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Old 01-24-2019, 10:45 PM   #1
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Does foam board trap moisture?


I will be removing the Fibreglass insulation from the rim joists and replacing with a rigid xps foam board. I currently see big signs of moisture and wasnt sure whether it's from condensation from the inside air coming in contact with the cold rim joist or if it's water getting in from a leak or capillary action. Since all the headers are all wet on all sides of the house I'm guessing its from condensation. Can anyone back this up by looking at the photos? We've also had some strange weather lately, it's winter and it rained today and at the time of the picture it got back down to below freezing outside.

My second question is, if water ever gets in contact with the rim joist OSB from the outside, would foam board trap the moisture and prevent he rim from drying out?
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Old 01-24-2019, 10:48 PM   #2
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Re: Does foam board trap moisture?


Whats the exterior look like? Where is the home?


XPS (1" Foamular) has a perm rating of about 1.1. Installed and sealed to the edges, it should do just fine.



I would still cover it with insulation as a standard, but it will work much, much better than did the fiberglass here.
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Old 01-24-2019, 11:03 PM   #3
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Re: Does foam board trap moisture?


That looks like water leak than water condensation. If that much condensation, you'd have more wet spots as well. Removed fiberglass also would be wet. What do you have around it? Condensation means moisture carrying air. To make that kind of spot, it would be something like leaky drier vent.
Using engineered rim joist and floor joist, I'd have thought better air blocking. XPS gives more value per thickness but more important is air sealing around ALL joints in the bay, then you could reuse the fiberglass insulation.
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Old 01-24-2019, 11:15 PM   #4
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Re: Does foam board trap moisture?


Thanks for the replies. I am located in southern Canada, zone 6. I might have to do some more investigating but i pulled 2 batts from opposite sides of the home and they looked exactly the same. That's why i'm lead to believe that it's condensation OR it could be water wicking up foundation or rain making its way behind the sill? If it's a condensation issue, foam board should solve the issue, but if water makes the rims wet when it rains or during the spring thaw, would a foam board prevent the rim from drying out, causing more mold and rot?
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Old 01-25-2019, 08:21 AM   #5
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Re: Does foam board trap moisture?


That does look excessive, but on the other hand, I don't think I see any mold. I would be inclined to agree that it is condensation.


How was the vapor barrier over the insulation? On my house, it looked like the plastic vapor barrier was cut and stapled by a blind man (and he was in a rush). A bit of batt fiberglass is going to do nothing to stop airflow. I only re-did a portion of my rim joist insulation, but I used a couple of layers of XPS (tight fit) and caulked all around. Definitely air tight.
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Old 01-25-2019, 08:30 AM   #6
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Re: Does foam board trap moisture?


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... but if water makes the rims wet when it rains or during the spring thaw, would a foam board prevent the rim from drying out, causing more mold and rot?

Wall always needs a method for water to drain, and a direction for the wall to dry to. I can't say your wall was done right, but since the old insulation would have had a vapor barrier over it (I fully expect), the plan would not have been for the wall to dry towards the inside. You are correct, XPS will not pass moisture.
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Old 01-25-2019, 08:41 AM   #7
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Re: Does foam board trap moisture?


Don't mix two system, water cannot be allowed to come in from outside. You can not insulate in anyway to deal with water coming in from outside.
Codes are changing but we for years have left gaps so moisture can get out , that is the science of house wrap.
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Old 01-25-2019, 09:14 AM   #8
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Re: Does foam board trap moisture?


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Don't mix two system
what do you mean by that?
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Old 01-25-2019, 09:19 AM   #9
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Re: Does foam board trap moisture?


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what do you mean by that?
If water can get in from outside you have deal with that. It can not be a consideration when insulating.
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Old 01-25-2019, 10:00 AM   #10
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Re: Does foam board trap moisture?


Neal means you need to find out where the water is coming from. If water is from outside, covering the area with something that will prevent any drying is a bad idea. It's like building a dam.
Check more bays. If it's only 3 bays, then why? What's above them? What's outside them? What kind of heating and appliances around there? Also search for images of joint between founation and wall. There is a little lip where underlayment meets the foundation. Siding must overlap this joint but may not be. Are there doors or windows above? Landscaping outside or gutter drain? How close is the ground and been snowing? Was the insulation wet?


BTW one way to check is leave the insulation out for a month. If the spots dry out, with about the same weather conditions outside, then you could suspect condensation. But why just there? Do you have a hot air ducts leaking? Not just there but from above? Air and water can move in mysterious ways.

Last edited by carpdad; 01-25-2019 at 10:02 AM.
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Old 01-25-2019, 10:08 AM   #11
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Re: Does foam board trap moisture?


Thanks Carpdad

i've looked at about 6 different rim joists and there all the same. I looked at some facing east, north, west, south - all wet.

We did have some unusually warm weather and rain 2 days ago though...

I would hope that it's only condensation, considering it's uniform between every single joist. however, since it rained, it leaves the possibility of there not being proper flashing and water getting in. i'm very discouraged at the moment!
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Old 01-25-2019, 10:50 AM   #12
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Re: Does foam board trap moisture?


Don't be discouraged, it's absolutely condensation. If it were a flashing problem the moisture would not be dead center on all sides of the house. Dead center would be the coldest spot with the most insulation between it and the basement. Remove a bunch (if not all) and watch the wood dry.

As for the rigid foam being a vapor barrier that isn't a problem. Pink or blue will allow a small amount of drying to the inside but foil faced will allow nothing. But those cavities are supposed to dry to the outside as long as they are not exposed to a lot of moisture, which is what you are seeing.

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Old 01-25-2019, 04:33 PM   #13
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Re: Does foam board trap moisture?


Centering makes sense. Once dry, if using foam boards, measure the bay then cut the board 1/2" short so you have a 1/4" space all around which you can fill with spray can foam. Where bay is too narrow, you can connect the straws that come with the can for longer reach. Use an awl to enlarge one end while heating it gently with a lighter. Then quickly insert another straw.


You should check where all the moisture is coming from. Gas fired furnace or boiler can make a lot, although the exhaust should all be going out the vent. If hot air, leaky ducts?

Last edited by carpdad; 01-25-2019 at 04:35 PM.
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Old 01-25-2019, 06:15 PM   #14
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Re: Does foam board trap moisture?


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Don't be discouraged, it's absolutely condensation. If it were a flashing problem the moisture would not be dead center on all sides of the house. Dead center would be the coldest spot with the most insulation between it and the basement.
I really hope your right! I also thought it was condensation because the wetness was mostly in the center, however, upon closer inspection, the sill is wet so maybe the water is getting in there and wicking up the centre of the OSB rim .... OR it could be condensation dripping down to the sill.

Really hard to tell at this point *sigh*
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Old 01-25-2019, 08:07 PM   #15
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Re: Does foam board trap moisture?


Here's a link from a well respected author, these are the type of people i trust.

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