Help! Condensation in Rim Joist after XPS Installation

Bought a home last year and after reading a number of articles on how to help insulate the home with plans to finish off the basement, over the summer I placed 1.5″ XPS in the rim joists, sealing edges using spray foam and put back in place the unfaced batt that was there.
We are in process of finishing our basement, and I happened to pull a section of the batt out and noticed condensation. I then spot checked around other areas of the basement and all direct to exterior rim joists have condensation forming on the xps behind the batt.
At this stage of the project walls are framed, hvac almost complete, electrical in a week. What can I do to remediate?
Would it be best to remove the batt all together and leave just the XPS to allow air to directly warm the interior side of the XPS? Ceiling joists will have mineral wool insulation and sheet rock when completed. I would look to do an additional layer of spray foam over the XPS but I would not have enough access to do so in some rim joist areas now that framing is up.
Any help or guidance would be appreciated!
Current conditions:
– Location Milwaukee, WI area so zone 6 I believe
– built in 2007
– basement has maintained around 65 degrees and 40-45% humidty.
– do run dehumidifier on occasion but humidity remains relatively consistent
– have had large swings in weather from above freezing to well below with a day over past few weeks.
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Replies
Grahlw,
Unfortunately the foam isn't thick enough to keep the inside face warm enough to avoid condensation. A good interior air barrier will help, but to be sure the two alternatives are to increase the thickness of the foam, or if that's not possible reduce the amount of permeable insulation (batts) inside.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
Thank you for the response!
Would using a system such as DAP Wall and Cavity Spray Foam kit over the existing xps in joist areas I can reach benefit or likely not be sufficient enough still? For joist areas I can not reach (only about one inch gap to access with walls framed) my only option then would be to fish out the existing batt leaving the xps barrier.
You could use one of the two-part spray foam kits here, or you could put some more rigid foam in (XPS or EPS) and foam it in place using canned one-part foam like Great Stuff. Either way works. Using one of the two-part spray foam kits (Touch'n'seal, FrothPak, etc.) would be less labor, but more money.
As Malcolm mentioned, you don't have enough rigid foam (about R7.5 or so) with only 1.5" of XPS to keep things warm enough in your climate zone to avoid condensation.
Bill
Grahlw
you wrote in your original post:
"I placed 1.5″ XPS in the rim joists, sealing edges using spray foam"
that doesn't look like "spray foam" to me, it looks like the type used to seal gaps around windows and doors ...
if the 1.5" of XPS is r-6, how much r-value did the unfaced batt have?
yes, I would buy a true spray foam kit & have at least 50% of the r-value (conservative) be in the XPS & spray foam (r-6 / inch)
1.5" of foam won't condense on the interior if the fiberglass batts aren't there keeping heat from the interior from reaching the foam surface. You can remove the batts, or you can stop the relatively humid indoor air from being able to freely circulate through the batts by adding an air control layer. A variable permeance membrane, painted drywall, wood or plywood would all work, as long as the edges are fairly well sealed.
Or you can remove the batts entirely and add more foam if you want additional R-value without having to deal with a separate air barrier.
Hey Mike, i have a very similar situation in my basement. I applied a single layer of xps and spray foamed around the edges. In a recent cold snap I noticed condensation. Due accessibility in some areas, I don’t want to / can’t add another xps layer. I am also doubtful spray foam would be possible in these areas as well. With all that said, I planned to pull out the insulation batts (some got wet) that I had placed against the xps. If I choose to not add any more r-value to the rim joist, what happens to the condensation when I close up the walls and ceilings with dry wall? In my mind, there will be trapped moisture in the air of that cavity. And the rim joist would still see some condensation. With virtually no air flow should I be concerned with the potential for mold? Can I trust a dehumidifier to work through the drywall once everything is all painted?
Thank you everyone for the responses! Removing the batts seems to have cleared the condensation. I'll look to use a froth pak over the xps and beef up the r value in the joist area.
So I just finished air sealing the mudsill/foundation connection with Prosoco joint seal caulk and insulating the rim joists in our basement as well. I used 4" rigid foam blocks, shimmed into the cavity with a wedge of foam so that I had about a 1/2 inch gap around the entire block and used canned spray foam around the perimeter to block any air flow from the conditioned basement into the rim joist cavity.
Today, a contractor came to do a bid on a sinking pier form in our encapsulated crawlspace and he told me that it was good I insulated the rim joist cavities but I shouldn't have sealed those blocks in because now my rim joist cavities can't breathe... I thought I was both trying to keep the warm air from our basement from contacting the cold rim joist by sealing the seams with canned foam and mitigating the temperature with the rigid foam block. Did I get this wrong?
holleecee,
No you got it right. They don't need to breath. They do benefit from a drying path to the outside, which I'm sure they have.
https://www.greenbuildingadvisor.com/article/insulating-rim-joists
The article you reference was my reference for doing the work and they do have the benefit of drying to the outside. I also watched the videos on Asiri Design's website. I appreciate all of the resources available here and there. I will note that, since air sealing the basement, our heater is not cycling on nearly as much as before. I'm very curious to see the energy bill next month.