Insulating Cantilevered Floor Joists

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

Insulating cantilevered floor joists.

4c as an alternative to floor insulation foundation walls of unvented crawl spaces. 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. Seal the perimeter of each piece of foam with caulk or canned foam. You can insulate cantilevered floors many ways.

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. The illustration shows rigid foam at the top and bottom of the joist bays. Installing cellulose via the crawl space is fairly simple as it is just blowing the material into the floor joists. Apply spray foam insulation to the bottom edge of the overhang.

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. Fiberglass must be cut precisely to fill in the batts in the floor joists. 4d extend insulation into joist space to reduce air flows.

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.