Foam + vapor barrier under slab?
egt
| Posted in General Questions on
Folks,
We’ve got a 4″ concrete slab with 15 mil stego directly underneath, then 2″ foam spec’ed for an upcoming addition project. My concrete guy wants to skip the stego, opting for taped foam only as the vapor barrier. Is this a bad idea?
TIA.
ET
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Replies
Hi ET,
You should not skip the poly sheeting beneath your slab. Though you didn't say what type of foam insulation you will be using beneath the slab, most allow some vapor diffusion. Poly is a class I vapor retarder and will stop more vapor diffusion than anything else you might put beneath your slab. You want to do everything you can to keep the slab from getting wet because that moisture is only going to move in one direction, up and into the building.
15 mil stego wrap is pretty thick. 10 mil is more common. it should be taped at seams and penetrations. edge detailing is finicky but something that warrants extra attention.
You may find this helpful: https://www.stegoindustries.com/blog/where-to-place-under-slab-insulation-above-or-below-the-vapor-barrier?hs_amp=true
stamant brings up good points about finicky detailing. But it's good to pay special attention to that. And that is one more reason to use the stego wrap. Imagine the difficulty of taping all those insulation seams to each other and to all the surrounding surfaces. Not only is the foam insulation unlikely to be a full vapor barrier, your builder's apparent attempt to save money by leaving out a step sounds like a false economy because he'll end up doing more work taping all those board seams.
Any 10 mil sheet will do the trick. There are likely cheaper options that Stego.
Do you have termites in your region? Keep that in mind with respect to any penetrations of the wrap and the slab.
You don't have to detail an under slab vapor barrier with as much attention to detail as you do with an air barrier. Simple overlapped seams between sheets is enough if it will stay in position during the pour. I like to tape just to make sure everything stays put, but allow a foot or so of overlap and then not a lot of worry about the tape job being perfect. Vapor barriers are not as a critical as air barriers: a small hole or tear here and there isn't nearly as big of a problem. I do still try to keep things as sealed up and damage free as possible though.
I wouldn't bother with Stego. Just use 10 mil polyethylene and you'll be fine. I would put the poly under the slab here if your insulation is going on top of the slab.
Bill