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Basement wall and rim joist insulation clarification

cward0625 | Posted in General Questions on

Hi, I have been reading this site and found it very informative but have a few specific questions/clarifications.

I live in Illinois and am going to finish my basement. The rim joists were insulated with fiberglass and walls with a fiberglass blanket and r11 fiberglass. From what I have read, if I just frame over this I will have moisture issues. My understanding is that I should remove the blanket and rim joist insulation. Then install foam board and great stuff to seal the rim joists. I am quite limited in space in my basement so every inch I can save helps too so I was going to use 2″ foam board in the rim joists but 1.5″ for the walls and caulk all around. Frame and use r13 fiber between the studs with the studs up against the 1.5″ board. My understanding from this site is to use no Vapor barrier either side of the stud and just put my drywall directly on the 2×4. Do I have this right? 
Secondly I have a slight complication, at 2 ends of my house the exterior joist/end joist has electrical conduit and gas pipe running tight up against it. A few years ago I found some condensation in the conduit when I pulled the fiberglass insulation back so I think this speaks to some of this issue with warm air meeting the cold air in the conduit as it is tight against the exterior board. With that said, I can’t fit foam board between the conduit and exterior board/end joist so is spray foam my only solution? Is there a kit recommended for this? Is it safe to use spray foam still… I thought I read some comments advising against it now somewhere stating it doesn’t allow the foundation to breath there?
Appreciate any help you can provide. 

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  1. Expert Member
    Peter Yost | | #1

    HI CW -

    Air sealing the rim joist asssembly is just as important as insulating this assembly. Many times in retrofit, the insulation/air sealing at the rim joist means that this assembly dries primarily to the exterior and that's fine.

    In terms of human health and spray foam: EPA has been grappling with this issue--in terms of both OSHA and worker safety and for DIYers such as yourself.

    Here is one recent GBA resource on this topic:

    And another from my former company, BuildingGreen:

  2. cward0625 | | #2

    Thanks for the links. I reviewed those and I feel the jury is out still on it's safety. If that is the case what are the alternatives to seal the rim joists and basement walls? I have a few asthmatics, sinus and allergy issues in the house, so reading those articles certainly raised some concern around spray foam to me.

    In regards to your statement 'the insulation/air sealing at the rim joist means that this assembly dries primarily to the exterior and that's fine.' I believe you are saying that it would be fine to spray foam over the pipes that run tight against the exterior joist above the basement walls then (if that is our only option and accept the potential risks of SPF). I just realized I am calling this joist a rim joist, but it is really the 'end joist' where this pipe runs along. Does that impact the process, does this still need air sealing, and is there any way to prevent condensation from building up in the conduit that runs tight against those end joists as they are very cold but then getting warm air in the electrical conduit still naturally through the conduit as it runs through the conditioned home? I tried including a photo but the site errors when processing it.

    One contractor, recommended pulling the blanket insulation down, leave 1 inch gap to the wall for it to breath, then 2x4 and R15 faced insulation between the studs then drywall over that. From what I have read on this site and in articles, I get the impression that isn't a good plan if my understanding is correct.

    Just having trouble as it seems you either accept a risk of moisture/mold, or risk of harmful chemicals but they prevent mold...I read mineral wool is safe, but not sure how I could install this to create an air seal in the situation with the pipes running along the exterior joist...I would think this would contribute to more condensation in the cold pipe as this insulation would just butt up against it and air would still leak around.

  3. Expert Member
    BILL WICHERS | | #3

    Mineral wool is good stuff, but won’t do anything for air sealing. You’re going to have to use some kind of sealant somewhere for air sealing. Personally, I think spray foam is safe, and I’ve used it myself, when it’s apllied properly. If you have a contractor that is just starting out, there can be problems with the mix if they don’t do it right. If you have an experienced contractor you should be fine. There have been many, many successful installations without problems. Just stay out of the house for a few days immediately after application so the fumes can dissipate. On my own house, I had the foam applied on Friday and then visited family out of town over the weekend while leaving some windows cracked open at home. No problems when I got back.

    The canned foam like greatstuf is much less of a concern for offgassing. It works great for air sealing but it’s much more labor intensive if you’re doing a large area. Sometimes caulk is actually better if you’re just trying to seal a small crack that you can’t get the canned foam straw into.


  4. cward0625 | | #4

    Is my strategy to seal the walls with 1.5" foam board/xps and the rest of the rim joistsand then stud up against that with insulation between the studs and drywall on that without an additional vapor barrier correct too (meaning the stud insulation bats should not be faced either?)?


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