Hi,
I’m closing in a screened porch. It’s got an above-grade slab floor sitting on footers and blocks. The slab is 3 1/2″ below the floor of the house. I’d like to maximize the insulation between the finished floor and the slab. So, two questions:
– if I lay down 2″ thick rigid foam insulation on the slab, is that a sufficient vapor barrier, or do I need an additional barrier?
– can I then lay 3/4″ subfloor directly on the rigid foam, with a 3/4″ finish floor over that?
Thanks,
Glenn
Replies
First, welcome to Breaktime. Lots of good info to be had here, occasionally some not as good - you decide which is which...
To be picky, you don't need a vapour barrier under a floor - those are to found in walls and ceilings, preventing water vapour created by room occupants from condensing as it meets cold exterior surfaces. A moisture barrier is placed under a slab, to prevent moisture from the subgrade material wicking into the slab and causing moisture problems inside the building. IMHO regular extruded foam insulation is too porous to act as a moisture barrier. You will need a layer of poly between it and slab.
I'd be uncomfortable with just rigid insulation and nothing to anchor to. Why not lay some pressure treated 2x2 on the flat at 16" centres fastened with concrete anchors, then 1-1/2" foam between those?
Wally
Wally,
Thanks. My only reason for considering not putting down the 2X2s is to maximize the amount of insulation between the finished floor and the slab. But I understand your reason for wanting something to anchor the floor to. I guess I was thinking of the "floating floor" model that so many of the newer finished floors used.
Glenn
Glenn,You can do the floor over the 2" foam without sleepers if you use two layers of 1/2" ply, joints offset. I did this for an acoustic subfloor and used runny titebond and billions of narrow crown staples. One big panel when you are done, and plenty of meat to catch the flooring nails/staples, too.If there was any doubt about the slab's dryness, I'd put down a vapor barrier just to be sure. It is terribly hard to retrofit later on.bill
2" of foam + 2 layers of 1/2" foam and then a 3/4" finished floor puts him 1/4" above the finish floor in the house. Not bad, but not a smooth transition either.
Glen,
Rip the 2x4s to 2" and tapcon them to the concrete, then put the 3/4" sub floor down, followed with the 3/4" finish floor. You will be happier with the more rigid feel of the finished job.
Dave