A. Blocking between joists directly below knee wall

B. Backing on other side of wall

C. Insulation batt with tabs stapled to face of stud
A. Blocking between joists directly below knee wall B. Backing on other side of wall C. Insulation batt with tabs stapled to face of stud

It’s not unusual for a client to ask a contractor to turn the attic space above a garage into a finished room. Done right, this bonus room can be a cozy addition to the home’s living space; done wrong, it can be chronically uncomfortable, either too hot or too cold.

The sad truth is that builders get more comfort complaints about these spaces than nearly any other room in the house, according to Brian Coble, director of building science at Advanced Energy, a Raleigh, N.C., building performance consultancy. He says that the most common response is to double down on space conditioning. “We have seen 500-square-foot bonus rooms with their own 1.5-ton HVAC systems.”

That response is not just overkill; it shows that the builder doesn’t understand the cause of the problem. A few simple framing and insulation details that a lot of builders never use can make that dedicated system unnecessary. Coble calls these details “a belt and suspenders” that prop up the insulation’s effectiveness. They might add $500 to $1,000 to the cost of the room but can save thousands of dollars in mechanical expenses.

Basically, the builder needs to lavish a bit more attention on the room’s knee walls.

The most important point to understand is that insulation will perform the way it’s supposed to only when totally encapsulated—that is, covered on all six sides, as in a standard exterior wall, and touching all of these six surfaces.

Often that’s not the case in bonus rooms. Many of these knee walls lack a top or bottom plate, and in most cases the insulation is exposed on the side facing the unconditioned attic space or garage space. The lack of a backing makes it easy for the insulation to pull away from the drywall toward the attic space, leaving a gap between the insulation and the drywall; the lack of plates allows air in this gap to circulate freely between the insulation and the attic space.

The problem with air circulation is that insulation works like a blanket: in order to provide warmth, it has to be in contact with the surface it’s insulating. “A blanket will keep me warm if it’s touching my skin,” says Coble. “But if I hold it away from my skin, cool air can move into that space.”

When insulation pulls away from the drywall, air in the resulting gap will be about the same temperature as air in the attic. (Drywall has no practical insulation value.)

In winter, warmed air will rise up through the space between the insulation and drywall 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—caused by naturally rising hot air or by air from the soffits blowing through the space behind the knee wall—will also make the insulation less effective.

The backing can be a panel product like ¼-inch plywood or drywall or a cardboard-like material made just for this purpose. Besides holding the insulation in place, the backing material raises the wall’s tolerance for an imperfect insulation job by acting as another block to airflow.

Backing is not something most people expect to have done in the attic, but it can really earn its keep. In fact, the Energy Star program considers backing so important that builders who want their homes certified have to put a backing on all insulated knee walls, ensuring that the insulation is enclosed on all sides.

While the most important details in a bonus room concern the knee wall, it’s also a good idea to add blocking to the floor system if the bonus room is above a garage or other unheated space. Often, the insulation contractor will install the insulation in contact with the drywall on the garage ceiling, leaving a 2-inch gap between floor and insulation.

Blocking adds some tolerance by stopping the airflow through that gap. The blocking should be installed at the very edge of the bonus room subfloor, directly beneath the knee wall.

An alternative to backing and blocking is to insulate between the roof rafters and make the roof system airtight, bringing the attic space behind the knee wall into the conditioned space.

Which approach makes the most sense for a particular builder or remodeler will depend on local material costs as well as the skill of the installers. The best bet, according to Coble, is “to determine what is the most likely thing your installers will get right.”

Regardless of where the insulation is installed, it’s not a good idea to inset staple the tabs on batt insulation to the sides of the framing. Some insulators do this so that the drywallers can find the studs more easily, but the result is a 1.5-inch space between the insulation and the drywall, resulting in all the thermal problems mentioned above.

The best practice is to staple batts to the face of the studs, so that the batts touch the back of the drywall. The drywaller may not like it, but the space will be more comfortable and the clients happier.