Sealing Floors Above Unconditioned Spaces

Insulation isn't enough – follow these instructions for blocking heat loss.

Floors above unconditioned spaces are a major part of the home's thermal enclosure, separating the conditioned living space from unconditioned space. A home without insulation in the floor will be uncomfortable and have higher energy bills. However, insulation alone isn't enough to stop heat loss. Air can leak in or out through any crack, gap, opening or porous material in the home's thermal enclosure. If a floor built over an unconditioned space isn't sealed to be airtight, it can cause unwanted heat loss in cold weather and heat gain in warm weather. The most common types of floors that have to be insulated and air-sealed include cantilevered floors, floors over garages and floors above unconditioned crawl spaces. Here’s how to seal each:

Cantilevered Floor

If a cantilever isn't properly sealed, it will leak air, causing the floor to feel cold. Condensation is the worst result of air leaks in cantilevers. Moist air in the home can pass into the floor cavity and condense on the coldest surface it finds—the back side of the sheathing or band joist—causing mold to grow there. Cantilevered floors don't receive direct sunlight, so they don't dry quickly, and condensation can cause the floor to rot.

Air-seal any place where the band joist meets the floor to prevent outside air from entering the cavity. Air-seal all gaps and penetrations in the cantilever. Air-seal the connection between the cantilever and the interior floor system to reduce the infiltration paths contributing to heat loss. After installing insulation, install continuous sheathing, such as insulating foam sheathing, on the underside of the cantilever. Then air-seal the sheathing material to the bottom side of the cantilever with caulk.

Floor Over a Garage

When the floor over a garage is poorly air-sealed, air can leak through a number of locations, including the band joists, structural framing and knee walls. Air leaks can lead to a host of problems, such as cold floors, freezing pipes and chilled duct runs that prevent the HVAC system from working properly. A poorly sealed floor cavity has a significantly reduced R-value and isn't much better than a floor cavity with no insulation at all. To air-seal the floor cavity properly, completely seal the band joist area and the subfloor and seal any penetrations in the floor with foam.

Floor Above an Unconditioned Crawlspace

Crawlspaces have many penetrations and mechanicals that aren't completely contained within the floor cavity. It's difficult to install an air barrier in a crawl space without moving the mechanicals into the conditioned space. The major side effects of poorly insulating and air-sealing at the floor line are cold floors and drafts from infiltration. Other side effects include issues with moisture, soil gases, condensation, and rusting or corrosion of the mechanicals.

Use a high-quality foam sealant to seal all penetrations in the floor above the crawlspace. A more complete approach would be to install spray-foam insulation directly against the flooring. The many mechanicals that run through a crawlspace make it difficult to properly insulate and air-seal the floor cavities. So consider making the crawlspace a semiconditioned space by insulating and air-sealing the walls and installing a continuous poly sheet or a combination of a poly sheet and slab.

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