Insulating under a raised floor
We are planning to build a timber-framed raised floor in one section of our house. Currently this section is 1-2 steps (7-14″) below the main level of the house, and is slab on grade. Long ago this was a garage.
We live in central NY, climate zone 6.
The 1970-s vintage slab is totally uninsulated to the best of our knowledge. We will add a plastic vapor barrier above it before placing the raised floor framing.
The question is about insulating the space under the new floor level. We have access to inexpensive recycled polyiso, so one approach is to cobble that in between the new joists, and/or potentially also underneath the joists in the 14″ raised section.
Another alternative could be to just fill the whole thing with blown cellulose, or even fiberglass. We could literally dump the cellulose in place without needing a blowing machine.
Looking for input on consequences (other than cost) which I might not be thinking about. Particularly moisture issues, if any.
Thanks!
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Replies
David,
I think that the best approach is to install multiple layers of rigid foam on the existing slab (or the polyethylene layer), so that the top of the rigid foam is about 4 inches shy of your desired foundation level. Then pour a new 4-inch slab on top of the foam.
Remember to insulate the perimeter of the new slab with vertical rigid foam.
Either EPS or XPS is preferred over polyiso for sub-slab insulation; green builders would choose EPS. That said, considering the fact that everything is above grade, polyiso might work here.
-- Martin Holladay
Hi Martin, thanks for your suggestion. Would you mind elaborating on why you recommended the insulation + topping slab vs. a wood framing approach? One concern I had had about the framing was if it were to create a dead air space where moisture or mold could potentially accumulate (despite best efforts to seal it off from moisture). Not sure if that was part of your reasoning however.
David,
There are several concerns with an old slab on grade like the one you describe, but the chief concern is moisture. That's why I would never recommend the use of cellulose between any joists installed on top of an old slab.
The second issue is thermal bridging through the joists, which limits the effectiveness of any insulation installed between the joists.
-- Martin Holladay