A vapor retarder does not prevent leaks; it prevents moisture from rising through the concrete slab into habitable space. A course layer of crushed stone also helps prevent water rising by capillary action. Avoiding fine sand directly in contact with the slab also helps if there will be flooring glued directly to the slab.

If it is polyethylene it should be cross-laminated and have a perm rating of less than .3. A common thickness is 10 mils. A mil is 1/1000 inch. Perhaps the architect specified .20 mm which would be about 8 mils.

I'm pretty sure the IRC says 6 mil VB on top of 4" gravel for any slab below grade for a habitable space. Call the architect and have him clarify what he wants.

The code also calls for a minimum 3.5" thick concrete slab so it often represents the threshold of bad rather than good design.

Sounds like a confusion of "Mils" vs. "Millimeters".

20 mil plastic sheets are used under slabs where there is radon gas.

Here is a link that might be useful: 20 mil vapor retarder

That is ABSOLUTELY why I love you guys, and you have been the only thing keeping me sane during the 2 years to plan this house! I'm used to dealing with mm at work, and looking back at the emails, I see that they are referring to 'mil'.
So it's apparently gravel, then this 10 or 20 mil vapor barrier, then concrete. It sounds like the radon gas is the only reason to use 20 mil over 10 or 15 mil? I still don't understand why 20 mil would be more brittle and prone to tearing? Isn't it still thicker, even though I got the units mixed up?

As an aside, does anyone know of a concrete additive that expands with moisture, then contracts when its dry, thereby helping to waterproof the basement? One contractor told me about it, but we didn't go with him, and now I can't find this stuff with a google search. THANKS!

WR Grace makes a waterproofing membrane (Prefrufe) that is laid under a concrete slab before the pour. It actually adheres to the slab. There are other materials like Bentonite and other clay panels that expand when water contacts them.

Here is a link that might be useful: preprufe

As the material gets thicker it loses flexibility and the ability to 'give' (stretch) without cracking.

The thickness of the vapor retarder should be determined by the necessary permeability and how sharp the compacted base will be. Based on testing over different kinds of base materials, the ACI determined that a polyethylene vapor retarder should be at least 10 mils thick although the 20 mil sheet performed better than any of the sheets and had no perforations at all.

The idea that a 20 mil sheet would be less puncture resistant or more brittle than 10 mil appears to me to be a convenient contractor fabrication probably due to missing the required thickness in the bid take-off.

Your architect should be able to deal with this. Is there some reason that hasn't happened?

Macv, it's interesting that you bring up the product Preprufe. This is a pretty high-end waterproofing product. It was my initial understanding that Preprufe was primarily a commercial application, but the more I've spoken with waterproofing experts, I'm learning that it does get used in high end residential projects. It's very very expensive, but it is the ultimate in below slab waterproofing. If you wanted the absolute best waterproofing, you would use Preprufe under the footings and under the slab and have the preprufe lap around the edges of the footings, then you would overlap the outside walls and outside edges of the footings with bituthene. The bituthene would overlap the preprufe at the footing and the entire foundation would be waterproofed. This is the best waterproofing money can buy, but you'll pay for it, as I believe that Preprufe is about $1000/roll and each roll covers about $300 sq. feet. I am in the initial planning stages of a build in Mass. and if money allows, I'll be doing my basement waterproofing in this method. Of course, on top of the bituthene you'll want 2" of XPS (R-10) and an exterior french drain and you'll never have basement flooding issues.
Macv, do you see much Preprufe used in your work on residences?

When a contractor wishes to substitute a different product than was specified it is usually necessary for him to submit the specification sheet (and sometimes a sample) for the architect to review and approve. That information would show whether or not the materials have different strength characteristics so there shouldn't be much discussion about it unless you're doing this without a contract and there is no procedure for substitutions and changes by the contractor. In that case there would be little reason for a specification.

I've specified it in the basement of a university dormitory and a library where there was no other option and I've seen it used in subway stations. If a house has that much sub-surface hydrostatic pressure it would be wise to build elsewhere.

thanks everyone, and mac, you too! They're trading emails, and one of them cc'ed me, so of course I came here to ask about it, because it didn't make sense. I can see how 20mil would be less flexible, but shouldnt it be stronger, and not more brittle? I'm staying out of it for now, since noone has asked my opinion. I guess I'll let them hash it out...sounds like they may compromise at 15mm.

Are there other builds going on in your area where you could see what they are using? How familiar is everyone who is involved with your project concerning soils type?

Sierra, I'm guessing the SF bay area has similar soil throughout, and the builder has built on a hill before, although not exactly in our town. We also had a soils report, and the foundation plan was done by the engineer, passed through the soils engineer (big bucks there!), and revisions done again. Thanks!

Just to follow up, we went out there today, and the sub told us that they were using 15mil, and putting down 2 layers! So they're at 30mil (if the math for vapor barriers is like math for my 1st graders). The contractor later told me that they had also decided to change the grade from 2% to 5% for the drains under the baserock, and the soils engineer had commented that it was a good idea. Thanks to you all again! Hopefully noone here thinks that's a bad idea, 2 layers of vapor barrier?

It seem unnecessary unless the sub-grade is unusually rough but it can't hurt and apparently you already own it in your contract.

Thanks mac! As long as it doesn't hurt, then I'm fine with it. I was hoping that you wouldn't come back and say that moisture could get trapped between the two layers, and make it worse, or something like that. BTW, thanks so much - I've learned alot from you, since your MA days, so thanks again for your help over the last 3 years! I"m sure I'll have more questions now that we've started construction!