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Remodel…random questions

Brandon S | Posted in General Questions on

We’re working on a remodel and I have a handful questions:

1.  The house has a florida room on slab that is 15″ below the main level floor.  We are finishing the room as living space and framing the floor over slab to be flush with the main level.  If I lay a vapor barrier over the slab can I throw loose fill cellulose over the slab and in between joists to insulate the new floor?

2.)  The exterior walls are 2×4 with brick.  We are adding 2″ of faced rigid foam to the interior with taped seams as air barrier/thermal break.  Any recommendations for a tape or caulk to seal the rigid foam to the subfloor?

3.)  We gutted the basement and are considering adding rigid foam over the slab.  Do we frame partition walls before or after the foam over slab?  

4.)  We’re building a detached garage on a frost protected mono pour slab.  Should the slab be sloped or is flat okay?

Thank you,

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  1. Expert Member
    Dana Dorsett | | #1

    The slab vapor barrier needs to extend all the way above grade, behind the wall foam and sealed to the concrete or brick with polyurethane caulk. The wall foam can be sealed to the vapor barrier with foam board construction adhesive, or polyurethane caulk if the foam has a heavy facer. Extend the wall foam all the way down to the slab level or an inch above.

    If there is any chance at all of the 15" subgrade level taking on water, use only polystyrene (EPS) not polyiso, since polyiso will wick and store moisture if the bottom edge is immersed for any extended length of time. It's fine to stop the polyiso at grade and use polystyrene below grade, if that's a concern.

    Don't fill in the space below grade with cellulose- even a minor leak would become a mess. Install batts between the joists instead. If it ever gets wet under the joists any dampeness in the rock wool or fiberglass batts can dry relatively quickly with a bit of ventilation and heat. Cellulose would need to be removed.

    For the basement, install any wall foam down to an inch above slab level, and install a continuous layer of floor foam under a subfloor. If there is going to be wall foam, butt the floor foam up to the wall foam. If no wall foam, leave a 1" gap between both the floor foam & subfloor to the foundation wall as a drain space for any seepage. The partition walls and exterior finish walls get framed atop the subfloor. If using 3/4" t & g subflooring it needs to be TapConned to the slab on an 18" or tighter spacing, otherwise seasonal humidity changes or seepage under the foam will cause "potato chip" curl or waves in the floor. Is using double-layered half inch CDX with the seams staggered by a foot or more and the layers glued & screwed to each other it can float on the foam layer, no floor fasteners needed.

  2. Andy S | | #2

    4.) We’re building a detached garage on a frost protected mono pour slab. Should the slab be sloped or is flat okay?

    Sloped to the garage door is so that when you pull a wet car in there the water will drain towards the door (and eventually out) instead of just pooling in puddles on the floor. If it's a shop and not going to have any vehicles, then flat is ok.
    But of course, all of this is dependent on the code where you build. They might require a sloped floor if there is a full size garage door even if you never bring a car in there.

  3. Brandon S | | #3

    It may not change the recommendations, but the florida room is about 6" above grade level now and is being raised with a framed floor to 21", which matches the main level floor height above grade.

    Regarding the basement, there is an existing load bearing wall in place. Are there any issues with butting the foam up to the existing sill plates?


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