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Mold/Mildew


Robert E Lee
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In the morning I am returning to look at a home (2007 vintage) I inspected this past summer. The client called with a concern about what he considers to be mold/mildew behind the vapor barrier in an unfinished area of the lower level. I said that I would be happy to come out and take a look, but what I expected was that condensation has collected on this vapor barrier (we had some very hot humid days this past summer) and what he is seeing is very typical and would likely be present in many areas of the house if we were to pull down all the sheetrock. My intention is to suggest he consider removing the vapor barrier and fiberglass insulation and replace it with closed cell foam insulation. Any other thoughts? Thanks in advance.

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In the morning I am returning to look at a home (2007 vintage) I inspected this past summer. The client called with a concern about what he considers to be mold/mildew behind the vapor barrier in an unfinished area of the lower level. I said that I would be happy to come out and take a look, but what I expected was that condensation has collected on this vapor barrier (we had some very hot humid days this past summer) and what he is seeing is very typical and would likely be present in many areas of the house if we were to pull down all the sheetrock. My intention is to suggest he consider removing the vapor barrier and fiberglass insulation and replace it with closed cell foam insulation. Any other thoughts? Thanks in advance.

I just recently had my first opportunity to watch several issues of Mike Holmes. It was closed captioned for the first time so I was able to understand it. The first episode had an identical situation - condensation behind a vapor barrier (0 perms) installed behind interior sheetrock. This was in a northern climate like yours.

Holmes said the cause was a lack of adequate ductwork to the room and insufficient r-value in the wall cavities which sounded far off course to me. But anyway, what's a vapor barrier doing behind the interior sheetrock of an exterior wall in an 07' house that's situated in MN? Shouldn't that be a vapor retarder instead.

Marc

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Marc,

In a northern climate the vapor barrier is placed on the interior to keep warm moist air from entering the wall cavity and condensing and freezing there. It doesn't work. There are far too many penetrations in the barrier to keep all the moisture on the conditioned side of the wall. In an old house with a ton of other leaks in the wall system there is enough air movement to dry out anything that makes it past the barrier. In a new house there is very little air wash in the stud cavities and any moisture that finds its way there tends to accumulate and create issues. I fully expect to see older homes exhibit these problems as more retrofit programs take hold and people start air sealing stuff that really shouldn't be.

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Marc,

In a northern climate the vapor barrier is placed on the interior to keep warm moist air from entering the wall cavity and condensing and freezing there. It doesn't work. There are far too many penetrations in the barrier to keep all the moisture on the conditioned side of the wall. In an old house with a ton of other leaks in the wall system there is enough air movement to dry out anything that makes it past the barrier. In a new house there is very little air wash in the stud cavities and any moisture that finds its way there tends to accumulate and create issues. I fully expect to see older homes exhibit these problems as more retrofit programs take hold and people start air sealing stuff that really shouldn't be.

Understood and thanks for your explanation. One question: If interior moisture gets thru this vapor barrier, which is installed behind the interior sheetrock, why would it condense just past the barrier instead of proceeding towards the exterior? Remember, the barrier is the primary restriction on the flow of moisture thru the wall and this moisture that we're talking about has already made it pass this barrier.

Marc

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In my climate, north of the Pacific Northwest, we have been installing a polyethylene vapor barrier on the inner side of the exterior walls and ceilings for 40 years or so. Since the 70's, the exterior walls have been usually 2X6 with fiberglass batt insulation. Problems with indoor moisture accumulating in the stud cavities are rare here. The vapor barriers do leak, but there are minimal problems with that, AFAIK. I think it may be because indoor heat helps to dry the cavity by pushing the moisture out? The outer siding has to breathe.

In recent years, we have been getting more diligent with sealing around electrical boxes with gaskets and tape, so the newer houses have less air movement through the walls. We use the Tyvek-type wraps, but there's still a lot of good old building paper being used on the exterior.

Mike Holmes is in Ontario, colder winters, so the building techniques seem to be slightly different there. Maybe a lack of heat in that room was preventing evaporation? There has to be heat on the interior wall, and that was probably lacking there. Without the heat, the drywall over poly becomes a mold factory.

Robert, if that area in your client's house is unfinished, could there be insufficient heat or air circulation in that part of the house?

I think if that area was heated, the moisture would not have been trapped in the wall like that. But what works in my climate may not apply in yours. Let us know what you find.

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In a tight assembly it does move to the exterior, but once there is condensation on the sheathing the insulation touching it becomes wet, the temps in the cavity drop and the dew point creeps inward. This is a little overly simple but you get the idea.

The reason this is a problem in a 2007 house and not a 1997 house is our energy code. Every pipe, cable, conduit that penetrates a top or bottom plate must be sealed, the exterior envelope is taped up tight, but there are no gasket requirements for anything that penetrates the GWB. Water vapor is introduced into the wall cavity at every electrical outlet and plumbing connection in the building and has no way to escape. Screw up one little detail in the tape or WRB and that water vapor is drawn into the wall.

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The reason I mentioned the hot humid weather this past summer in my original post, is that I believe the slight staining that is present on the exterior side of the vapor barrier is the result of condensation that has collected within the wall cavity. Cool basement, hot/humid outdoors, when that humid air encounters the vapor barrier, condensation forms and mold/mildew follows. I don't consider it a big problem, in fact as mentioned earlier I think this is a condition that would be found through out the building if the sheetrock were to be pulled off.

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