Insulating an Attic Floor
A poorly insulated attic sends heat right through the roof in cold weather and serves as a solar collector on hot, sunny days. The solution is to insulate the attic floor. If the space will be used only for storage, insulate the floor. If the space is finished or you intend to finish it later, insulate the ceiling and walls.
First look for leaks that might damage insulation, and make sure you have adequate ventilation. You may choose to cut summer heat buildup with an attic fan or whole-house fan. If your attic has a floor, you have two options: pull up sections and work the insulation under, or hire a contractor to blow in loose-fill through holes bored into the floor. If the attic lacks a floor, bring up planks or plywood panels to set atop the joists. This will allow you to move around without stepping through the ceiling.
If there already is a vapor barrier, use unfaced materials so moisture doesn’t get trapped between the two barriers. Be sure not to cover recessed light fixtures or exhaust fans; doing so could cause a fire. Instead, install baffles that keep the insulation about 3 inches away on all sides. NOTE: Some insulating materials, such as fiberglass and mineral wool, are harmful to lungs and skin. Be sure to wear a painter’s mask, gloves, and long sleeves.
Tools: Utility knife, staple gun, rented blower for loose-fill.
Allow for ventilation. When placing batts or blankets, take care not to jam them against the roof; leave space at the eaves for airflow.
Cut pieces to fit tightly. To cut a blanket or batt, place it on a piece of plywood with the facing side up. Use a piece of 2x4 as a guide and to compress the insulation. Cut with a utility knife. When you encounter bridging or other obstacles, cut for a snug fit.
Vapor Barriers - When warm air meets a cold surface, condensation results, producing moisture that can damage insulation as well as a house’s structure. A properly installed vapor barrier—which may be the paper, foil, or plastic facing of the insulation or a sheet of plastic—keeps moisture from collecting. In most areas, the vapor barrier should be placed on the side of the insulation that faces inside. In the Southeast, however, the vapor barrier should face the outside. Check with your building department if you are not sure.
Loose-fill insulation. For loose-fill, nail baffles at the eaves. Or buy special plastic baffles that will fit between joists and ensure airflow. Insulation should cover the wall top plate but not obstruct airflow. Pour some insulation between joists, then level it with a board. Be sure you don’t leave low spots or voids.
Insulating a Finished Attic
In a finished attic, the space above collar ties, coupled with gable louvers at the ends, gets rid of winter condensation as well as summer heat. If you don’t have collar ties, install them. If your attic already is finished, you’ll have to cut holes in the knee walls and the ceiling to gain access to insulate these areas.
Tools: Utility knife, chalk line, hammer or screwdriver, stapler. More tools may be needed to add collars or to refinish walls.
Install collar ties. Add collar ties if your attic has none. Use 2x4s for spans of 6 feet or less; use 2x6s or larger for longer spans. Plan the height, and use a chalk line to mark the locations on the joists. Cut the ties to fit, then nail or screw them to the rafters. You may also choose to install knee walls.
Staple blankets. Staple insulation blankets to the collar ties, with the vapor barrier side facing correctly. Continue down the roof and the knee wall to the floor.
Fill the knee-wall space. In a finished attic, make openings to get at spaces above collar ties and behind knee and end walls. Lay loose-fill or blanket material between joists in the knee-wall space. Use a broom handle to push into “unreachable” spots.
Fill the ceiling. Don’t insulate between the roof rafters in the knee-wall space. Place material between the wall studs, with the vapor barrier facing correctly. Pour loose-fill material between the rafters in the sloping part of the ceiling. Finish off with batts in the attic above the collar ties.
Add to existing insulation. If you’re adding to existing insulation, cut away its stapled flange and push it to the back of the cavity. Slash the old vapor barrier. Place the new insulation over the old. Staple flanges to the rafters, with the vapor barrier facing correctly. Lap the insulation joints (as shown above) as you go.