Tag Archives: insulate cantilever

The Evil Cantilever

Our house is a really cool back-split style house, but along with that comes some insulating and air sealing issues that are unique to our house. In 2010 when my Grandmother still lived here, I helped her replace the boiler as well as perform a whole-house insulation upgrade (the costs and rebates were so good she had all the work done for just the cost of the boiler and cut her heating bill in half.) These guys did blown cellulose in most walls (there was none previously) and air sealed a large portion of the house.

One thing they did not air seal – and I didn’t realize at the time to insist it be done – was the cantilever. The evil cantilever. You see, a cantilever adds additional space on the second floor of your house (yay!) but until recently they were almost universally constructed in such as a way that they are places where enormous amounts of energy (heat or cool) leak from your house. Our house was no exception. The cantilever in our house was so porous that during the winter the circulating air in the space between the family room and upstairs bedrooms was so significant it actually chilled the floor. Not a lot of fun – so I had to fix it.

This would qualify as a medium-scale project for me. It was pretty messy and took about a week to complete. I started by removing the soffit pieces on the cantilever and pulling back the wood that closes the cantilever up. Once inside, I took pre-cut pieces of foam insulation and fit them into the cavities between each set of joists above the wall and, using spray-foam insulation, foam sealed them into place. Once the spray foam expands and dries it forms a nearly air-tight barrier. By doing this I effectively “cut off” the cantilevered space from the interior space of the house – this would be sufficient to stop the air flow problem – but I still would have another problem that the outside 1.5 feet of floor in each bedroom upstairs would be exceptionally cold now.

To solve this, I picked up some mineral wool insulation (WAY better than fiberglass) and fit them into the cavities underneath the floor. This should insulate that last 1.5 feet of floor and keep it nice and warm. In one spot I had a pipe for the baseboard heat that poked into the space. To solve that, I had to use foam board and build a “box” around it.

Once finished I put the wood cover back up and caulked it appropriately to provide a first line of air sealing, cleaned everything up and reinstalled the plastic soffit pieces. The finished product came out nice, but of course I was doing this in late spring which means we don’t get a chance to see how much better the floor feels until winter. Based on some previous air sealing on another section of the flooring, we are expecting the change to be pretty significant.

Here is a photo chronology of the work: