A floor cantilever must be air-sealed and insulated well because its bottom is exposed to the weather. Rigid foam and cavity insulation can achieve R-40 or more, but stopping air is even more important. Floor bumpouts can be cold spots in a home if you don't air seal and insulate cantilevers.
A little while ago we went over how to frame a floor with cantilevers in it. We also went over how to cover that floor with subflooring, which is an important step because people could get hurt walking around on the framing every day.
Framing connections are leaks until they are sealed
Today, we will cover how to seal the underside of that cantilever. Because above it is going to be living space, if it is not sealed and insulated, that floor will sabotage all of the other energy-saving measures that you are gonna take with the walls and roof. Every seam between a joist and a block leaks until it is sealed. The long gap between the mudsill and foundation is a leak until it is sealed. Yes, the sill seal gasket helps, but it’s not enough.
Airseal and insulate cantilevers one step at a time
We will seal rigid insulation against the underside of the subfloor as a primary air barrier. The block should be cut about an inch smaller than the opening so that there is a gap around the perimeter that can be sealed with canned foam. Don’t be stingy with the gap, canned foam’s primary job is to expand into the gap. And it is good at its job. Do this for every joist bay. Now, you can fill the cavity with regular insulation, like fiberglass or mineral wool batts. Run a bead of sealant under the framing and cut another piece of rigid insulation to fasten against the framing.
Final water management steps
Now, go build some walls. When it’s time to sheathe those walls, run the sheathing down to the bottom of the rigid insulation and tape the edges with flashing tape. Along the back edge, run another bead of sealant. Depending on your siding type, install a rigid finish soffit material.
Housewrap overlaps the flashing tape, and the trim or siding at the bottom should extend below the soffit to act as a drip edge. Seal the soffit to the trim and the foundation, and you’re ready to step back and grab a cup of coffee cause it’s break time.
