Insulating Basement Walls
I have read just about everything I can on insulating basement walls on the internet. Unfortunately there is conflicting opinions and recommendations just about everywhere which makes the entire thing more confusing. Figured I should ask my specific question here to see if I can get any additional help.
Background about my house:
My house is one year old and in south central Wisconsin (Zone 5). The house faces north and the basement is half exposed on the south side with a ~3.75′ concrete wall below the windows/framing/insulation. The east wall I will be finishing has no exposure. The exterior of the foundation was insulated with some type of foam board but I don’t know the rating of it. The rim joists have spray foam from when the house was built. There is no water or condensation issues in the basement that I have noticed in the year we have lived here. We are in the middle of winter with consistently freezing temps and the concrete walls are cool to the touch, but not freezing cold. The builder did finish one bedroom and bathroom in the basement, but I am about to start finishing the family room and second basement bedroom. From what I can tell, it looks like the builder only used unfaced fiberglass batts in the 2×4 walls of the finished part of the basement. The walls he built have about 3/4″ gap between them and the concrete walls, but it does look like the batts come into contact with concrete in some places but they are not smashed completely against it.
My plan:
My original plan was to finish the walls similar to how the builder did his. My understanding was that there was less of a concern with the batts since the exterior of the foundation is insulated. However, the more I read about it the more confused I get. Now I am thinking about using 1″ XPS foam board on the concrete walls, building the 2×4 walls up against it, and then using fiberglass batts before covering with drywall. But again, the more I read about it the more confusing it gets.
I have a few specific questions I am hoping to figure out and hopefully get some recommendations.
- Is my original plan of just 2×4 walls with unfaced fiberglass insulation about 3/4″ – 1″ away from concrete sufficient since the exterior is insulated? My understanding is that exterior insulation reduces moisture in the concrete walls, but does that mean they won’t ever produce condensation?
- Is using 1″ XPS foam board on the interior walls problematic because there is already foam board on the exterior? This means the concrete is sandwiched between two layers of insulation and it is unclear to me if this is acceptable or if it can cause problems
- Everything I read online says to avoid vapor barriers in the basement. However, my understanding is that the XPS board is a vapor barrier so I am confused why this is recommended so often. Will using XPS foam board trap moisture between the concrete and foam board causing issues?
- What would you recommend? I want to avoid spray foam due to cost so I am looking for solutions with fiberglass and rigid foam board.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
1- A lot of this depends on how much insulation is on the exterior. If you don't know, then assume it's not enough for your climate zone -- assuming you have LESS than the minimum is the safer option. Regardless of how much you have, I wouldn't use batts here myself. I never use batts in basement walls. Rigid foam is much less risky in all situations.
2- There is no issue using XPS on the interior. Concrete does not need to "dry". This being a green building site and all, note that XPS is the least green option you have here. EPS is better, and polyiso will get you more R per unit thickness. I would use polyiso here.
3- If anything is trapped between the foam and the concrete, it's also encapsulated between the foam and the concrete, so it can't really cause any problems. If you are worried about this, use EPS on the interior which is more vapor open than XPS, and especially more open than foil faced polyiso. EPS will allow some vapor diffusion, so it allows for a small amount of drying.
4- I would not use fiberglass. I would use EPS or polyiso for the reasons outlined above. Basically if you want maxium R in minimum space, go with polyiso. If you want to use the most vapor open rigid foam for drying potental, go with unfaced EPS.
Bill