Published on June 10th, 2014 | by Henri


Insulating a cantilevered floor

Q.    I have a two-story colonial. The second floor extends two feet over the first floor on the front side of the house. The problem is heat loss. The section of floor  on the upper level that overhangs is often cold, especially when the wind blows.

What is the proper way to insulate this overhang? Currently the underside of the overhang has non-perforated aluminum. I believe fiberglass insulation is packed in between the floor joists and the insulation sits directly on top of  the aluminum. What would you recommend to improve this?

A.    Since the condition is worse when the wind blows, it tells me that the non-perforated aluminum is not tight, and the wind penetrates around its perimeter. One possibility is that  the entire depth of the projecting joists may not be filled with fiberglass insulation. Another possibility is that the band joist may not be fully insulated.

Since fiberglass is a porous filter that works in slowing down heat loss only when tightly enclosed, the wind robs it of that property and cold air circulates through it. The easiest solution is to remove the aluminum soffit, make sure that the entire joist depth is filled with fiberglass insulation (if not, add some or replace the existing insulation with the correct size) and install plywood, properly sealed, around all edges.

If there is enough room between the bottom of the joists and trim board at the base of the wall, add rigid insulation–as thick as will fit–fastened to the bottom of the joists. Then caulk the perimeter and cover it with the plywood.

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