Start with a sketch.
This sketch will serve as a reference point once you get into the attic and will help you locate areas of air leakage. Note dropped soffits over kitchen cabinets or bath vanities, slanted ceilings over stairways, areas where walls (interior and exterior) meet the ceiling, and any other dropped-ceiling areas. These areas may have open stud cavities leading directly into the attic and can be huge sources of air leaks.
Look for common locations of attic air leaks.
- Behind and under kneewalls
- Attic hatch
- Wiring holes
- Holes for plumbing and pipes
- Dropped soffits that are open to the attic
- Recessed lights
- Furnace flue or duct chaseway (the hollow box or wall feature that hides ducts)
Plug the big holes first.
Don't worry about finding and sealing all the little holes in your attic; your biggest savings will come from plugging the large ones. Once in the attic, refer to your sketch to locate the areas where leakage is likely to be greatest: where walls (inner and outer) meet the attic floor, dropped soffits (dropped-ceiling areas), and behind or under attic kneewalls.
Look for dirty insulation, which is evidence that air is moving through it. Dropped soffits may be filled or covered with insulation and hard to see. Push back the insulation and scoop it out of the soffits. You will place this insulation back over the soffit once the stud cavities have been plugged and the soffits covered
Step 1. Create stuffed bags. Cut a 16-inch-long piece from a batt of unfaced fiberglass insulation and fold it into the bottom of a 13-gallon plastic garbage bag.
Step 2. Plug open stud cavities. Fold the bag and stuff it into the open stud cavity. Add more insulation to the bag if it doesn't fit tightly. Plug all open stud spaces, and then cover the soffit.
Step 3. Cover dropped soffits. After removing insulation from a dropped soffit, cut a length of reflective foil or other blocking material (rigid foam board works well) a few inches longer than the opening to be covered. Apply a bead of caulk or adhesive around the opening. Seal the foil to the frame with the caulk or adhesive and staple or nail it in place, if needed.
If you have a finished attic, seal behind the kneewalls.
Finished rooms built into attics often have open cavities in the floor framing under the sidewalls or kneewalls. Even though insulation may be piled against or stuffed into these spaces, they can still leak air. Again, look for signs of dirty insulation to indicate air is moving through. You need to plug these cavities in order to stop air from traveling under the floor of the finished space.
Step 4. Seal behind kneewalls. Cut a 24-inch-long piece from a batt of fiberglass insulation and place it at the bottom of a 13-gallon plastic garbage bag. Fold the bag over and stuff it into the open joist space under the wall (a piece of rigid foam board sealed with spray foam also works well for covering open joist cavities). Again, cover with insulation when you're done.
Seal around furnace flues.
The opening around a furnace or water heater flue or chimney can be a major source of warm air moving into the attic. Because the pipe gets hot, building codes usually require 1 inch of clearance from metal flues (2 inches from masonry chimneys) to any combustible material, including insulation.
These gaps should be sealed with lightweight aluminum flashing and special high-temperature (heat-resistant) caulk. Before you push the insulation back into place, build a metal dam to keep it away from the pipe. Use the same technique for masonry chimneys. Caution: Furnace flues (the pipe that removes your furnace exhaust) can be very hot.
Identifying Attic Pipes
| Flues/Vents/Pipes |
Made Out Of |
Seal Around With |
| Furnace/Water Heater |
Galvanized Metal or PVC |
Aluminum flashing or sheet metal and high-temperature silicone caulk |
| Chimney |
Masonry/Metal |
Aluminum flashing and high-temperature silicone caulk |
| Plumbing |
Cast Iron or PVC |
Expanding foam or caulk, depending on the size of the gap |
Step 5. Cut aluminum flashing. Cut aluminum flashing to fit around the flue. For round flues, cut half circles out of two pieces so they overlap about 3 inches in the middle. Press the flashing metal into a bead of high-temperature caulk and staple or nail it into place. If there's no wood, staple or nail it directly to the drywall, but be sure not to staple or nail through the drywall.
Step 6. Seal with silicone caulk. Seal the gap between the flue and metal flashing with special high-temperature caulk. Do not use spray foam.
Seal small gaps.
Even though most of the gaps spilling warm air into your attic are buried under insulation, you might be able to find evidence of them. Look for areas where the insulation is darkened. This is the result of filtering dusty air from the house. In cold weather, you may see frosty areas in the insulation caused by warm, moist air condensing and then freezing as it meets the cold air in the attic. In warmer weather, you might find water staining in these same areas. Although the insulation is dirty, it is still okay to use. There is no need to remove and replace it. After sealing the areas, just push the insulation back into place. If you have blown insulation, a small rake can be helpful to level it back into place.
Use expanding foam or caulk to seal the openings around plumbing vent pipes and electrical wires. Be sure to wear gloves and be careful not to get expanding foam on your clothes, as the foam is very sticky and can be hard to remove once it sets. When the foam or caulk is dry, cover the area again with insulation.
Step 7. Form an insulation dam. Form an insulation dam to prevent insulation from contacting the flue pipe. Cut enough aluminum from the coil to wrap around the flue plus 6 inches. Cut slots 1 inch deep and a few inches apart along the top, and bend the tabs in. Cut slots about 2 inches deep along the bottom and bend out the tabs. Wrap the dam around the flue and secure the bottom by stapling through the tabs. Now put insulation back right up against the dam.
Step 8. Find attic bypasses. Check for gaps in your attic that facilitate air movement by checking for dirty insulation. Seal the gaps with caulk or expanding foam. When complete and dry, push the insulation back into place.
Step 9. Fill holes with caulk. Fill wiring and plumbing holes with expanding foam. Caulk around electrical junction boxes, and fill holes in boxes with caulk.
Step 10. Stuff gaps with insulation. If the space around your plumbing pipe is wider than 3 inches, you may need to stuff some fiberglass insulation into the space to serve as a backer for the expanding foam. Once the fiberglass insulation is in place, follow the directions on the can to foam the space around the pipe.
Considering an attic sealing project?
Some attics have vermiculite insulation, which may contain asbestos. Vermiculite is a lightweight, pea-size, flaky gray mineral. Do not disturb vermiculite insulation unless you have had it tested by an approved lab to be sure that it does not contain asbestos. Contact your local health department for the name of an approved lab.
