Fiberglass insl. frozen to basement wall

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Old 03-09-11, 09:32 PM
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Fiberglass insl. frozen to basement wall

I was running the wiring for my basement finish and came across some insulation frozen to the vapor barrier.
Located in minnesota, built in 2003. The poured concrete wall is half below grade.
The design from the builder was- poured concrete wall, vapor barrier (6 mil poly with cut outs at the top) studs (1" from wall) with fiberglss batting, then an outer vapor barrier of 6 mil poly.
It gets cold here obviously. When I pulled off the vapro barrier and some of the batting it was frozen to the poly that was up against the poured concrete, so the batting was touching the wall. The moisture was from the inside not outside so its not a moisture issue from the outside in. i recently installed a whole house humidifier so this could have played a role in moisture building up and freezing the batting to the vapor barrier.
Question is, do I install 2" rigid foam between the studs and make sure it does not touch the vapor barrier or install rigid directly to the wall via vapor barrier between them. Or is the batting ok even though it will be nearly impossible to keep it from touching the vapor barrier against the poured concrete?
I'm a bit perplexed and have tried to get ideas from some other posts.
any suggestions?

Thanks
 
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Old 03-11-11, 06:28 AM
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You should not have a double vapour barrier. That will trap moisture inside the wall and create the problem you are describing. I would take it all down, remove the vapour barrier against the wall, apply 2" of xps, then frame and further isulate with Roxul Comfort Batt. No need to add a vapour barrier on top of the Roxul as the XPS also doubles as a vapour barrier.

I had the same issue as you (without the double vapour barrier) and researched it extensively. I have just finished redoing the basement as described above.
 
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Old 03-11-11, 02:11 PM
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Hi wirez, hryckman is correct, no VB. With your cold location, the 2" of rigid will prevent the surface of the rigid from getting too cold and promoting condensation. However, if you install too much insulation on the conditioned side, you will move the dew point towards the inside and if it finds warm moist basement air, it may still cause a problem. So don't skimp on the rigid and don't overdo it on the Roxul. As a precaution, avoid gaps and air leaks that will allow cold air to seep out and thus pull warm air in behind the wall. Roxul is very dense as opposed to fiberglass and will do most of the job. Here is a dot gov article on basements, building science dot com has info also. http://www.eere.energy.gov/buildings...s/db/35017.pdf

One correction, the rigid foam board isn't a true VB, they use the term "vapor diffusion retarder" Here is another link on that subject. Energy Savers: Vapor Barriers or Vapor Diffusion Retarders

Bud
 
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Old 03-11-11, 02:40 PM
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I agree with the comment not to overdo it on the Roxul for the reasons stated. I read somewhere that you should have a ratio of 45% xps and 55% insulation (by R value). 2" of xps is R10 while the 2x4 batts of Roxul are R14 so the ratio is pretty close.
 
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Old 03-12-11, 10:40 AM
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Great info.
Few questions in response.
I have 4.25 - 4.5" to work with to place the rigid and Ruxol. I'm thinking 1" Rigid then Ruxol? Or do I do 2" rigid and then try to split the Ruxol to half its thickness? Too messy?
Also, I believe building code in my area requires that I have a VP on both sides. I could cut it out just behind the Rigid and then again right behind the rock but there would be some 'visible' to satisfy the inspector. And/Or is ok to leave the VP that is there between the poured concrete and the Rigid? I believe the answer is no.
If I put the rigid between the studs and fill that gap, what do I place right behind the stud between it and the poured wall (1.5" gap) where the rigid won't fit? Do I buy cans of great stuff and start filling that gap, or go with 1" rigid that I can squeeze behind the studs to completely cover the wall and leave the Ruxol to fill between the 2x4's.
Thanks
 
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