Cantilevered Floors
Air seal the top, bottom, and sides of a cantilevered floor cavity and ensure that insulation is in full contact with all sides without voids.
Install a rigid air barrier or other supporting blocking to separate the cantilever from the conditioned space above. Seal all seams, gaps, and holes in the air barrier with caulk or foam.
Block and seal any open floor joists abutting the cantilever floor cavities.
Install insulation without misalignments, compressions, gaps, or voids to fill the cantilever floor cavity, making full contact with the top, bottom, and sides of a cantilevered floor cavity
Cover the bottom of the insulated cantilever floor cavities with a rigid, weather-resistant solid blocking material such as plywood or house siding.
A cantilevered floor is a floor that sticks out past the foundation or supporting wall below. Air barriers must be put in place across any open floor joist bays to form an air barrier between the cantilever and the rest of the house to prevent air from blowing through the insulation, which renders the insulation ineffective. Blocking material (rigid foam, OSB, plywood, or drywall) should be installed between the cantilever and the primary structure. Air sealing and insulation materials may be installed by framers, insulators, and/or siding installers. This task should be included in the contract for the appropriate trade depending on the workflow at specific job sites.
Figure 1 - Rigid foam and caulk are used to seal each joist bay cavity beneath a cantilevered floor.
Figure 2 - All seams are caulked to completely air seal the subfloor of the cantilever.
Figure 3 - Fill the joist bays of the cantilevered floor with fiberglass batt or spray foam.
Figure 4 - Cover the cantilevered floor cavity insulation with rigid foam.
Figure 5 - Cover the rigid foam on the underside of the cantilever with painted plywood.