(Sorry, this is a repost–I originally posted in wrong forum.)

 

Hi,  My first post here–did some digging, but didn’t find *exactly* what I was looking for in terms of options for a basement remodel.  Please point me to existing info, if applicable, otherwise I look forward to hearing some discussion about our situation, which is:

– 1950’s era house in Seattle with walkout basement, no moisture barrier under slab

– we installed interior french drain to deal with major moisture issues, and all good (no surface water) in last few years

– we’d like to install insulation and finish flooring over slab, but have following restrictions:

– – limited ceiling height, so want to limit floor height to 1.5″ if possible

– – Calcium chloride moisture test reads 3 to 4 pounds per 24 hours

 

I have read some great answers to questions about installations of XPS (extruded polystyrene) with plywood subfloors, but these installations are contingent on “no significant moisture issues.”  My question is, can we quantify significant moisture issues?  Most manufacturers seem to say their products are okay with <3 pounds of water vapor per 24 hours.

 

Our plan is to layer, on slab:

1)  0.5″ XPS (we’d love thicker, but limited by ceiling height)

2)  19/32″ plywood sheathing (seems more breathable and mold resistant than OSB?)

3)  some engineered 13/32″ cork product

 

We’ve read the installation requirements for some flooring and found most of them say <3 pounds of water vapor per 24 hours.  I’m hoping our XPS will reduce moisture vapor emission from slab to <3 pounds per 24 hours so we fulfill the installation requirements of the cork flooring.  Our concerns are:

a)  potential moisture-related issues, such as mold, plywood, or top layer buckling? 

      – is fastening plywood to concrete advisable (we are thinking powder actuacted nails), or can we float and adhere them to each other with biscuits?  floating sounds preferable to us, and Building Science article seems to indicate that floating is possible, although our layering may be less substantial (due to ceiling height).

b)  is a 19/32″ subfloor on XPS sturdy enough?  No pool tables or anything, although washer and dryer, etc…

c)  is 0.5″ XPS thick enough to make it comfortable and reduce moisture vapor transmission?

 

Thanks in advance for any help on this!

-noah