I'm looking for advice on insulating the rim joists in my century-old basement. I want to replace the old fiberglass insulation with rigid foam board and use spray foam to seal the edges. However, I'm concerned about trapping moisture in such an old house. Would that be an issue, or is it mainly a concern if I fully spray foam all the cavities? I live in the Midwest where winters are quite severe. Also, some parts of my basement ceiling are finished, making access to the rim joists difficult, and my HVAC runs through there, complicating insulation efforts. If I can only insulate about 50% of the rim joists, is that even worth doing?
3 Answers
From my experience, insulating the basement walls could be more beneficial than just focusing on the rim joists. Replacing the fiberglass insulation may not yield significant improvements alone. Insulating the walls can help create a warmer and more welcoming space, and reduce overall heat loss. However, without more context about your specific setup, it’s hard to give definite advice.
If there's plumbing or heating in your basement, it's classified as a conditioned space. Removing that old fiberglass is a step in the right direction. We recommend ensuring a moisture barrier is present on the basement floor and extending it up the walls. Insulating the walls is crucial, as this keeps the basement warmer overall and helps reduce heat loss. Even if you can't get to every spot, taking steps to insulate at least some areas is worth your while, as it will still contribute positively.
It's just cement walls, so I'd have to frame it to insulate, and I don't plan on finishing it since it floods. I'm more concerned about improving the rim joists for air leaks.
It's definitely a good idea to tackle the rim joists, even if you can't do the entire job. Focus on sealing those edges well; cutting the foam board at bevels can help achieve a good fit for the spray foam. Ideally, go for at least 2 inches of insulation in the Midwest, and layering it can make it easier to handle. Remember, even a partial insulation job is better than nothing!
I'm in a similar situation with a cement basement. I plan to remove the old fiberglass for sure, and I'm leaning towards rockwool for moisture resistance.