Expert: Daniel Humphrey - 12/24/2007

Question
Hi Dan,


I have a cement slab that covered by a roof. I wish to close it in to make it part of the existing bedroom. The cement floor of the patio is about 5" lower than the bedroom floor so I need to raise it.
My question is, do I cover the cement floor with some type of vapor barrier before laying down the floor joists onto the cement.
The joists will be anchored at both ends but the wood will be in contact with the cement(or vapor barrier) in some areas since the cement floor is a slight slope to it.
Also, is it wise to use insulation between the floor joists to keep the floor a little warmer?
I plan to cover the floor joists with plywood and carpet over the that.

Any help is appreciated.

Budd

Answer
Bud,

Yes, the concrete should have a vapor retarder over it, especially if the ground in your area is wet.  Remember, you will also have moisture migrating from inside the new room into that space, and it has to get out  somehow.

So probably most important is that the space beneath the floor is vented. In five inches, this could be difficult, especially if you insulate it. I've created screened venting through the rim joist into the air gap between the wall sheathing and siding. It's not much, but it's all you have.

I try to isolate the wood joists from direct contact with the concrete. Rubber pads (3" square) made from conveyor belt material works well, placed every few feet to keep at least a half-inch gap, or whatever you need to level up.

Avoid batt insulation here. I would use a foil reflective layer for the radiant porton of the heat loss, at a minimum.  A thin sheet of high-density foam with a foil backing might be another good choice inserted between the joists and decking. Don't try an infill--just place your high-density sheet goods over the joists and under the plywood. Make sure your plywood is T&G.

Dan