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I'm planning to install an engineered hardwood floor on a concrete slab that is very very cold. The climate is northern New Mexico. The slab perimeter is not insulated, and doing so is cost-prohibitive at this time as it requires removing more concrete that abuts it, some of which is structural. The slab's thermal mass is not contributing much if anything to moderating internal temperature conditions due to the house's siting being unable to take advantage of much direct solar irradiance. All it's doing is making the house cold.

So I'm thinking of applying polyiso foam insulation board over the top of the slab but below the new floor, basically cutting off the thermal mass of the slab from the interior climate. If I were to do this, where should the moisture barrier go? Between the bare slab and the insulation board? Between the insulation board and the wood floor, perhaps with a moisture-barrier-type underlayment? Would I even need a moisture barrier at all with this setup?

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I'd consider XPS, and I believe, if the seams are taped, that will act as your barrier (not technically a barrier as much as a retarder, but essentially serving the same purpose in this particular instance). I'm never a fan of true moisture barriers in basements. –  DA01 Feb 5 at 22:38
    
The slab is on grade. I'm leaning more towards polyiso due to the better R-value per inch because this will be a very height-constrained install. –  iLikeDirt Feb 5 at 22:43
    
while R-value is always nice, the floor isn't really a major source of heat loss (compared to walls, windows and ceilings). Underfoot, it's less about R-value and really just about creating a thermal break between your stocking feet and the slab. So if space is a concern, I don't think you need to overdue it regardless of the insulated board type. –  DA01 Feb 5 at 23:03

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I am curious what you consider to be "very very cold". Here in Maine, 45 to 50 degrees at slab level 4 feet below grade is common. So interested in what temps you may have and the average relative humidity in that space. With those questions asked; the straight answer to your question is yes, you should use a vapor barrier when installing any wood type floor over concrete. Both methods you mentioned would work. A one inch foam insulation, closed cell, waterproof, under the subfloor would be fine. If you use the tongue and grove style and fit it carefully, additional vapor barrier should not be needed. You can add a sheet of 6 mil plastic under the insulation for good measure if you feel the humidity is high in your area and are concerned with condensation. Another option is a paint on barrier such as Hydra-loc or any low pressure concrete coating used for interior waterproofing. Again, use this under your foamboard insulation. The one step method is the interlocking subfloor systems with a built in moisture barrier. this works well but is very pricey.

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Without a laser temperature gun or anything, I can't give you precise numbers, but I can tell you that right now when it's about 25 outside, the bare slab is painfully cold to walk on barefoot when the heat has been off. After the heat has been on for a while, it gets better probably because it's soaking up all the heat, but the heat bleeds outside to the concrete touching the slab perimeter that rings the house. When it snows after an overcast day (so the exterior concrete hasn't absorbed much if any solar heat), the snow melts as soon as it touches the concrete but accumulates on the ground. –  iLikeDirt Feb 5 at 22:11
    
The average humidity inside is about 15% right now and will probably never rise above 50%. This is a pretty arid place. As for the placement of the moisture barrier, what governs whether it should be above or below the insulation board? –  iLikeDirt Feb 5 at 22:11
    
you always want to avoid trapping any materials between the vapor barrier and concrete. Concrete is like a big sponge, and in damp locations can wick moisture even if it is 20 years old. In your situation as long as flooding is not an issue, you should be fine –  shirlock homes Feb 5 at 22:27
    
Fine without a moisture barrier, or fine with the moisture barrier between the concrete and the insulation? Also, the slab is 42 years old. –  iLikeDirt Feb 5 at 22:40
    
if you use foundation foam with T&G, and fit it well, you will not need extra vapor barrier. –  shirlock homes Feb 5 at 23:20

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