Home Weatherization: Sealing Air Leaks That Drive Up Energy Bills

Before buying any materials, do a visual walkthrough of your home's interior and exterior. Look for gaps where different building materials meet: where siding meets the foundation, where pipes and wires enter the wall, around window and door frames, and at baseboards along exterior walls. Write down or photograph each location.

Common leak sites include electrical outlets on exterior walls, recessed light fixtures in ceilings below attic space, and the band joist (the framing member at the top of your foundation wall). Tasks like these are easy to overlook but matter year-round.

Light an incense stick or thin candle and hold it a few inches from each suspected gap. Move it slowly along window frames, door edges, electrical outlets, and pipe penetrations. Watch the smoke carefully: if it drifts toward or away from the wall rather than rising straight up, air is moving through that spot.

Work methodically from room to room. Mark confirmed leaks with a small piece of painter's tape so you can return to them with materials.

New sealant bonds poorly over crumbling or peeling old material. Use a putty knife or utility knife to scrape away deteriorated caulk from window and door frames, making sure the surface is clean and dry before you apply anything new. For weatherstripping on door stops or sashes, peel it away completely and wipe the channel clean.

Use a caulk gun loaded with paintable latex caulk (for interior trim and painted surfaces) or silicone caulk (for areas that get wet or face extreme temperature swings, such as exterior window frames). Cut the nozzle at a 45-degree angle to match the width of the gap — typically no wider than ¼ inch.

Apply a steady, continuous bead along the gap, then smooth it with a dampened finger or a caulk tool. Wipe away excess before it skins over. For window and door weatherization detail, our guide to managing window and door drafts covers specific techniques.

Weatherstripping seals the gaps around moving parts that caulk would just crack. For exterior doors, foam tape or V-strip (tension seal) weatherstripping works well along the sides and top of the door stop. For the bottom gap, a door sweep screwed to the interior face of the door provides a reliable seal against the threshold.

Cut weatherstripping to length with scissors or a utility knife. Press foam tape firmly into clean channels; nail or staple V-strip in place if needed. Test the door: it should close firmly without requiring extra force and should show no light gaps along the edges.

Expanding spray foam is the right tool for irregular gaps larger than about ½ inch — particularly where plumbing pipes, electrical conduit, or HVAC lines pass through walls, floors, and ceilings. These penetrations are often the worst offenders in older homes.

Shake the can, attach the straw nozzle, and apply foam in short bursts. The foam will expand to roughly double its initial volume as it cures, so use less than you think you need and let it expand fully before trimming any excess with a utility knife after curing. Seal gaps at the band joist in the basement the same way.

Once all materials have cured (usually 24 hours for caulk, immediately for foam), repeat the smoke or candle test at each location you sealed. Confirm that smoke rises straight up without deflection. Check door sweeps by sliding a sheet of paper under the closed door — you should feel slight resistance as you pull it out.

Air sealing works best as part of a broader weatherization strategy. Pairing it with adequate insulation addresses the full picture: sealing stops uncontrolled airflow while insulation slows heat transfer through surfaces. See insulation types and where each one belongs for guidance on what comes next.