Cold Climate
ICF basement
5" slab with 4" of insulation board under hte slab

Would it be a good idea to put poly UNDER the foam to prevent the concrete from wicking moisture from the ground if it made its way up beyond the foam insulation or am i going overkill?

I've been looking at slab construction and what I've seen is poly under the slab. Sheets overlap and sealed together with silicon caulk.

Most now do argue for there being a poly sheet right under the slab, with the XPS foam under that. Previously some folks would have argued in favor of a sand layer under the concrete, to absorb excess water from the concrete when it is poured. However, those who understand the moisture issues say definitely no sand layer next to the concrete, as it becomes a water reservoir that would take ages to dry upward through the slab.

You might find some differences of opinion on whether the poly or foam should be directly in contact with the concrete. Proponents of foam over the poly argue that it protects the poly from punctures from the slab pour, while proponents of poly over the foam argue that it needs to be tight to the slab so as to be most effective against moisture. XPS is a fairly low-perm vapor retarder itself. My own feeling is that the poly should be something tougher than ordinary 6-mil, but it should be over the foam.

For what its worth, I would but the poly over the insulation and directly under the concrete. I always install several inches of clean wash rock, with drainage, under the insulation. This diverts ground water away from the insulation and slab, helping keep the insulation dry. My main reason for this arrangement is that it keeps the moisture from draining out of the concrete when it is poured. Keeping the moisture in the concrete slows the curing process and incrreases the strength of the concrete. Concrete finishers don't always like this, because it requires them to be on site longer, during the finishing process. However, the added strength and stability is worth a little of their time.

We show plastic on top of the insulation in our construction detail library. Moist people recommend against sand on top of the plasrtic, as Dick points out, but some engineers in extremely dry climates, like Las Vegas, do it because it helps the slab to cure better. Just about everywhere else, this can be a big mistake.

Isnt 4 inches of foam a little bit of a over kill? I mean cost wise do you think you can see a payback in energy savings for the extra foam?

Here is some POLY for thought. Always put as much Insulation under the slab that you can afford.

I've also wondered about the optimum amount of foam to put under the slab. It's a matter of both comfort (warmer slab) and heat loss. So I try to relate the heat loss to the ground underneath to loss out a well-insulated wall to cold air. Consider a cold climate, with, say, 70 degrees inside, 0 (F) outside air, and 48 in the ground. If you figure you'd like to have a superinsulated level R40 wall, then a comparable heat loss per sq.ft would require R= 40 * (70-48)/(70-0) = 12.6, which would round up to 3 inches of XPS (R5/inch).

Arguments for more: the heat loss to the outside air is peak for limited periods during the winter, whereas the loss through the slab is fairly constant through the whole heating season. Perhaps the ratio to the loss through the wall should be based on the heating degree days for the location and the equivalent (very even) heating degree days for the slab-ground loss. That alone would tend to push the proper thickness up toward 4". Further, if the slab has radiant heat in it, then the slab temperature may be another 8-10 degrees warmer.That would up the required insulation thickness by 40% or more, so that 6" beneath a radiant heated slab arguably is justified.

Argument for less: without the sub-slab ground being exposed to winter air, it might be argued that at least parts of the soil beneath the slab would equilibrate to a higher temperature than the deep ground annually averaged temperature. How much this effect would be should depend on depth of the slab below grade and the thickness of the sub-slab insulation. This thought of ground temperature elevation was expressed by someone else on another forum recently. I'd like to know of any good modeling or actual measurements that quantify this temperature elevation.

So it seems the consensus is that you have the GROUND then FOAM then POLY then SLAB

Yes. I don't think that it can be stressed enough: If you want to try and attain the maximum strength of your concrete, you don't want anything to reduce the water within the mix. Poly is a good way to prevent that. When pouring directly on grade, I wet it down so that it won't suck as much water from the concrete. And, of course if you are really into strength, you'll use some method for retaining the moisture on the top of the slab. Poly works for that also.

You spray the Poly with water just before pouring the slab ? Did I read that correct?

No. If we're not using Poly we spray the subgrade prior to the pour. With Poly there is no need, as the poly will prevent the 'soil' from leaching any water out of the concrete.

Ok thanks, im going to go this route

The way I'm doing it is undisturbed GROUND, then ~6" CLEAN GRAVEL (1-1/4" river rock) with drainage to daylight,  then ~2" 2# density EPS FOAM, then ~10 mil POLY vapor barrier, then REINFORCING STEEL (6"X6" #6 flat panel wire plus 1/2" GR 60 rebar around the perimeter), then SLAB.

Arkie6,

I am just curious, but why are you using the 6" x 6" flat panel wire?  Is this just regular welded wire?  Or is it Welded Wire Reinforcing like Varigrid structural mesh that is deformed and available in various diameters and has a ksi of 80?
 
In other words, are you using welded wire for strength or for a non-structural anchor for a radiant tube system?

I don't know all of the particular specs regarding this wire, but it is a 6"x6" welded #6 deformed wire and comes in 8' x 20' flat sections.  I'm getting it from the same place I get my GR 60 rebar.  I'm using it for strength/crack control in my slab.  I am not installing a radiant tube heating system.

Arkie6,

Do you plan to use chairs to keep the structural wire at the proper height in the concrete slab?

I have never used this type of wire but I am seriously considering it.  Please let us know about any problems in using this type of wire.

Yes, I plan to use lots of plastic chairs to hold the wire ~1" above the bottom of the slab.

Most slabs in my area are done with 6x6x6 wire mesh for added strength i did this and its great for zip tieing your tubes to as well

The designator for the mesh that I am using is 6 x 6 x 6/6 (W2.9 x W2.9). This is #6 deformed wire welded on a 6" x 6" grid. I am getting it at CMC Rebar in N. Little Rock, AR.

Note that most residential slabs around here use the 5'x150' rolls of 6 x 6 x 10/10 (#10) welded wire mesh which is about half the weight of the #6 wire. It is also a pain to unroll it and get it to lay flat.