Air Sealing and Insulating the Attic

If you do one energy project in an old house, make it the attic. The National Park Service ranks attic insulation among the most effective measures, with little impact on historic materials ¹, and ENERGY STAR calls attic air sealing and insulation a do-it-yourself job if the attic is accessible ². The order is the same as always: seal first, insulate second.

Three stop signs

Before you start, look for three conditions that mean stop and get help ²:

• Vermiculite. A lightweight, pea-size, flaky gray mineral insulation, common in old attics. It may contain asbestos. Don’t disturb it until it has been tested by an approved lab; your local health department can name one ².
• Knob-and-tube wiring. ENERGY STAR flags it as a fire hazard when in contact with insulation ². Deal with it first (see the Old Wiring module).
• Moisture. Wet or damp insulation, moldy or rotted rafters, or a history of ice dams mean a leak or serious air leakage that needs fixing first. So does a bathroom fan or dryer vent dumping moist air into the attic instead of outdoors ².

Plug the big holes first

Don’t try to find every pinhole. Your savings come from the big openings ². Take a sketch of the floor plan up with you and look where walls meet the attic floor.

Open wall tops and soffits. In an old house, interior and exterior walls may be open at the top, and dropped ceilings over kitchen cabinets or stairways (soffits) can be open boxes into the wall cavities below. ENERGY STAR’s trick: fold a piece of unfaced fiberglass batt into a plastic garbage bag and stuff it into each open stud bay. Cover open soffits with foil-faced board or rigid foam sealed with caulk or construction adhesive ². Kneewalls in finished attic rooms often hide the same open cavities ².

Chimneys and flues

The gap around a chimney or furnace flue can be one of the biggest leaks in the attic, but you can’t stuff it with insulation or foam, because the pipe gets hot. Building codes usually require 1 inch of clearance from metal flues and 2 inches from masonry chimneys to any combustible material, insulation included ².

Instead, cut aluminum flashing to cover the gap, set it in a bead of high-temperature caulk, and fasten it. Then wrap a sheet-metal dam around the flue so the insulation stays back the required distance ².

Wires, pipes, and the hatch

• Seal around wires and plumbing vents: caulk for gaps up to a quarter inch, spray foam for gaps from a quarter inch to about 3 inches. Stuff a little fiberglass in wider gaps as a backer for the foam ².
• Look for dirty insulation, which marks hidden leaks ².
• Old recessed lights need air around them to stay cool, so get advice before sealing them, or replace them with IC-AT-rated fixtures made to be covered and airtight ².
• Weatherstrip the hatch, add hook-and-eye latches to compress the weatherstrip, and glue rigid foam to its back. Treat it like an exterior door ^{2 1}.

Keep the eaves breathing

A vented attic needs air to come in at the soffits and out at the ridge or gables. The most common insulation mistake, ENERGY STAR says, is blocking the soffit vents ². Before insulating, staple rafter vents (baffles) between the rafters where the roof meets the attic floor. They hold a channel open so you can insulate right out to the edge of the wall below ². The Park Service adds that unfinished attics must be ventilated ¹.

How much insulation

ENERGY STAR’s retrofit recommendation for cold climates like Wisconsin’s is R60 on an uninsulated attic floor, or R49 added on top of 3 to 4 inches of existing insulation ³. (For comparison, Wisconsin’s code calls for R-49 ceilings in new houses ⁴.) Where batts are hard to fit, blown insulation gives more complete coverage ¹. If you add batts over old insulation that already reaches the tops of the joists, use unfaced batts laid across the joists ¹. A vapor retarder isn’t necessary in attic floor insulation ¹.

Finally, have your combustion appliances checked after sealing ².

Try it

Measure the depth of your existing attic insulation in a few spots, especially along the eaves where it’s often thinnest, and note its type. Then count your big leaks: open wall tops, soffits, the chimney, flues, the hatch. That count, and the depth, tell you how big a weekend you have ahead.