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Community and Q&A

CMU / Slab Hybrid Retrofit

joninva | Posted in Energy Efficiency and Durability on

Hi, I’m rehabbing a CMU/slab on grade small house in eastern (not coastal) NC, climate zone 3.
My tentative plan is foil faced poly both on the slab and on the walls.  The slab foam will get 2 layers of glued and screwed OSB or ply as a floating floor, and the walls will get 2″ of poly with 1x furring screwed into the block.

Two main questions;

1) Can I drylok/paint the exterior block without creating moisture trapping issues between that and the interior poly?  If so, do the outside with what, vapor permeable mineral paint?

2) is the foil backed poly sufficient on top of the slab (which is very unlikely to have plastic under it already), or should I put down a moisture barrier on the concrete first?

There’s also the question of best practices for running electrical behind/in/whatever the walls, but that’s a bit later ..

Thank you in advance!


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  1. Expert Member
    PETER Engle | | #1

    I would not use polyiso insulation on the slab. It is somewhat sensitive to moisture and there are just too many ways for it to get wet. EPS/GPS foam would be a better and cheaper choice. If possible, I would also put EPS/GPS on the exterior walls, though polyiso would work fine there and give the highest R-value for the thickness. Putting the polyiso on the exterior rather than interior would bring the mass of the wall inside the thermal envelope and help to even out temperature swings a bit. It would also keep the masonry closer to interior temperature and moisture. You can skip the paint on the CMU, but of course you would have to add cladding. Inside, simple furring on the walls can give you a cavity for utilities.

    1. joninva | | #2

      Great advice, and I would really like to do the exterior, but I have two blockers - the additional expense of cladding, and the fact that I can't really think through a good, economical way to address the thermal bridge through the foundation and up the concrete wall. It seems less expensive and more effective to try and get the envelope inside the block. Especially in a climate where it's usually above or below interior temperature both night and day, so not as much benefit from a thermal mass wall.

      Several sources of factory second or reclaimed poly around here, so that's the clear cost winner, but I was afraid using it on the floor was pushing it. If I go EPS I'll probably have to drop to 1", which is still more than code (none) and better than nothing.


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