Are you all willing to help out an architect? I am designing an addition with a 400 sf basement, partially below grade (in a hill). It will have a slab on grade over sand and vapor barrier, and walls will be carefully waterproofed, so I am hopeful we will avoid major moisture issues. The clients want a hardwood floor, and I am concerned about whether I can detail it so as not to have it haunt me in a few years. What is the best way to install wood / engineered wood to avoid problems in this space? I was going to go with wood over plywood underlayment over polyethylene over 2x4 treated sleepers set in mastic on the slab. Will that do it, because I am still worried.

Your help will be greatly appreciated.
Lee

I'd test it just to see where the baseline moisture emissions are.

Then I'd make sure the concrete is flat, as below grade or sunken living rooms are never flat!! ( 3/16" / 10' )

Then www.WFTaylor.com and use the Taylor 2012 lock-Down sealer and the 2071 adhesive, with Meta-tec.

Your not liable for floods, are you?? Is it in your contract to seal out all ground moisture issues?

I hope you can meet their expectations..

I would NOT use a solid wood floor. They are not for below grade, even partially below grade. IMO, the BEST system for your situation is a quality, engineered floating floor like Kahrs. After getting the substrate flat per manufacterer's specs, a 6 mil polyethylene vapor barrier is installed over the entire wood floor area and sealed at the seams with duct tape. It is rolled up the walls about four inches and the excess is trimmed off after floor installation. The beauty of this installation system is that the floor is completely isolated from the slab and one does not need to concern themselves about moisture emmisions, generally. It is also inexpensive to install and does not require expensive sealers and adhesives. There are many brands and styles to choose from. If I were bidding this job, this would be the flooring type I would promote.

Gary, you would be surprised how moisture got on top of the plastic!!!

Think dew point, and a glass full of water, and the plastic is the "glass"

Condesation can happen!!!

I'd float or direct glue an engineered. I'd still seal the slab with lockdown.