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Mold in my basement on the rim joist

14783 Views 12 Replies 7 Participants Last post by  Gary in WA
I have recently started finishing my basement and have a question for some knowledgeable people. First let me say I live in central WI and it is anywhere from 10 degrees to 30 this time of year. I live in a 4 year old house and recently started insulating the rim joists with 2" XPS foam. The builder had used fiberglass insulation to stuff in there when the house was built. After doing a number of rim joists without any problem I came across 4 in a row that had mold growing on the band joist.(pictures attached) Not a lot of mold but enough for me to take concern and want to fix it. I also noticed all 4 of the spaces that had mold also had rusted nails sticking through them from the outside nailed in.(I imagine the cold wicked through the nail to the warm air inside and condensated on the OSB) My basement is dry this time of year, my humidity gauge says 40% and the temp is 63 degrees with all the heat vents closed in the basement. The bare concrete walls measure around 55 degrees and the bare band joist is anywhere from 45-55 degrees. I talked to a worker at menards and he recommended i use Concrobium Mold Removal then seal it with a product called 1-2-3 which is a primer/sealer, then use the 2" foam and seal around it. I guess the whole point of my post is to see if I am doing things correctly or if I am missing something. any input is appreciated


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100% sure it's mold? Hit it with some bleach and see if it goes away.
I've often times seen some weird ink stains all over OSB before.
Now that you mention it I noticed one of the fiberglass batts when I pulled it down had what looked like blue ink on it a little, but didnt think much of it. What are the chances it is ink? and why would there be ink blotches?
If there was going to be mold that's one of the places I would have looked, you have cold outside air on one side and the other is in a warm moist area.
Test it and see.
If it's mold you should be able to wipe it off with a finger or just spray some bleach or even a bathroom cleaner with bleach in it and it should just dissapear without even wiping it.
I tried wiping it off with a paper towel with no luck. I also tried spraying concrobium on it twice and maybe it got a little more dull in color but didnt make it go away like Ive seen other reviews say it would. Im starting to wonder if it isnt actually mold, idk what else it could be. attached are the new pictures i took after 2 treatments of concrobium


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If you would spray some bleach on it and just let it sit, If it goes away it's mold, if it stays it's just ink.
Looks like mold but bleach of a mild mildewicide will kill it.

Be sure to insulate those areas properly and get some moisture (vapor) control in place.
Sure looks like mold. Surprisingly your guy at Menards has most of it right. Spray it with a detergent and scrub it with a brush. Then wipe it down with a rinse solution. After that treat it with your fungicide. After it has completely dried, seal it with the 1-2-3. After that has dried reinsulate and seal it so no warm moist air can reach the cold surface.
It's mold.

The best product I've ever used is Microban. Hands down, better than anything I've used to kill & contain mold.
Do you run a dehumidifier in your basement? I put one in this summer and what a difference! When I had the insulation foam sprayer the guy couldnt believe how dry the rim joists were.
I've seen ink spots similar on TJI's and rimstrand rim board (engineered). Many times; when I used to/was younger, frame houses.

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