Retrofit Existing Cantilever Floor
Recently purchased a home that has main floor which cantilevers past foundation walls approximately 30”. Building zone R5/R6. – i have spent hours combing thru details for this condition, but i have some concerns with different methods. My concern is trapping condensation inside the joist spacing.
The cantilever is approximately 18” above the exterior grade, so there is some clearance for working from the outside.
I would like to remove the existing sheathing and place rigid insulation on the underside of the floor joists and fill the cavities with batt insulation, however I am concerned with condensation settling on top of the rigid insulation?
One detail shows rigid foam at the underside of the floor and vertical at the rim joist (with sealed edges) and also at the underside of the Joists to mitigate thermal bridging (some showing cavity filled
with batt insulation as well)- would this not create a potential for moisture to be trapped in the cavity space with rigid insulation on all sides?
Would filling the cavity with batt insulation and only using rigid on the underside of the joists, followed by the sheathing, create potential for moisture to settle on top of the rigid and soak into the batts?
Being an existing condition and older house, my concern is with air infiltration into any cavity space, and trapped moisture – as my strong preference for any method to also include the rigid insulation on underside of joists. In this existing condition, is it best to just use rigid or spray on the underside of the flooring and at rim joist and just “live” with the thermal bridging at the floor joists?
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Replies
If I'm understanding your situation correctly, I would attach rigid foam to the underside and rim, and seal the edges with spray foam. Similar to the attached picture. In the event of an interior spill, it should still dry inward. I wouldn't worry about thermal bridging unless I was building a new ultra high performance house. Most houses have bridging at every wall stud. I would avoid air gaps between layers of foam as that could block drying the cavity as well as batt which tends to hold water and critters.
Floors are not roofs or walls. In heating climate, stack pressure is typically pulling air through an overhang into the house, so the cavity tends to be very dry. There is no issues with rigid on the outside.
I would keep it simple. As much fluffy as you can fit in the joist space (if you have access from the basement, dense pack cellulose is even better) and layer rigid under with seams taped. Also tape to the sheathing and to the foundation for air barrier continuity. Cover with with plywood as critter barrier before putting back the metal panels. For a small area like this, you can also use a couple of sheets of Zip R instead of foam+plywood.
Air barrier continuity is the most important part, so make sure you seal properly to your wall and foundation. Fentrim or liquid flash is your best option for this.