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Old 08-22-2016, 01:43 PM   #1
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Rim Board/area sealing question


I have been debating the proper way to seal the rim board area of my basement, as i think doing so could cause moisture to accumulate within the existing basement wall assembly.

I would like to know if my thinking on this is correct..

A bit of info:
-House is built 1960 and is a single story bungalow.
-The Basement walls are poured concrete and the floor joist (2X10) are cast/set into the top of the wall. The top of the concrete wall stops about 1/2" from the underside of the floor sheathing. This method of construction is fairly common for the time. The gap is filled with some batt insul stuffed in there
-The existing basement wall framing is 2X4's with batt insulation and vapour barrier up to the underside of the joists. The vapour barrier appears to not be sealed at joints or at top/bottom
-The area between joists is not insulated or finished. The vapour barrier does not extend through the floor joist area.
-The basement RH in the summer is about ~60% (i use a dehumidifer to bring this down to between 50-55%. In the winter RH is 25-30%.

The wall studs are held 1" away from the concrete and this gap is open to the joist bay above (and into the conditioned space). It's this 1" of space that has me wondering.

Right now there appears to be no sign of any mold or moisture in the wall. I believe this is because of the 1" space behind the wall that is open at the top allowing air movement behind the wall.

My thinking is that if I were to seal the rim board area this would close off this gap at the top of the wall and any moisture that accumulates within the wall would be trapped within the wall.

Thoughts?

Sorry for the long post

Last edited by davidmr; 08-22-2016 at 02:09 PM.
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Old 08-22-2016, 02:35 PM   #2
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Re: Rim Board/area sealing question


A difficult one to easily answer. Modern thinking is to avoid that gap because the circulation not only bypasses the insulation but can carry moisture from the lower area of the wall up to the above grade area where in winter it can form ice.

As you know having those joists buried in the concrete is not the best and doubtful they used any form of treated wood back then.

You said the basement wall "is 2X4's with batt insulation and vapour barrier up to the underside of the joists. The vapour barrier appears to not be sealed at joints or at top/bottom". Is there drywall on the inside or just exposed vapor barrier?

From your description of your humidity readings it sounds like there is a lot of outside air leaking into that basement. Is sealing that your objective?

Bud
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Old 08-22-2016, 02:56 PM   #3
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Re: Rim Board/area sealing question


Quote:
Originally Posted by Bud9051 View Post
A difficult one to easily answer. Modern thinking is to avoid that gap because the circulation not only bypasses the insulation but can carry moisture from the lower area of the wall up to the above grade area where in winter it can form ice.
I agree the gap sucks, but its the existing condition. The formation of ice in winter at the top is my concern. By the gap currently being open at the top it is preventing this from happening IMO.. If i seal the rimboard it will become trapped.

Quote:
Originally Posted by Bud9051 View Post
As you know having those joists buried in the concrete is not the best and doubtful they used any form of treated wood back then.
True, but the buried section (once rim joist area is insulated) would be able to dry to the outside.. Chance of a mositure issue is really very small there.


Quote:
Originally Posted by Bud9051 View Post
You said the basement wall "is 2X4's with batt insulation and vapour barrier up to the underside of the joists. The vapour barrier appears to not be sealed at joints or at top/bottom". Is there drywall on the inside or just exposed vapor barrier?
The wall is drywalled and painted (latex) up to underside of joists (or just below).


Quote:
Originally Posted by Bud9051 View Post
From your description of your humidity readings it sounds like there is a lot of outside air leaking into that basement. Is sealing that your objective?

Bud
There is. It's a 1960's house. I have been wondering if its even worth doing.. As well the windows are the old aluminum sliders that surely leak. There is a sump pit (top isnt sealed) that runs periodicly (more so during wet/rainy times)

I have thought that my best option might be to spray foam the 1/2" gap at the top of the wall and caulk around the buried joists..

Last edited by davidmr; 08-22-2016 at 03:05 PM.
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Old 08-22-2016, 03:43 PM   #4
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Re: Rim Board/area sealing question


Quote:
Originally Posted by davidmr View Post
True, but the buried section (once rim joist area is insulated) would be able to dry to the outside.. Chance of a mositure issue is really very small there.
On second thought I take this comment back about buried joists. Your right, it is a concern.

Simply air sealing might be my best option...
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Old 08-22-2016, 06:07 PM   #5
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Re: Rim Board/area sealing question


As an example, a related issue with super insulating a wall is that you are no longer keeping the sheathing warm enough to keep it dry. The air sealing you would like to do can have a similar unintended results and it is beyond me to guess what those might be.

One solution would be to air seal and insulate from the outside, hard to justify unless you were going to install new siding anyway. But a couple of inches of rigid on the outside (very common) would extend to cover the rim joist area in question, both adding insulation and air sealing.

Do you know if the house has board sheathing, plywood, or other? After years any board sheathing will have dried leaving many large gaps between boards. Covering that with the rigid would be another benefit.

Bud
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