Vapor retarder at rim joist?
rshuman
| Posted in Energy Efficiency and Durability on
I recently applied spray foam along the rim joist of my house for air sealing/insulation and will be placing roxul in the joist bays for additional insulation. The ceiling of the mostly finished basement will be drywalled after this is done. Is there any need to place a vapor retarder along the inside of the rim joist after I have installed the roxul? I am thinking not but would like a second opinion. The house is located along the coast in Lubec ME.
Thanks for any and all replies.
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Replies
Was that closed cell foam applied to rim?
Yes, closed cell foam.
As little as an inch of closed cell foam is sufficient to protect the rim joist itself. Whether an interior side vapor retarder is desirable (or essential) depends on the R-value of both the foam layer and rock wool layer. R7.5 foam would be sufficient dew point control for R15 rock wool at the foam/fiber boundary IF there were an interior side Class-III or tighter vapor retarder & air barrier.
If you have 2" foam (R12-R14) the need for interior side air barrier dissipates unless going for more than R15 rock wool.
Thanks Dana. So, I generally have less than 2" of foam and am placing R23+ worth of roxul in the cavities. In this situation it sounds like you would say yes to a retarder or barrier. If that is correct, something like membrain or plastic sheeting? The latter is easier to pull off (rapid availability, cost) but plastic generally makes me nervous. Your thoughts?
It's really hard to detail sheet goods as an air barrier in joist bays. Cobbling in some air tight gypsum board or OSB and meticulously sealing the edges would be more reliable, but painstakingly slow. OSB is it's own "smart" vapor retarder, but with gypsum board painting it with "vapor barrier latex" half-perm paint works.
Adding another inch or so of closed cell foam and backing off to R15 rock wool is usually easier and more reliable. I'm generally not a big fan of closed cell foam (especially when used in copious amounts as the primary insulation), but in this limited situation it could be the best option.
Is the lesser of two evils installing sheet goods within the joist bays as well as possible (but realizing absolute air tightness will not be achieved) or making no attempt to install a vapor barrier (leaving the foam/Roxul filled bays exposed to the above-ceiling air)? Does the fact that the half of the basement ceiling that is sheetrocked will communicate directly with the other half that has a suspended ceiling have any bearing on the best path forward (from the standpoint that the entire ceiling could, perhaps, be considered quite leaky as a result and, therefore, affect the supply of interior moisture)?
Rob,
What do you mean by "installing sheet goods within the joist bays"? Are you talking about rigid foam, gypsum wallboard, plywood, OSB, or something else?
For more information, see "Insulating Rim Joists."
Plastic, perhaps membrain.
Rob,
If you have a drywall ceiling separating the basement from the joist bays, the chance of condensation problems are significantly reduced. I think that the idea of not installing any vapor retarder will have a low risk.
That said, Dana's suggestion is best: Just increase the thickness of the spray foam layer. You can buy a two-component spray foam kit at most lumberyards.