Open cell vs closed cell in a basement?
I am looking to insulate the basements worth of band joists in Northern Vermont (climate zone 6A). There’s a couple different things going on here and I want to be sure I’m not goofing it up.
On 2 walls of the house, which is a 1970’s addition on a 1790’s cape, work has been done before me getting there which has 2″ of XPS covering the poured concrete foundation on the exterior. This is covered with a heavy-duty coil stock metal in lieu of parging. The XPS joins 2″ of foil-faced polyiso foam over the walls. I don’t know much about the connection of these foams, but I’m assuming it’s decently well done – 1 part can foam sealing the seam, at least. So, that’s the South and West walls. I am retrofitting the East wall and am taking a different approach. I noticed the drywall has a poly vapor retarder (a la 1970’s style) under it followed by fiberglass batts in a 2×6 wall. I have stripped the siding and am going to install BlueSkin VP100 over the sheathing followed by 4″ of RockWool Comfortboard IS, strapped and sided.
So, that’s the background. I am planning on having a foam contractor come and install 3″ or so of closed cell spray foam into the band joists of the East wall, where I am working. I was also planning on having them spray roughly the same depth of open cell spray into the band joist bays of the South and West walls in the basement. I have air sealed the band joists and mud sills of these two walls with a mixture of sealant and 1 part foam, depending on the gaps. I didn’t air seal the East wall since I feel safe using closed cell there.
When I was discussing the strategy with the foam contractor he felt very concerned about using open cell in a basement. He said he would do it if I insisted, but has never done anything like it to this point, and he does a lot of work in the area. Needless to say, this shook my confidence in my approach, though I really appreciated his sticking up for his sensibilities and find him to be a “keeper” on account of this. My thinking was that with the XPS with it’s very low perm rating and metal flashing with it’s nil perm rating on the exterior, the band joists need a direction for drying potential. I would go light with the open cell (3″ or so) to not go over the 60/40 rule of thumb I have been led to understand for innie v outie insulation. Is there something I’m missing? Are basements too damp in general for this to be a safe option? I don’t believe this basement is particularly damp, and regrading and installing ground gutters and french drains is part of the overall scope of work, so we’re doing everything we can to mitigate any moisture problems. I am also open to using other insulations (e.g. ComfortBoard) if they are more appropriate.
I’m trying hard to not create any petri dishes here, and greatly appreciate any advice or encouragement anyone has to offer, depending on whether I’m thinking about this all correctly or not.
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