Insulating Cantilevered Floor Joists

4b slab floors built directly on the ground.

Insulating cantilevered floor joists. Seal the perimeter of each piece of foam with caulk or canned foam. At the top of the joist bays insert narrow rectangles of rigid foam between each joist and secure that foam to the subfloor with a compatible adhesive. 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. Cover the horizontal surface of the overhang as well as the joints where the horizontal surface comes into contact with the air dams.

You can insulate cantilevered floors many ways. Block and seal any open floor joists abutting the cantilever floor cavities. Apply spray foam insulation to the bottom edge of the overhang. The illustration shows rigid foam at the top and bottom of the joist bays.

Fiberglass must be cut precisely to fill in the batts in the floor joists. Also insulate 4a any portion of the floor in a room that is cantilevered beyond the exterior wall below. Cellulose cantilevered floor insulation cellulose insulation can be installed in the cantilever floor joists two ways through access from the crawl space or by drilling holes through the bottom of the overhang. Installing cellulose via the crawl space is fairly simple as it is just blowing the material into the floor joists.

You should only put in fiberglass insulation if you can access the cantilevered floor joist from a basement or crawl space.