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Community and Q&A

Open cell vs closed cell in a basement?

davidbailey | Posted in Energy Efficiency and Durability on

Hi All!
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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  1. GBA Editor
    Martin Holladay | | #1

    You are clearly conscientious and thinking things through. The basic reason to avoid open-cell spray foam at rim joists is to prevent interior moisture from diffusing through the foam and accumulating on the cold rim joist. But when a rim joist is insulated on the exterior with 2 inches of XPS, the rim joist won't be as cold, so moisture accumulation is unlikely. So if a rim joist has exterior rigid foam, interior open-cell spray foam is less risky than it would be if there were no exterior rigid foam.

    Just because open-cell spray foam is less risky, though, doesn't mean it's necessary. Either type of spray foam would work in this scenario. Researchers have measured the moisture content of rim joists that are encapsulated with rigid foam on the exterior and closed-cell spray foam on the interior, and these measurements show that the rim joists are dry.

    There are always ways to get into trouble, though. Every foundation wall needs a capillary break between the foundation and the mudsill. This capillary break is usually provided by the sill seal. Hopefully the 1970s builder knew to include sill seal. If there is no sill seal, you have to consider whether there is a risk of capillary rise.

  2. davidbailey | | #2

    Thanks, Martin! That is extremely helpful information.
    You ask a good question about the capillary break. The 1970's builder used fiberglass under the mudsill. There is an area that had some sill repair done to it that now has felt paper under the mudsill. Neither of these are a capillary break, to my mind. Maybe the felt paper is to some degree? The mudsill isn't pressure treated anywhere except a small area where I made a repair on the East wall.
    Does this information change the picture in any way?
    Again, thank you for your attention and knowledge in these matters!

  3. GBA Editor
    Martin Holladay | | #3

    Asphalt felt provides an imperfect but adequate capillary break, in my opinion.

    Fiberglass is useless as a capillary break, unless it is a type of sill seal that consists of thin fiberglass in a polyethylene jacket.

    Considering the fact that some areas of the foundation lack a capillary break, open-cell spray foam would be less risky than closed-cell, because it would allow the rim joist to dry to the interior of it ever got damp from capillary rise.

    Other methods of improving the situation include removing all vegetation on the exterior side of the problematic area (so that sunlight can reach the area) -- a method that works best if the rim joist has no exterior insulation -- and jacking up the house a 1/2 inch so that you can slip a capillary break between the foundation and the mudsill. (For more information on this somewhat daunting task, see "Rubble Foundations.")

    The risk is hard to evaluate, however. If the existing foundation wall seems dry, most builders would accept the small risk of capillary rise rather than jacking up the house.

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