notes, adding that he finds knee walls are more tolerant of errors.