Sealing Air Leaks: Basement

Inside the DIY Guide

Stopping the Chimney Effect

Outside air drawn in through basement leaks is exacerbated by the chimney effect created by leaks in the attic. As hot air generated by the furnace rises up through the house and into the attic through leaks, cold outside air gets drawn in through basement leaks to replace the displaced air. This makes a home feel drafty and contributes to higher energy bills. After sealing attic air leaks, complete the job by sealing basement leaks, to stop the chimney effect.

Seal All Gaps and Cracks around Rim Joists

Though you may not be able to see cracks in the rim joist cavities, it is best to seal up the top and bottom of the inside of the cavity. Also, rim joist air sealing is especially important at bump out areas such as bay windows that hang off the foundation. These areas provide greater opportunities for air leakage and heat loss. Caulk is best for sealing gaps or cracks that are 1/4 inch or less. Use spray foam to fill gaps from 1/4 inch to about 3 inches.

We also recommend you seal penetrations that go through the basement ceiling to the floor above. Generally, these are holes for wires, water supply pipes, water drain pipes, the plumbing vent stack (for venting sewer gases), and the furnace flue (for venting furnace exhaust).

Materials Checklist for Basement Sealing

  • Silicone or acrylic latex caulk and caulk gun
  • Expanding spray foam
Caution: When sealing the furnace flue (which will be encased in a metal sleeve) use high-temperature caulk. Run a bead of high temperature caulk around the pipe sleeve and around the metal frame.

Areas to Foam or Caulk

  • Along the gap between the sill plate and the foundation
  • At the bottom and top of the rim joist on each end of the house
  • All electrical, water, or gas penetrations and any venting ducts that pass to the outside

What About Insulating the Rim Joist?

After air sealing the rim joist area it is relatively easy to insulate each cavity with rigid foam insulation or fiberglass batts. If using batts, just cut the insulation to fit and place against the rim joist without compression, gaps, or voids. If using rigid, foam into place.

This could also be done in conjunction with finishing the basement, when you would insulate the basement walls floor-to-ceiling. Attic and basement air sealing will go a long way to improve your comfort because your house will no longer act like an open chimney.

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