poly over basement concrete slab vs no poly inside basement walls — can someone explain the difference?
I have read a number of articles explaining why there should never be poly or anything vapor impermeable directly against the interior of basement walls — for example: “Basement wall systems should never include any polyethylene. You don’t want poly between the concrete and the insulation; nor do you want poly between gypsum drywall and the insulation. You don’t want poly anywhere.” from https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall.
I thought I understood the logic. But then I am having trouble understanding why poly *IS* appropriate on a slab in a basement. For example: “All basement floors need a vapor retarder….If you live in an older house with a basement slab that lacks any sub-slab polyethylene or sub-slab insulation, you can install polythylene above the slab, followed by a continuous layer of horizontal rigid foam and a plywood or OSB subfloor.” from https://www.greenbuildingadvisor.com/article/vapor-retarders-for-basements-and-crawlspaces
Would someone be willing to explain why the vapor retarder is prescribed on the slab but proscribed on the wall…? Not trying to nitpick, genuinely trying to understand…
Thanks in advance!
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The wall article is from 2012 and I think it's a little dated. There used to be a belief that basements had some ability to dry to the exterior, I don't think that's believed any more. To the extent a basement wall can dry, it's to the interior only, so it has to be vapor-open to the interior. At the same time it needs to be vapor-closed to the concrete, otherwise you're trying to dry all the moisture in the soil through your walls.
I'm not a big fan of anything permeable in basement walls, because there is so little drying potential. Moisture moves from warm to cold and from wet to dry. The interior of a basement is going to be warmer than the walls year-round. To the extent there is drying the interior air has to be a lot dryer than the air inside the wall to overcome the vapor drive from the temperature difference..
DCcontrarian,
Many thanks for this. I think I didn't follow all of this, though. Re walls, maybe I understand: Rigid foam is semi-vapor-permeable, so it will allow the concrete wall to dry to the interior (good thing) but air impermeable so it will prevent interior moist air from getting to the interior of the concrete (good thing). If that's what you meant, I think I understand.
But then I didn't understand what you said in your second para about air inside the wall, as what I was referring to is poured concrete basement walls. Also, I didn't follow wht the logic would be for vapor retarders against floors...
Would you be willing to explain again for a beginner..? :)
Allowing the concrete wall to dry to the interior is never a good thing.
Moisture doesn't hurt concrete, it makes it stronger. You want to keep moisture out of your basement. The soil behind the wall is an effectively limitless reservoir of moisture, you'll never dry it all out. You want a good vapor barrier between the concrete wall and the interior. You want an impermeable layer against the concrete, so that moist interior air can never contact a cool surface. The rest of the wall has to be vapor-open to the interior.
Thanks again, DCcontrarian --
My idea about the interior rigid insulation allowing drying to the interior comes from the 2022 edition of Joe Lstiburek's Builder's Guide to Cold Climates, which describes a basement interior insulation assembly that he recommends (p. 246):
"Concrete Basement With Interior Rigid Insulation:
The key to this assembly is the use of non-water sensitive rigid insulation on the interior that still permits drying to the interior. The recommended permeance of the interior rigid insulation layer is approximately 1 perm.".
Sounds like you would disagree with this? (if I am understanding you correctly).
The concrete must not be allowed to dry to the interior. The rest of the wall assembly must be allowed to dry to the interior.
"But then I didn't understand what you said in your second para about air inside the wall, as what I was referring to is poured concrete basement walls."
By "inside the wall" I meant between the concrete and the interior drywall.
otinkyard,
Martin clarified his advice on poly later:
"In my opinion, this type of vapor retarder, when installed on a crawlspace wall, is unnecessary but harmless. The important work is being performed by the rigid foam, which not only limits inward vapor drive; it also helps limit summertime condensation on the walls (something that a vapor retarder alone cannot do)."
Malcolm --
Thanks very much, as usual.
I did actually see that advice on poly from Martin (it's in the second article I linked to), but interpreted it as applying only to crawlspace walls and not to basements. But if I understand you correctly, it applies to basement walls as well.
But even with that change, I'm still struggling to understand what seems to me an asymmetry between the wall vs floor recommendations (in that same article). For the floor (assuming no sub-slab insulation and no sub-slab vapor barrier), poly is *recommended* as the first layer against the slab, to be followed by rigid foam on top of the poly. Why is poly then not *recommended* against the wall (with rigid foam next), but merely considered *harmless* (provided there is rigid foam next)..?
I don't mean to ask you to speak for Martin Holladay, but rather would value your own thoughts on this difference!
Crawl spaces are different in that the walls have an ability to dry to the outside. Rather, above-ground walls are different from below-ground walls.
otinkyad,
Crawlspaces are just short basements. Their below grade (and above grade framed walls) should be constructed the same way in both cases.
The moisture drives and risk of condensation of concrete stem-walls and basement slabs seem the same to me, and should be treated the same. The only difference is that the walls experience a gradation of outside temperatures from their base to the top.
When you get down to the foam layer being 1 perm or less, I don't think you get any useful drying to the interior, but conversely I don't think the amount of moisture that moves to the interior is enough to worry about.
What may govern whether you include poly rather than rely on the foam, may be the practicality of adhering the layers in an airtight manner over the concrete.
Malcolm --
I'm going to be Contrarian here and say there is no such thing as "useful drying to the interior" when it comes to concrete. If there is moisture in your concrete -- and there always is -- that moisture is better left in the concrete than making its way to the conditioned part of your house. Always. In all climates, in all seasons. You want an absolute vapor barrier between the concrete and the interior. You want that vapor barrier to be on the concrete side of any insulation. One perm isn't bad, but zero perms is what you should be shooting for.
In all but the warmest climates, a concrete wall is a "wrong-side" vapor barrier -- you're forced to put the vapor barrier on the opposite side of the prevailing vapor drive. In this way it's like a cathedral ceiling or an exterior wall in a cold climate with a cladding that is a vapor barrier. You have to prevent condensation within the wall assembly, and ensure that to the extent drying happens, it can happen to the interior.
DC,
I agree. "useful drying" is a poor way to put "there is no point in drying to the interior".
Unlike US codes, ours classifies every material under 1 perm as a vapour-barrier. When you get down to that level I'm not sure it makes much difference.
Malcolm --
I don't mean to be nit-picky, but "no point" just clangs on my ears. It's like saying there's no point to having a roof preventing water from entering the house, or in a cold climate, no point to having the walls keep the cold out.
Maybe it's my bias as someone who lives in a climate where fighting humidity is a year-round struggle, but it's not that there's no point to allowing moisture into your basement, it's that the whole point is keeping that moisture out.
And I don't get why people who are otherwise really smart when it comes to building science, like Joe and Martin, seem to have a blind spot here. I know that in the first edition of his book Joe said that concrete should dry to the interior, then recanted after getting pushback. And now it seems to have snuck back into his latest edition.
DC,
I row back to any language you feel appropriate, and agree entirely. It's one of those things where you feel like you must be missing something, because on the face of it, it makes no sense.
Malcolm -- This is immensely useful (as usual) -- thank you! The point about airtight adhering of poly to concrete wall vs concrete slab gives me a principled reason for the difference.
And thanks to both DCcontrarian and you for the additional discussion about vapor barriers against concrete -- this is helpful for me as I try to sort out (and understand) do's and don'ts.
Thanks again to you both!
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Where I live, Washington, DC, it is not uncommon to see dew points in the high 70's in the summer outside. If the air conditioning is off for any reason -- say, the inhabitants go away on vacation -- condensation will occur in any thickness of fluffy insulation and I don't recommend that it be used at all in basements. There's just not enough vapor drive to get that condensation out once it happens.