Heat will travel through your attic by both heat transmission and air leakage. Whenever the attic is colder than the house, heat will flow from the house to the attic through the drywall, plaster, or paneling that covers the ceiling. In summer, heat will travel in the opposite direction down into your home. Heat flows through wood framing members, too. But another important heat-flow mechanism is air leakage, since air carries heat into and out of your home. Air can leak around your chimney, plumbing pipes, wires, and other penetrations in your ceiling. Air leakage can account for a surprisingly large portion of your winter and summer utility bill.
 |
| Any penetration or irregularity in an attic is a likely location for an air leak. |
So insulation reduces heat transmission and air-sealing reduces air leakage. Air sealing should be done first because the installation of attic insulation obstructs air sealing. Attic insulation slows heat transmission but doesn’t seal air leaks because it is a loose material and air passes right through it. If you insulate without air sealing, you may never be able to seal the air leaks once you cover them with the insulation.
Before you insulate your attic, locate each and every penetration in the ceiling of your home. You’ll do this from within the attic. You may find a main plumbing vent and one or two smaller plumbing vents that lead to the roof. If you have a gas furnace and water heater, you will have at least one and possibly two chimneys. You could have another chimney for a wood stove or fireplace. The places where these vents and chimneys penetrate the ceiling are often the home’s largest air leaks. You may find cavities that lead into closets or cabinets, too.
You can seal small holes in your ceiling with caulking. Liquid foam-in-a-can is useful for the larger holes and gaps you find. The biggest gaps can be covered with drywall or plywood. To seal around chimneys, use fire-proof materials such as aluminum sheeting and high-temperature silicone caulk. Once your attic is sealed from your living space, then you can go ahead and add attic insulation.
Both Residential Energy: Cost Savings and Comfort for Existing Buildings and Saturn Energy Auditor Field Guide contain more information on diagnosing and fixing air leaks in attics.