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:

5 thoughts on “The Evil Cantilever

  1. John

    The second picture shows brick cantilevered on the side elevation, was this a change from the drawings that shows siding? Typically the cantilever is covered in siding as drawn. Guess you have some type of steel support for the brick? Our parents 1962 split level had the brick stopping at the underside of the cantilever, we also had to add additional batt insulation in the overhang along the basement walls.

    1. Douglas Camin Post author

      John – Sorry I didn’t see your comment until now for some reason. I just took a look at the plans again to see what you were referring to. The exterior of the house is significantly different than what was drawn in the plans in terms of materials used. The plans had four different materials – brick and stone (front) and wood shingles (sides and back first floor) and vertical siding (cantilevered portion on back and sides.) As built, the house is all brick (except a stone accent in the front) and vertical California redwood siding only on the back wall of the cantilevered portion.

      The story about how the house came to be brick goes like this: My Grandfather’s mother (my great Grandma) called up my Grandfather from her place in Chicago when he was planning the house. She asked some questions about the house, including what the exterior materials were going to be. He told her, and she said “Artie, brick houses are better.” (their house in Chicago was brick.) He explained that to make the whole house brick it would be too expensive. She asked how much and he said about $300. She mailed him a check for that amount to make sure the house was brick. That is how my grandmother told me the story, at least!

  2. [email protected]

    I know this is an old post, but what is the order of operations to get to the sofit/cantilever, without damaging the siding? I’m guessing you start with the flashing on the far left, then slowly remove the screws holding in each piece of sofit/cantilever cover material?

    1. Douglas Camin Post author

      This really depends on how your cantilever is set up. As this house was originally built, there was vertical siding on the face and then a thin plywood soffit board underneath. But in the 90s my grandparents had the soffits and fascia boards “wrapped” with aluminum and plastic soffit material put over the plywood boards. This eliminates the need to paint these. But because they were present, there was an edge wrap piece on the vertical siding (the new soffit material lowered the effective height about 1 inch from original.) So for my case I took off the edge wrap and steadily worked my way down the line, pulling the plastic soffit pieces off one at at time by sliding them down the track they hang in to the end after taking out any screws used to keep them from rattling in the wind. You can also bend these pieces somewhat, so I was able to work around obstacles or bend the piece just enough to get it out anyways. Taking the plastic off exposed the old plywood, which I carefully removed. Once I was done getting the insulation into the cavity I put everything back and made sure to caulk the edges of the soffit material. The whole goal here is to minimize any airflow, even though the “outside” portion of the cantilever bay – it’s that airflow that will make your floor chilly.

      Hope that helps!


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