Interior Insulation of Embedded Joists in Cold Climate Basement
I was about to pull the trigger on installing some insulation in my basement until I read about the “problem” of insulating embedded rim joists and have seen a lot of conflicting information.
First, I live in SE Wisconsin (Climate Zone 6) and have a 1930s house with cinder block basement walls, concrete floors, embedded rim joists, no HVAC sources in the basement, and likely no exterior insulation or waterproofing. Basement is almost entirely below grade (even the joists are mostly at or below grade).
My goal is to add drywall walls around the perimeter of the basement and some of the ceilings to brighten things up but not looking to make it a fully finished area or necessarily seek to meet the code requirements thereof.
My plan is/was to simply install 2″ of XPS rigid insulation on the walls around the perimeter of the basement, and then 2×4 stud walls with 1/2″ drywall and no insulation in the cavity (again, not looking to make this a finished space). I understand that level of insulation would be enough to provide protection to the studs and drywall from moisture intrusion and thick enough to avoid condensation issues that can come from using thinner XPS insulation with a lower R value.
Then I started reading about embedded joists, which I have (here is a photo of mine: https://bit.ly/38bOHyD). The joists appear to rest on the top of the cinder block walls and then concrete or some type of masonry is pretty tightly infilled between the joists all the way to the top, and this vertical surface is flush with the cinder block wall surface below. The joists do not show any visual signs of prior water damage or rot.
I had thought I was going to run the 2″ XPS wall insulation up into this joist space and seal the XPS around the joists and floor, but then started reading about concerns with this causing the joists to rot. I’ve read some different things about what to do here, such as max 1″ XPS insulation (per Canada’s energy website). Some say don’t do it at all. Some say add insulation, but leave a gap between it and the joists.
I’d like to know what the realistic options are here for interior insulation that are least likely to moisture or rotting issues. Not looking to insulate the exterior, cut off joist ends, etc. Again, I’m looking more to protect the studs and drywall walls I’m building from moisture than to provide energy efficiency. Above all, I want to avoid doing anything that could create moisture/mold/rotting issues.
1) Are there any risks if I install the 2″ XPS from the floor to the bottom of the joists and put no insulation in the joist space?
2) Are there any risks if I install the 2″ XPS from the floor to the bottom of the joists and install 1″ XPS in the joist space? If so, should I airseal the XPS around the joists and subfloor? Or leave air gaps between the joists and XPS? I’ve read both ways….
3) Are there any risks if I do same as (2) but with 2″ XPS in the joist space?
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