# Does foam board trap moisture?



## PatFlemming (Jan 10, 2017)

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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## Windows on Wash (Aug 30, 2011)

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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## carpdad (Oct 11, 2010)

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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## PatFlemming (Jan 10, 2017)

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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## SPS-1 (Oct 21, 2008)

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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## SPS-1 (Oct 21, 2008)

PatFlemming said:


> ... 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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## Nealtw (Jun 22, 2017)

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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## PatFlemming (Jan 10, 2017)

Nealtw said:


> Don't mix two system


what do you mean by that?


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## Nealtw (Jun 22, 2017)

PatFlemming said:


> 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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## carpdad (Oct 11, 2010)

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.


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## PatFlemming (Jan 10, 2017)

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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## Bud9051 (Nov 11, 2015)

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.

Bud


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## carpdad (Oct 11, 2010)

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?


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## PatFlemming (Jan 10, 2017)

Bud9051 said:


> 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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## Bud9051 (Nov 11, 2015)

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

Bud


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## SPS-1 (Oct 21, 2008)

Just that I would not agree with the spray foam. Spray foam makes a total mess. I cut XPS to snug fit, then put acrylic caulk around it.


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## spitz1234 (Jan 1, 2019)

unfaced FOAMULAR XPS 250, 1” thick, has a rating of 1.1 perm. 2” thick has a rating of 0.70 perm, or, about equal to that of 15/32” oriented strand board sheathing (OSB).

Sent from my SM-G960U using Tapatalk


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## Windows on Wash (Aug 30, 2011)

SPS-1 said:


> Just that I would not agree with the spray foam. Spray foam makes a total mess. I cut XPS to snug fit, then put acrylic caulk around it.



Agreed. Just tough to reach in some spots. 

If they are doing all the bands, invest in a decent foam gun and use window and door foam (Closed Cell Variety) for the perimeters. You will make less of a mess. 

Foam is a must in the case of I-joists as nobody is cutting them to fit flush to the top and bottom chords in those cases.


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## PatFlemming (Jan 10, 2017)

Windows on Wash said:


> Foam is a must in the case of I-joists as nobody is cutting them to fit flush to the top and bottom chords in those cases.


Exactly. I have I joists which would make cutting the foam boards much more work - possible but much more work. However, I'm really leaning towards the foam board because I'm not convinced of the safety of sprayfoam. I have a 3 year old and a newborn on the way in March. I don't want to put my family at risk.

If I do end up having the courage of cutting each board to size around the I-joists, I'm still confused with which product to use to sealing the gaps. Caulking doesn't last forever and doesnt the canned sprayfoam have the same potential toxic considerations?


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## spitz1234 (Jan 1, 2019)

Frothpak. Most new construction is foamed now

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## Windows on Wash (Aug 30, 2011)

PatFlemming said:


> Exactly. I have I joists which would make cutting the foam boards much more work - possible but much more work. However, I'm really leaning towards the foam board because I'm not convinced of the safety of sprayfoam. I have a 3 year old and a newborn on the way in March. I don't want to put my family at risk.
> 
> If I do end up having the courage of cutting each board to size around the I-joists, I'm still confused with which product to use to sealing the gaps. Caulking doesn't last forever and doesnt the canned sprayfoam have the same potential toxic considerations?



The potential for off-gassing and poor mixtures (i.e. the biggest issues with rig spray foaming) do not exist with canned foam. 



Get a gun, extension straw, and use window and door foam (closed cell) and it should be great.


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## Bud9051 (Nov 11, 2015)

To cut the foam board a tip from the forum was to sharpen the edge of a 3" drywall knife and use that to cut. I had an old one with a angled blade so gave it a shot. Much easier as it can cut deeper than any utility knife and being wider it draws a straight line easily.

As for the i-joist you might cut some pieces of foam to fill out each side so the fill piece becomes just a rectangle. 

Bud


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## PatFlemming (Jan 10, 2017)

Thanks for info everyone!

Bud, that's smart thinking. Probably better than trying to cut each fill peice to fit snug between the I-joists. 

Cheers to all


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## Windows on Wash (Aug 30, 2011)

You would be notching the 4 corners which isn't that hard once you figure the size that you need if you want a really tight fit. I always default back to just getting the largest one that I can in there and foam gun the rest.


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## Spyder (Jan 11, 2019)

Bud9051 said:


> To cut the foam board a tip from the forum was to sharpen the edge of a 3" drywall knife and use that to cut. I had an old one with a angled blade so gave it a shot. Much easier as it can cut deeper than any utility knife and being wider it draws a straight line easily.
> 
> As for the i-joist you might cut some pieces of foam to fill out each side so the fill piece becomes just a rectangle.
> 
> Bud


What I do is use a razor knife with a sharp blade and either a framing square or a drywall square depending on cut size. Run it down quick tight to the square, then remove teh square and push hard as you score it down. Then you break it at the cut by holding the board down and banging down on the piece quick and it snaps, essentially like drywall. Sounds hard, but it goes quick. This works especially well on long cuts on big pieces. cutting off more than a couple of inches.

i've used a Japanese saw on the stuff and it works great when the razor doesn't. finer teeth cuts through quick and makes less of a mess.

https://www.amazon.com/Stanley-20-3...BT/ref=olp_product_details?_encoding=UTF8&me=

IMHO, measure rough and spray foam it. honestly very little to be gained by getting it perfect if you have the foam.
for the I joists I haven't done it, but I'd measure the tallest and widest parts subtract 1-2"s then just cut each corner triangle off and foam.


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## Windows on Wash (Aug 30, 2011)

If they are all the same size, and you have a bunch of them, I would set up a table saw jig. We do that and it makes much shorter work of the cutting if you have more than a handful to do.


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