The dreaded vapor barrier question
Hello,
let me start by saying i have read almost every single vapor barrier post one this site and many others and am still confused, or over thinking this because im an engineer and thats what we do. It also seems that the answer is a case to case basis so here goes.
I want to finish part of my walk out basement for a play room for my kids and i am concerned with mold/rot problems in the future if not done correctly. I live in central NH which is climate zone 6 and the house is not Air conditioned but is heated and the basement will be heated with a pellet stove.
The room will have two exterior walls both of which are 4 foot of concrete and 4 foot of 2×4 construction with unfaced batts. the make up of the house is vinyl siding, house wrap, osb, 2×4 construction with unfaced batts, currently there is moisture behind the insulation on the interior side of the osb (which scares me)
one wall i want to finish flat and eliminate the concrete step so my plan is to either replace the unfaced batts with kraft faced, or use a 6 mil poly vapor barrier then build the wall out with rigid foam untill even with the concrete, then run rigid foam from floor to top sealed well, frame a 2×4 wall infront of that with unfaced batts, then sheetrock.
the second wall would have a shelf so it would just be vinyl siding, house wrap, osb, 2×4 framed wall with kraft faced insulation or unfaced batts with vapor barrior then sheet rock.
i am concerned with the wall being able to dry, should i use the vapor barriers or not and what are my other options, spray foam is not in the budget.
also how should i insulate the rim joist.
sorry for the long first post but i wanted to get as much information in as i could
Thanks in advance for all the help
Never mind on the rim joist, just read the question a few below mine, thanks
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Replies
In zone 6 a wood sheathed wall without exterior insulation to raise the temp of the wood sheath requires something tighter than a class-III vapor retarder (such as interior latex paint), even with back-vented siding such as vinyl. (In zone 5 you could have gotten away with it.) Kraft faced batts would be adequate if air-tight, but it's nearly impossible to reliably get air-tightness with kraft facers. Unfaced batts are a code violation unless you either put air tight wallboard with a "vapor barrier latex", (about 0.5 perms), or air-tight 6-mil poly (0.05 perms) or a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus (variable permeance). The smart retarders offer the most resilience, since they inhibit wintertime vapor diffusion, but becomes vapor open whenever the assembly has enough moisture to make the RH inside the cavity more than 40-50%, increasing the rate at which it can dry. See:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm
Building the studwall portion out to make it flush with the concrete works- use unfaced EPS, which is still about 1perm @ 3", which is the edge of class-II vapor retardency.
For the concrete wall section you can skip the interior vapor retarder if your floor to ceiling foam is at least R7.5 without any risk at all, but since there isn't wood between the new batts & foam you can actually cheat that a bit- the stud edges are still warmed due to the higher thermal conductivity of the wood- it's not exactly the same situation as with a wood sheathed assembly. With 1.5" of EPS you'd be at R6.3, which would be just fine with unfaced R15 rock wool, even without interior side vapor retarders. But if you're nervous 2" (R8.4) would give you margin.
Air tightness is more critical than vapor retardency, and under NO CIRCUMSTANCES should you put inteiror side polyethylene, which could trap ground moisture in the assembly at the below-grade sections. A smart vapor retarder would be fine though, and may be needed if you use only an inch of EPS between the studs & concrete on above grade concrete.
Put an inch of EPS under the bottom plate of your 2x4 studwall as both a thermal & capillary break. too.
So I can omit the poly barrier with three plus inches of eps? Where is membrane available?
http://foursevenfive.com/
Would it be beneficial to caulk between the studs and OSB in all the exterior bays and between the concrete and sill plate?
Membrain is available at my local Menards and their website. Would guess other big box have it or can order it.
Richard....475 doesn't appear to carry it. Based on their intello price it would be over Menards price even if they did carry it.
Matthew,
The most important fact to remember is this: to keep interior moisture out of your wall assemblies, the most important thing you can do is to pay attention to airtightness. The vapor permeance of the layers doesn't matter much.
That said, you don't want any polyethylene anywhere in your wall assemblies.
One more point: You wrote, "The second wall would have a shelf so it would just be vinyl siding, housewrap, OSB, 2x4 framed wall with kraft-faced insulation or unfaced batts with vapor barrier, then sheetrock."
Even if you eliminate the vapor barrier -- and you should -- this is a particularly bad wall because it is so poorly insulated and because it includes the worst kind of insulation (fiberglass batts) ever invented. The wall is thin and there is no provision or reducing thermal bridging through the studs.
If I were you, I would improve this wall by switching to dense-packed cellulose, or including a continuous layer of rigid foam, or both.
Matthew,
Q. "Would it be beneficial to caulk between the studs and OSB in all the exterior bays and between the concrete and sill plate?"
A. Yes.
Thank you very much Martin, I will eliminate poly everywhere, concentrate on air tightness and add an inch and a half of rigid foam to the "bad wall"
Foundation Insulation Effectiveness: Basement building science
Pat Huelman, Cold Climate Housing Coordinator with the University of Minnesota Extension discusses the issues associated with insulated foundation walls.
https://www.youtube.com/watch?v=kwn0Vjw_ji0
Did you watch minute 24 on... regarding foam instead of batt being better... or better yet (exterior foam). Or minute 29 regarding spray foam on the interior being one of the least risky?
His comment regarding the moisture in rim joist being most related to moisture of foundation wall was odd... since most new houses for the last 20yrs that i have seen have a complete sill seal - which prevents moisture drive into bottom plate.... but i suppose his research didn't look at any particular style/year of home.
Nick- the foamy sill-seals do not make a very good capillary break, or even a reasonable vapor retarder between concrete & wood. It really takes EPDM, heavy polyethylene sheeting, sheet metal flashing, or some other true vapor barrier to moisture-isolate the concrete from the foundation sill.
Worse still, the cheap foamy sill gaskets usually aren't thick enough to really deal with just how sloppy the foundation tops are finished in most new construction, and don't even serve the air-sealing purpose for which they were designed. They're basically "better than nothing", but to get good air sealing requires flatter more even foundation tops and/or a heavy more compliant (and highly vapor retardent) EPDM sill gasket.
oh - no doubt they are not a airtight option - like say EPDM gasket with super flex tape on the outside sheathing as well. In-lieu of a good gasket/tape seems like most modern homes around here are then spray-foamed interior to seal that whole mess up.
However, i have to imagine a 2x6 located on a block wall will have a higher moisture content most of the year then a 2x6 set on a cheap sill polyethylene foam gasket. (.25") ? same low perm materiel as poly sheeting?
I guess all the interior walls in any basement i have seen are just greentreated on slab... i'm in preplan on finishing mine, was planning to also use sill seal to get off the ground a touch.... Not worth it for cap break? I see BSC does recommend this product for use as a cap break and thermal break when framing basement walls. So i will stick with my plan.
Dana,
Thank you for addressing that critical detail.