Frost, Ice, & Condensation @ RIM JOIST

I live in Michigan. I built and moved into a brand new home in November of 2005. We are going through our second winter in the home. We are preparing to finish the basement and I noticed something disturbing last night that I have a question to ask the forum.

I have been sealing the sill plates (construction was foundation wall, foam vapor barrier, pressure treated sill plate) and bottom portion of the rim joists around the entire basement with silicone sealant. I am also in the process of replacing all the R-13 insulation that the builder installed between each joist along the perimeter of the basement (not cut right, some pieces missing kraft paper, etc).

When I got to the 2 fresh air ducts plumbed to the outside of the home (the first one supplies fresh air to the area of the furnace) (the second one supplies fresh air to the return duct work), I noticed that ice is building up on and around the metal duct work that is stubbed into the home. Flexible & insulated duct is connected to the metal duct work and is plumbed to each area described above. I also found ice on the rim joist in each area, and because I assume the warm basement air is coming into contact with the cold metal duct and plywood rim joist, I also noticed some small water droplets on the pressure-treated sill plate. The insulation that was installed around the duct work was damp/wet, not soaked, and basically was only wet around the circumfrence of the duct.

Once I found this problem, being curious I went around the rest of the basement to inspect the rim joist and what I find is even more disturbing/confusing. A majority of the spaces between where the floor joists and rim joists meet are completely dry. However, approximately a quarter of everywhere else I inspected, I noticed frost and/or ice buildup on the plywood rim joist, and where it is condensing, dropletes of water on the pressure treated sill plate. The insulation in these areas is also not wet, however there are areas or “patches” on the kraft paper that have “wet spots” from what I assume is the frost/ice melting onto the kraft paper.

Another observance is that in the spaces that have frost and/or condensation I am noticing that the tips of nails are begininning to rust.

Another observance and/or question is that the builder installed the insulation (R-13) with the kraft paper (vapor retarder) facing towards the exterior of the home and not facing me when looking at the insulation in the basement.

Can someone please explain to me what I can do to try and alleviate this frost/ice problem. My concern, and what I worry about most is the rim joist becoming ruined from condensation/water and MOLD forming in the spring/summer time once things begin to thaw because of the insulation getting wet now and not drying later.

My only thoughts were to completely caulk all corners/intersections along the rim joists and between each floor joist, and maybe remove all the R-13 insulation and replace it with R-19 (or higher???) insulation.

Can someone also please put to rest what is the CORRECT direction (kraft paper towards the exterior or kraft paper towards the basement) for the insulation to be installed in the rim joist area. I have read conflicting reports that each way is correct, and I have a hard time believing this.

I am sorry that this is so long, but I thank you for the help in advance!

If someone needs me to take some photos and post them, please let me know.


The vapor barrier (kraft paper), in Michigan, should face the heated side of the insulation.

It sounds like you have moist heated air coming in contact with the cold rim joist and condensing to form water droplets and moisture in general. Insulate (with R-19 min.) those areas of the rim and seal them up as best you can to prevent heat loss. Caulk at the joints helps. Do the same with the combustion air ducts.

Here is a start for info on finishing you basement:


Thank you for taking the time to write back.

I will seal up the rest of the joints in the rim joist area. I assume that silicone, like I used previously is still fine.

I will also rip out all the old R-13 and replace it with the R-19, with the kraft paper facing me while looking at it in the basement.

Thanks you also for the links.


Sean, Vapor barrier is always to go on the warm side. Another option is to get an Insulation company in and spray foam these areas. It does all three in one easy step…insulates, vapor barrier, and seals up all cracks and crevices, no matter what materials joining what. It’s more expensive, but is by far, the most efficient at all three tasks.IMO

Sean, I would check to see if the exterior sheathing overlaps the foundation and does not end at the bottom of the plate or top of the foundation. Try to look or feel up under the siding and make sure it overlaps.