Make Bonus Rooms Comfy

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.

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.

But encapsulation doesn't solve the whole problem. As with a blanket, the insulation must be in contact with the surface in order to do its job. “A blanket will only keep me warm if it's touching my skin,” says Coble. “If I hold it away from my skin, no matter how small the distance, cool air can move into that space.”

Unfortunately, some insulators inset staple the tabs on batt insulation to the sides of the studs so that the drywallers can find them more easily. But doing so leaves a space of about 1½ inches between the insulation and the drywall. The air in this space will be about the same temperature as air in the attic. (Drywall has no practical insulation value.) In winter, this warmed air will rise up through the space between the insulation and dry-wall and escape over gaps at the top of the insulation, carrying away most of the room's heat while drawing in cold air through gaps at the bottom of the insulation. In summer, airflow through this space also will reduce the effectiveness of the insulation.

Coble recommends stapling the batt to the face of the studs so that it will hug the back of the drywall. This will make life a bit more difficult for the dry-wallers, but it will make the space a lot more comfortable.

Gaps are also why a bonus room above a garage (or other unheated space) needs blocking in the floor system to stop airflow. “Often, the insulation contractor will install the insulation in contact with the dry-wall on a garage ceiling,” says Coble. “That leaves a 2-inch gap between floor and insulation. Blocking adds some tolerance by stopping the air flow through that space.” The blocking should be installed at the very edge of the bonus room subfloor, directly beneath the knee wall.

ENERGY EXPERTS: John Tooley (left) and Brian Coble (right) work for Advanced Energy, a North Carolina company that helps builders solve building performance problems. Tooley is the company's senior building science consultant, and Coble is a quality control technician.

AE provides training for Masco's Environments for Living (EFL) program, which encourages builders across the country to apply building performance principles to their homes. As part of that program, John Tooley, senior building science consultant at AE, has worked with some of the country's largest builders. He says that about 75,000 houses have been built using its principles, which include blocking and backing for bonus rooms. To help motivate builders to make the change, EFL guarantees that the temperature at the center of the bonus room will be within three degrees of the thermostat setpoint. If not, the company will work with the builder until it is.

According to Tooley, it takes most builders about 90 days “to get to where they understand the process and are doing it well.” There's nothing new for insulators and framers to learn—the builder just needs to change its scope of work so that these subs know what to do, then put the process in place for making sure it's done correctly.

Charles Wardell is a freelance writer based in Vineyard Haven, Mass.