Most basements have multiple air leakage locations. Because of the physics of the stack-effect (causing warm air to rise), basements are a common source of cooler air leaking in through cracks or penetrations in the foundation or unsealed joints where floor framing rests on the foundation wall. Air then rises through holes around chimneys, vents, wires, and plumbing into the rest of the house. Careful sealing will help to limit warm air entering the attic.
Sill plate and band or rim joist
Caulk cracks between the sill plate and foundation wall using a masonry caulk, and fill cracks between the sill plate and band or rim joist. Then insulate the band/rim joist area by spraying with expandable foam or by cutting foam board insulation to fit and then caulking in place. Do not use fiberglass, as it does not stop air infiltration and because moisture from the basement can get between the fiberglass and the rim joist, leading to mold and mildew.
Chase for plumbing vent stack(s)
Seal vents and ducts - Vents for dryers, exhaust fans, or high efficiency water heaters or furnaces that exit through foundation walls or rim joists should be caulked or foamed to prevent air leakage (above). Furnace ducts that rise from basements through walls should be sealed where the metal meets the wood framing (below).
Sealing the chase at the basement end, as well as the attic end, will reduce the flow of air to the other floors in the house.
Vent flues for furnace and water heater
As in the attic, close the gap between framing and chimney/vent flues with metal; seal the edges with red fire-stop caulk. Flues made of PVC (plastic) piping can be sealed directly with spray foam or regular caulking.
Hatch or door to crawl space
Weather-strip the edges and insulate the back of the hatch or door.
Openings through basement ceiling
Seal with caulk or foam the hole where the bathtub drain comes down and other holes for plumbing or electrical wiring in the basement ceiling. You may need to use filler material for larger holes.
Using a masonry caulk or spray foam, fill cracks where the frames of the windows are set into the walls. Windows that are not used for ventilation or as fire exits can be caulked shut permanently.
In homes with air ducts, there may be gaps where they pass through ceilings, floors, and walls. Caulk or foam where the metal duct and the ceiling, floor, or wall meet.
Seal cracks or holes in the foundation with caulk, foam, or the appropriate patching material. Don’t forget to check where the electrical, gas, water supply, telephone, and cable lines enter the house, and seal on the inside and outside.
DON'T SEAL COMBUSTION AIR SUPPLIES!
Water heaters or furnaces may have a flexible duct that supplies fresh air for combustion. These must be kept open! A “J” loop near the duct end can reduce cold air in the basement.
Air leaks in the occupied part of your home are the ones you are most likely to notice. The drafts of air infiltration can be felt around windows, doors, outlets, and fireplaces. The results of these air leaks can frequently be seen as condensation or frost build-ups on windowsills or the bottoms of doors.
Window and door frames, trim, and baseboards
Exposed laths indicate a direct hole into wall and ceiling cavities. Repair with plaster or cover with new drywall.
Cracks in plaster and drywall
Repair cracks using the appropriate patching material, and repaint.
Windows and doors
Check the weather-stripping on all windows and doors, and repair or replace as necessary. Check the threshold on doors, and replace if worn or leaking air. Replace broken glass and reglaze or putty loose windowpanes. Caulk around moving parts of windows with nonpermanent caulk that can be removed easily. You can also install inexpensive window films (inside is best) to cut air leaks.
Electrical switches and outlets
Install foam gaskets and child-safety plugs on all switches and outlets—even on interior walls. Although this may have a very small effect on heat loss from air infiltration, it can have a noticeable effect on comfort, by reducing drafts. Feeling a draft can lead to turning thermostats higher than necessary.
Other holes in exterior walls
Caulk or foam around all ceiling fixtures, heat registers, medicine cabinets, bathtubs, kitchen cabinets, drains, and water pipes where they enter the wall in the kitchen and bath. Also seal any other holes in exterior walls.
Seal holes, cracks, leaks - Anything that penetrates your walls or ceilings is a potential location for an air leak. Gaps around windows, doors, fireplaces, electrical boxes - even cracks in plaster - can allow air to move between the inside and outside of your house. Seal them up with caulk, weather-stripping, or plaster. And foam gaskets on outlets and switches can increase comfort, avoiding unnecessary energy use.