Wet and Smelly Basement Wall

TeeeGeeeNovember 10, 2013


We have noticed a strong musty odour coming from our basement wall for the past few months. We had our basement finished about 18 months ago (wall parged, delta wrapped, insulated with roxul, vapour barrier, and drywall) and had no smell until about three months ago. We noticed some cracks in the cement outside our house where water was draining into right by the wall with the smell. We decided to have a contractor replace the cement and also dig down and delta wrap the outside of our house. In the process we did uncover an old broken drain that may have been leaking water into this wall. We cut some holes in the drywall and have not found any mold on the studs or the drywall. It seems the smell is coming from the foundation wall itself, from behind the delta wrap. My concern is that because the wall is now delta wrapped on the inside and outside that we may have trapped the water in between. How can we get rid of the musty smell and dry out our wall? Can we do this without having to rip out the drywall and insulation and inside delta wrap? Please help!

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I hope it wasn't an alleged professional who designed this system!

Pause for a second to consider how Delta MS and competitive bubble membranes work as waterproofing on the exterior.

First, the membrane keeps water away from any cracks in the wall. Second, it provides a drainage plane for water, both on the outside of the membrane and on the inside, i.e., the space between the membrane and the foundation wall. But this water has to be removed. If you're in sandy soil, it can drain away down to the water table. Otherwise, you have to provide weepers that drain away to a sump, far off the perimeter of the building or into a municipal storm system of some sort.

If the bubble membrane is used on the inside of the foundation wall, it must be used in conjunction with an internal weeper system. If you just wrap the interior wall with the membrane, all you're doing is trapping water behind it that will eventually find its way into the basement.

As you've discovered, the water problem was best attacked from the outside--excavate, repair cracks, fix or replace weepers, then cover the foundation. The plastic membranes are the simplest, but not the only material that can be used.

Unfortunately, you're still left with a mould-prone wall due to the moisture trapped and /or accumulating behind the interior membrane. And compounding that mistake, you have used an inappropriate vapour barrier and insulation.

Mouldy fiberglass insulation behind vapour barrier in basement wall. Credit: Building Science Corp.

There should have been neither plastic vapour barrier nor fibrous insulation. (The fibrous insulation is only appropriate in conjunction with foam on the interior of the foundation wall.) The plastic vapour barrier traps moisture in the wall when the moisture drive in the summer is to the interior in the above grade portion of the wall. While rockwool itself isn't affected by moisture, any moisture in it supports mould growth.

You may indeed have to take down the wall and rebuild it.

However, before that, try lots of mechanical dehumidification to keep the relative humidity below 50%.

And, if you have to rebuild, take a look at the linked piece from the US Department of Energy.

I built and renovated a number of basements in the '80s and '90s that used the vapour barrier to the inside strategy--indeed, it was Code!--but they were still mould free to most noses as long as mechanical dehumidification was employed from day one.

Good luck!

Here is a link that might be useful: Basement Insulation Systems

This post was edited by worthy on Mon, Nov 11, 13 at 17:36

    Bookmark   November 11, 2013 at 5:21PM
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