Don’t do a slab. Do a crawl space with proper vapor control. You’re going to have to dig down under the frost line anyway. I wouldn’t think a slab would be very viable with that issue.

Use pressure treat instead of regular dimensional lumber. Apply two coats of polyurethane/epoxy roll on moisture barrier. The advantech would need to be sanded with a 40 grit prior to applying the moisture barrier.

The majority of our new construction is over conditioned crawl spaces. That said, we have had good experience installing rigid foam insulation, treated wood sleepers, and subfloor over concrete slabs for remodels. We also have had good experience installing glue down engineered hardwood floors over basement slabs using waterproof adhesive.

There are a couple of different insulated subfloor panels on the market which integrate the insulation and subfloor; I don't have any experience using them. Perhaps others will chime in. Here's a link to one such product:

http://barricadesubfloor.com/barricade-subfloor.html

If you're going to use pt lumber, consider that it is usually stored wet. Either you'll have to buy a dry batch or purchase ahead and stack it so it can dry suitably before use. I believe pt also requires the use of stainless steel or heavy galvanized fasteners.

You can use stainless or non-corrosive fasteners, combined with Advantech's polyurethane foam adhesive. The adhesive is designed for low temp and wet install. So, you can skip the drying process for the PT.

"you can skip the drying process for the PT"

Couldn't that end up trapping moisture under the wood floor?

Moisture escapes along the perimeter, where the expansion gaps are located . Most likely behind walls, if, walls are built on top. Either way, it’s deflected towards the wall.

Thanks for all the replies.

I spoke to the builder today and he has a different approach: use Rockwool (Roxul). He gave three reasons:

1. If you go with EPS then you need to do the walls also. It’s all or nothing. Not sure why.

2. It’s much cheaper than EPS.

3. It allows some airflow. Because you can’t guarantee that it will be airtight the best thing is to let a little airflow happen to allow any moisture to escape. Seems old school but I think he has a point.

Mackdolan: No room for a crawlspace. First floor is only 8 or so inches above grade

G & S Floor Service: I mentioned PT but I need to clarify this is what he will use. He is orienting the 2x4’s edge wise so the insulation will be 3.5 inches.

You don't want an air space under the floor. You want rigid insulation that is a moisture barrier. Roxul doesn't meet that criterion. What you choose to use for insulation in the wall cavities is independent of the choice of rigid foam under the floor.

Charles Ross Homes: If I have a vapor barrier below the concrete then why do I need another vapor barrier on top? Wouldn't that trap any moisture that does get into the concrete?

Good to know the walls and floors are independent. That makes sense.

Water is an ingredient in concrete which is liberated over a long period of time as concrete cures and concrete can absorb moisture from other sources as well. Unless the polyethylene sheeting is placed at the top surface of the slab, the migration of any moisture between it and the slab surface is unretarded (note that it's a vapor retarder not a barrier.) Second, a hardwood floor over sleepers with cavities between is not an air-tight assembly. Air (which contains some level of moisture) will get into voids. If the concrete is at the dewpoint, you can expect condensation which may give rise to mold or mildew.

I suspect your builder has no little or no experience installing hardwood flooring over concrete slabs and he's inventing his own technique to reduce the cost. You're the one who has to live with the result. Get a guarantee from the builder or hardwood installer in writing.

Your builder is wack. He’s creating a failure right off the bat. Get someone who knows what they are doing.

To add to Charles Ross Homes said, the plastic sheeting underneath the slab can - and will (eventually) - rupture at some time in the future. A well educated builder - with plenty of education in the moisture mitigation area - will recognise this and take extra steps against this concept.

In the building sciences we assume that ALL vapour barriers under slabs are compromised the day we show up on site. That's the BEST approach to take when working with flooring. With that assumption the contractor will take ALL NEEDED PRECAUTIONS (which are paid for by the client...of course) to mitigate AGAINST moisture disasters.

Any floor installed over a concrete slab that is at or below grade (on the surface or below the surface of the yard) NEEDS a vapour barrier. Any floor. Even ceramic tiles have issues with a damp slab (porcelain has much less...but a bad slab can still cause problems with porcelain).

So...try to find someone who understands slabs, flooring AND wood. That's your best approach. It will cost more, but you will get what you pay for.

XPS and poly under the slab will slow/stop the moisture drive upwards. The finish floor you choose will determine any further above-slab treatments. In my little part of the world sleepers haven't been "traditional" since the 1970s.

BTW, below slab 2" of XPS and 6 mil poly is what is speced on our latest build, with tiles above. Schulter tile underlayment will be speced to avoid cracking.

After not guaranteeing such use, Roxul now promotes its insulation under slab. I think the jury is still out on that one. (Here's the problems one Passiv house builder encountered.)

See Building Science Corp. on basement insulation.