Understanding and Improving Cantilevered Floors for Energy Efficiency: A Guide by Foam Comfort

A cantilevered floor is a floor that sticks out past the foundation or supporting wall below. It may be a first- or second-story bump-out, a bay window, or a room over an open porch. Floor joist bays that extend from the house out under the cantilevered floor are sometimes left unsealed and uninsulated by the builder, allowing outside air to flow through the home and conditioned air to escape. Sometimes cantilevered floors are insulated but not air sealed. 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 such as spray foam should be installed and when designing the home, HVAC ducts and plumbing pipes should not be located in cantilevered floors.