BLANKET ADVICE 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.
GUARANTEED RESULTS 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.