Air Sealing Your Home: Finding and Fixing Drafts, Gaps, and Leaks

Where Air Leaks Hide

The biggest leaks are usually not at windows and doors — they are at structural penetrations through the building envelope. The top of the house (attic floor) and the bottom (basement rim joist and sill plate) leak far more than the walls in most homes.

Attic bypasses are the worst offenders. Every wire, pipe, duct, and chimney that passes through the attic floor creates an opening. Recessed can lights are particularly bad — each one is essentially a hole in your ceiling. Plumbing vent stacks, electrical chases (the hollow spaces around wiring), and dropped ceilings or soffits above kitchen cabinets or bathtubs are all direct paths for conditioned air to escape into the attic.

In the basement, the rim joist (the board that sits on top of the foundation wall, where the floor joists connect) is the primary leak point. This joint between the wood framing and the concrete foundation is rarely sealed during construction. Every joist bay is a potential gap, and the sill plate below the rim joist often has daylight visible behind it.

Walls leak at penetrations: electrical outlets, light switches, plumbing pipes, dryer vents, exterior hose bibs, cable and phone line entries, and anywhere different materials meet (siding to foundation, siding to trim, window frames to wall sheathing).

Finding Leaks

A professional blower door test is the gold standard. The technician mounts a calibrated fan in an exterior door, depressurizes the house to a standard level, and measures the total air leakage. Some energy auditors use infrared cameras during the test to pinpoint exactly where leaks occur. Blower door tests typically cost $200 to $400 and are often available through utility rebate programs.

For a DIY approach, use the incense stick method on a windy day. Close all windows, doors, and fireplace dampers. Turn off exhaust fans and the furnace blower. Light an incense stick and slowly move it along the edges of windows, doors, outlets, light fixtures, attic hatches, and any visible penetration. Where smoke streams horizontally or gets sucked toward (or blown away from) the surface, you have found a leak.

Check the attic during daylight. Turn off the attic lights and look for pinholes of light coming through the attic floor — these are penetrations you need to seal. Also look for dark staining on insulation; this marks spots where air movement has deposited dust in the insulation fibers over time.

Sealing Materials and Where to Use Them

Caulk works best for gaps up to 1/4 inch in stationary joints — where two materials meet but do not move relative to each other. Use silicone caulk on exterior joints (it stays flexible in temperature extremes) and paintable acrylic latex caulk on interior joints. Apply caulk around window and door frames, where trim meets walls, around pipe penetrations through walls, and along baseboards where walls meet floors.

Spray foam fills larger gaps (1/4 inch to 3 inches). Use low-expansion foam around window and door frames — high-expansion foam can bow the frame and make the window or door inoperable. Use high-expansion foam for large openings like rim joist bays, attic penetrations around plumbing stacks, and gaps around ductwork.

Weatherstripping seals the movable joints around doors and operable windows. V-strip (tension seal) is the most durable for doors. Adhesive-backed foam tape is the simplest to install but wears out fastest. Tubular rubber or silicone gaskets offer a good balance of durability and ease of installation.

Rigid foam board (1 or 2 inch extruded polystyrene or polyisocyanurate) covers large attic openings like dropped soffits, open chases, and attic hatches. Cut to fit, press into place, and seal the edges with spray foam or caulk. Use fire-rated caulk or intumescent foam around anything within 3 inches of a chimney or flue pipe.

Attic Air Sealing (the Highest Priority)

Seal the attic before adding insulation — insulation slows heat transfer but does not stop air movement. Pull back existing insulation to expose the attic floor. Work from the perimeter toward the attic hatch so you do not crawl over freshly sealed areas.

Seal all wire and pipe penetrations with caulk or spray foam. Use fire-rated material within 3 inches of any flue, chimney, or other heat source. Sheet metal flashing and high-temperature caulk work for chimney surrounds.

Seal recessed can lights from above. If the fixture is IC-rated (insulation contact), build a box from rigid foam or drywall scraps around it, seal the seams, and cover with insulation. If it is not IC-rated, maintain the required clearance (usually 3 inches) — seal the wire penetration into the housing but do not cover the fixture with insulation. Better yet, replace old non-IC fixtures with airtight IC-rated models.

The attic hatch or pull-down stairs are among the leakiest points in the house. Add weatherstripping around the hatch frame, attach rigid foam insulation to the back of the hatch panel, and install hook-and-eye latches to hold the hatch tightly against the weatherstripping. For pull-down stairs, build an insulated box that sits over the entire assembly.

Basement and Crawlspace Sealing

Seal each rim joist bay individually. Cut rigid foam to fit snugly in the bay, press it against the rim joist, and seal all four edges with spray foam. This creates both an air seal and insulation in one step. A can of spray foam and a sheet of rigid foam can seal an entire basement rim joist for under $100 in materials.

Seal the sill plate (where the wood framing meets the concrete foundation) with a bead of caulk or spray foam. This joint often has gaps large enough to see daylight through.

Seal around all mechanical penetrations through the basement ceiling into the living space: plumbing pipes, electrical wires, ductwork boots, and HVAC refrigerant lines. These are just as important as attic penetrations but often forgotten.

Crawlspaces with vented foundations need the floor above sealed and insulated. Crawlspaces with sealed (conditioned) foundations need the walls insulated and the ground covered with a sealed vapor barrier. Either way, seal every penetration through the boundary between conditioned and unconditioned space.

Safety Considerations

Do not seal combustion appliance venting. Gas water heaters, furnaces, and fireplaces need combustion air and proper draft. If you dramatically tighten your home's envelope, you may need to verify that combustion appliances still draft correctly. A worst-case scenario is back-drafting, where exhaust gases (including carbon monoxide) are pulled back into the house. Have a technician check combustion safety after major air-sealing work.

Do not seal controlled ventilation openings. Bath fans, range hood vents, HRV/ERV intakes and exhausts, and fresh air intakes for combustion appliances are intentional openings. The goal is to seal uncontrolled leaks, not to make the house completely airtight without mechanical ventilation.

Wear appropriate protection in attics: N95 mask (insulation fibers and dust), long sleeves, gloves, eye protection, and knee pads. Attics in summer can exceed 140 degrees — work in early morning or wait for cooler weather.

Frequently Asked Questions