Insulating Cantilevered Floor Joists

Block and seal any open floor joists abutting the cantilever floor cavities.

Insulating cantilevered floor joists.

Apply spray foam insulation to the bottom edge of the overhang. 4d extend insulation into joist space to reduce air flows. 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.

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. 4c as an alternative to floor insulation foundation walls of unvented crawl spaces. This material has a few drawbacks as it still allows some airflow through the floor and is known to trap allergens and moisture which can leave you with mold growth. Seal the perimeter of each piece of foam with caulk or canned foam.

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. Fiberglass must be cut precisely to fill in the batts in the floor joists. Cover the horizontal surface of the overhang as well as the joints where the horizontal surface comes into contact with the air dams. The illustration shows rigid foam at the top and bottom of the joist bays.

4b slab floors built directly on the ground.