Rim joist insulation, embedded mud sill
hello,
Im looking to finish part of my basement and started to insulate by gluing 2” XPS to the walls. The original plan was to cover rim joist and top of the walls with 2” XPS as well and seal them with spray foam. I used this method last year when I build my wife an office in the basement.
Im now realizing I may have gone about insulating the rim joist area wrong when I built my wife’s office, due to the fact I have embedded rim joist(without a capillary break)and my floor joist make contact with the top of the concrete wall.
My concern is trapping condensation where the joist and sill contact the concrete causing rot. I’m wondering if I should change my methods? Possibly installing a poly sheet under the effected floor joist (probably wouldn’t do much for my mud sill)or injecting borate rods.
-house with built circa 1994
-The rim joist was originally all insulated with 1.5” EPS and sealed with calking. I tore a couple pieces out and didn’t find any moisture.
-bottom of the rim joist is 7-15” above grade, seems to have good drainage, have never had any moisture issues in the basement.
– I assume I have a PT mud sill but is hard to say for sure.
-no exterior rim joist insulation, just ship lap siding with tar paper
-house is located Nova Scotia, not sure of climate zone.
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Replies
You are probably okay with what you did. The EPS insulation allows for some moisture to move through should the rim become wet. The majority of the floor joist is still exposed and can dry.
In the photos you included, I do not see a mudsill, it appears your floor joist are placed directly on the concrete foundation wall. The joists look to be in excellent condition. I wouldn't worry, just periodically inspect the space, if you have a moisture meter, periodically check the level of moisture in the joists. If you start seeing readings in the mid to high teens, the trusses are getting too wet.
Randy
Here’s a better picture of what I’m calling my mud sill. I’m thinking it is actually pressure treated, but does not appear to have a capillary break that I can see. Hopefully it is PT and that will be enough to keep it from rotting.
You don't have a mud sill. If you had a mud sill, you'd see a board on top of the masonry wall, and the joists would rest on top of that board. Your joists are directly on the masonry.
You're probably fine with what you did, just check things periodically. Everything looks good in your pics (no signs of dampness or rot), so you probably haven't had any water/moisture issues in the past. Insulating rim joists is most risky when the masonry is prone to wicking water up to the wood, and you don't appear to have that problem -- at least not in the area where you took the pics. I'd do periodic (annual or so) checks looking for signs of moisture, but you'll probably be fine.
BTW, you can save a step and make things easier by skipping the step with canned foam before you put in the EPS insulating panel. What I do is cut the EPS panel about 1/4" to 1/2" short all around, to leave a gap in that range on all sides. I then inject canned foam into the gap while the panel is held in place with a piece of wire. In this way, the canned foam both seals the air leaks and any gaps around the perimeter while at the same time securing the EPS panel in place. This works well, and goes reasonably fast. If you can cut the edges of the EPS panel at an angle, you can make the expanding foam help to press the EPS panel against the rim joist too for a tight fit, which is what you want -- you don't want any air space between the rim joist and the EPS panel. Using canned foam this way, you don't need to bother with the fussy step of caulking the EPS panel in place.
Bill
Thanks for the tips!
the EPS that’s in the picture is what was done by the previous owner. My plan is to the leave the existing EPS and add XPS to it, using a similar spray foam method to the one you had mentioned.
I would use EPS, not XPS here. XPS costs more and is less green, and offers no advantages. EPS is cheaper, more green, and allows for a very slight amount of drying, all of which are benefits over XPS in this application.
Bill