Hoo boy, someone needs to calm me down. I'm having a passive solar slab-on-grade home built, with the slab over 5" (R20) of Plastispan. I was away the day before the pour when I thought the 6-mil poly was going on top of the foam. To my surprise when I came to observe the pour, there was no poly layer. The general contractor says that it isn't needed in this situation, given that we have (a) super-permeable sand/gravel 'soil', (b) the foam as a partial vapour barrier (it isn't taped), and (c) horizontal sheets of foam running 4' outwards from the house perimeter and a foot below grade, sloped away from the house for drainage. The pour went ahead.

So my big question is, is he right or am I in big doo-doo?

The soil is indeed super permeable. From the well record, we have 40' of sand then 60' of gravel before reaching bedrock. We're 100' from a tiny lake, 40' above it. The winters are cold, central Ontario kind of like Minneapolis. I've just been surfing various sites on this, and some say it may not be needed in such situations but it would have been worth the low cost to put it down just to avoid risks of diffused water vapour rising. Any thoughts? I doubt there's anything I can do post-pour except accept that I may have to run the HRV's dehumidifier more often (?).

On the side, this is a big lesson in communications. I originally had 6-mil cross-laminated poly in the plans, and when doing a round of cost-cutting the general contractor told me he could save $700 by not using this unnecessarily-strong plastic. I said go ahead, assuming he meant replacing it with basic unlaminated 6-mil poly -- but he meant removing it. My fault for not noticing it wasn't on the modified plans. To make things worse, I didn't insist on taping the foam sheets since I thought there would be the 6-mil above them. At least the two layers of 2.5" foam are offset so there are no clear air channels from the stone below to the gravel above.

Another concern is radon, but I have a radon stack from the stone layer below the foam, and there are no records of radon being found in the region.

So, can I calm down or should I panic over this?
Thanks,
...Terry

I think you can calm down.  Just be sure to verify everything from here on with this contractor.  He may be the most honest contractor in the world but at this stage we do not know.  A failure to communicate effectively can result in this type of un-intended change.  By the way, if you do still have concerns about having a damp slab, then ask the contractor for the cost of applying epoxy to the concrete floor.

Thanks, Alton. They did spray a thin coat of CPD Cure and Seal Solvent on the slab after finishing, as it was a hot sunny day and they said that would slow down the cure. I imagine that will also reduce moisture passing upwards.
...Terry

Very very little water vapor can get through 5" of foam. I think you were correct to save the money.

If the slab gets moist, it is because of condensation from the air.

Posted By terrynew on 07 Jul 2010 12:25 PM
Thanks, Alton. They did spray a thin coat of CPD Cure and Seal Solvent on the slab after finishing, as it was a hot sunny day and they said that would slow down the cure. I imagine that will also reduce moisture passing upwards.
...Terry

Terry;

your cure and seal will probably flake off as it ages, it is not intended as a permanent moisture barrier. While it may be an honest mistaek. I would be concerned with moisture comimg thru insulation joints. For what little  poly costs, it would have been prudent to install it and never have a question.

Why is it that concrete guys are always trying to remove the plastic?

I checked and 5" of foam is that same as 2 mil plastic in terms of vapor transmission. Given your soil, I can't see you ever having liquid water to worry about. Without that, gaps only matter if you have some positive pressure differential to cause upward air flow - and I can't see that happening if you have a radon system creating a negative pressure.

2 mil is too thin and the real concern is the foam joints that would open up while the concrete is placed. I am curious as to what the approved building permit plans call for and why an inspector would not question this before the pour

5" foam permeability = 0.151, recommendation is .3 or lower. I agree on the potential of cracks if you didn't have the radon system.

Posted By jonr on 10 Jul 2010 09:05 AM
5" foam permeability = 0.151, recommendation is .3 or lower. I agree on the potential of cracks if you didn't have the radon system.

jonr;

thast fine,  poly is lapped and taped to prevent moisture penetrations , the foam joints at every 4 ft will allow a path for penetration. It can trully cause havoc with some floor finishes

You're right, Chris, the approved plans had the 6-mil. This is a small rural town. I think the contractor verbally notified the inspector of the change, and the inspector said it was fine since he has high confidence in this contractor.

I'm just glad I have the radon system, high-perc sand-gravel soil, and the 4' umbrella insulation around the perimeter. Again, my big lesson is ensure good communication (and check everything!).

Thanks, everyone, for your input.
...Terry

Terry;

High confidence is no substitute for sticking to the plans and codes, shame on the inspectors attitude you will pay the price later for the six mil you paid for and did not get.

If the job starts this way, it usually ends this way with things being ommited

It depends on a lot of factors whether this will be an issue or not. I like to see 6mil poly under the foam.As long as it is wetter under the slab vapor pressure will constantly drive moisture thru the plastispan. If I was installing a moisture sensitive product I would use a spray on moisture barrier and I probably would still do it even if I it was not....Eric