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Old 08-28-2013, 10:07 PM   #1
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Wet Insulation due to Vapor barrier?


All I am new here an seeking some advice.

I have an unfinished lookout basement and I live in a suburb of Minneapolis MN so our summers are hot and humid and winters are fridgid.

I have fiberglass insulation on the exterior walls that are above grade with a plastic vapor barrier. The insulation gets wet/damp in the summer months then dries in the colder months. The insulation appears to have small amounts of mold growing on it. The home was built in 2005 and I purchased it in 2010. When I purchased the home, I had it inspected and the home inspector noticed the damp insulation and pointed it out to me. His comment at the time was that I should eventually get it replaced/fixed but it isn't a major problem. He believed that the mositure was comeing from the hot humid air out side coming through the sheathing and condensing as soon as it hits the vapor barrier as the vapor barrier is ice cold from being in the basement.

My questions are these:
1) Since this is happening in the basement isn't the same thing happening in the rest of the house because the rest of the house is air conditioned. Or does having sheetrock up prevent enough cold air from directly hitting the vapor barrier ther condensaition in the finished walls isn't a problem?

2) How do I fix the condensation issue with the insulation? It has been suggested to me that there is a semi-perminable vapor barrier on the market that allows the somehow allows water vapor into a house but keeps liquid water out similar to gortex and that would prevent the condensation. Alternatively, it has been suggested to me to have all the fiberglass insulation and vapor barrier torn out and have spray foam insulation installed. Would any of these solutions work? If not do you have ideas on what would.

3) How big a deal is this in the short-term (2-5 years)? Is this causing structural damage by potentially rotting the studs, window framing and other support structures?

Any thoughts would be much appreciated.

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Old 08-29-2013, 10:56 AM   #2
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Wet Insulation due to Vapor barrier?


1. Tough to say without pulling it apart but it is certainly more common with basement walls and the plastic vapor barrier. There is traditionally a bunch of air leakage along the band and that is causal to the issue.

2. Rip it off and cover the walls with rigid foam. There are a ton of threads on here about this.

3. You need to address it ASAP.

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Old 08-31-2013, 07:51 AM   #3
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Wet Insulation due to Vapor barrier?


"I have fiberglass insulation on the exterior walls that are above grade with a plastic vapor barrier."

Well, I think that what you have there is standard, minimum code builders insulation, which may or may not just end below the grade line, forming what is called a 'diaper'; the purpose is to provide at least some insulation in the areas above the frost line that lose the most heat in your basement. It is a temporary form of insulation, quick and easy for builders to put up, in order to comply with minimum code...but something you'll have to look into down the road.

Now you may be seeing mould - or you may be seeing just the effects of (dirt-laden) air movement travelling through this insulation summmer and winter. Usually, we see just that - the remnants of dirty air movement - and this is why we say that air-sealing is more important that vapour- sealing. What you have is a vapour barrier, when an air barrier is really what you want.
"1) Since this is happening in the basement, isn't the same thing happening in the rest of the house because the rest of the house is air conditioned. Or does having sheetrock up prevent enough cold air from directly hitting the vapor barrier then condensaition in the finished walls isn't a problem?"

No; the rest of your house has airsealing built in, by the sheetrock on the inside of your upstairs walls and the housewrap on the outside. And yes, sheetrock - and even the layers of paint - act as an airbarrier too. So upstairs you don't have as much air movement "through the walls" as you do in the basement, so to speak.
"He believed that the mositure was coming from the hot humid air outside coming through the sheathing and condensing as soon as it hits the vapor barrier as the vapor barrier is ice cold from being in the basement."
Not sure I fully agree with your inspector, here...but only minor difference. When you get around to removing that fibreglass insulation, as Windows says, replace it with polystyrene boards stuck directly to the concrete walls, no gaps, with taped joints - or spray foam it once and for all.
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Old 08-31-2013, 10:07 PM   #4
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Wet Insulation due to Vapor barrier?


In basement on the walk-out wood frame wall? If so, the poly should have been covered with drywall right away, or left off. It was installed to meet code at the time of construction. Interior basement air will condense there (due to ineffective sealing), as you found out. Once the mold is in f.g., throw it all away.

Either a Class 2 (asphalt-faced batts) or Class 1 (poly) is acceptable for your location, Figs. 4a-4c; http://www.buildingscience.com/docum...nd-wall-design

The variable-perm retarded mentioned; http://www.bestofbuildingscience.com...3-4_p37-40.pdf

Yes, it does condense on the rest of the house, if the drywall is not air-tight, holes around electrical wall boxes, etc.; http://www.buildingscience.com/docum...wall-approach/

The basement is feeding the "stack effect" of air because of the pressure differences; http://www.wag-aic.org/1999/WAG_99_baker.pdf hence the advice of sealing the rim joists for infiltration.

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Last edited by Gary in WA; 08-31-2013 at 10:11 PM.
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