Rim Joist & Foundation Insulation in Boston, MA

Rim joist and foundation insulation stops the cold air that enters around the bottom of the house. The rim joist is the band of framing resting on the foundation, and in most older Boston homes it is bare on the inside, with gaps at the sill where wood meets brick, stone or concrete. Cold air comes in along the whole perimeter, chills the first floor and feeds the upward airflow that pushes warm air out at the top.

Closed-cell foam on the rim

Closed-cell spray foam seals every gap around joists, pipes and wires and insulates the rim in one pass. At typical thickness it is a vapor retarder, so humid basement air does not condense on cold wood. HomeGuide lists rim joist foam jobs at $2,000 to $2,500 nationally. It is often the best value in the whole house.

Brick, stone and concrete walls

Boston basements range from fieldstone and brick to poured concrete. If you plan to finish a basement, insulating the walls first is far easier. The Massachusetts stretch code summary lists R-19, R-13 plus R-5 continuous, or R-15 continuous for basement walls in Zone 5. Closed-cell foam works on poured and block walls. On old fieldstone or brick that stays damp, we sometimes recommend insulating only the rim and leaving the masonry to dry inward, and we explain why.

Water and filled land

Wikipedia notes that more than two-thirds of inner Boston's land was filled marsh and tidal flat between 1631 and 1890, and many basements near the water and in low areas run damp. Foam is not waterproofing. If water comes through the wall or up through the floor, drainage or sump work comes first. We look for staining, efflorescence and damp sills before quoting.

Floors over cold spaces

Rooms over unheated porches, garages and bump-outs have the same problem from the other side. Foam in those floor bays is often part of the same job.