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Insulation Help Required

JeffBush's picture

How do I properly insulate between floor joists of a cantilevered floor and tie it into the basement wall without trapping moisture in the air space between the concrete and the basement walls?

Please see attached sketch.


thanks in advance



jeff (post #207424, reply #1 of 4)

One way-

Install rigid styrofoam (dow) against foundation wall all the way from floor to top of foundation.  Foam in place, use styrofoam glue-you shouldn't need fasteners.

This will keep "cold condensation" from forming on the foundation wall and to some extent, keep moisture from migrating


Moisture sealing should really be done outside on the exterior of the foundation and footing.

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Cantilever (post #207424, reply #2 of 4)

Cantilevers should be spray foamed as you have it drawn, bottom and rim esspecialy.  The rest can be dense packe with cellulose or BIBS.  Keep mechanicals out of the the cantilever if you can, and as Calvin pointed out insulate the foundation wall inside, behind the stud wall and the insulate the stud wall as you would the rest of the house.  The spray foam serves as an excellent air barrier in that space. It should be brought ou fa enough to completely cover the foundation plate and the top plate of the inerior stud wall.

One slight add (post #207424, reply #3 of 4)

I would use 2 layers of rigid foam (probably XPS) with staggered, taped seams, directly against the concrete, full height of the concrete. Use furring strips attached through the foam to hang the drywall. I would not use fiber insulation in framed wall cavities--get all your insulating done with the foam.

Question to all responders (post #207424, reply #4 of 4)

Why would you not completely fill the area between joists in the cantilever and then completely seal off teh cantilever with soldi blocking or solid foam at the sill plate and foam that in place?  Just seal it off and avoid any air leak into that spcae.  If there are any air leaks into the cnatilever, which could eaily be the case you are covered.