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Basement mold and possible solutions–full post!

2014green | Posted in Green Building Techniques on

Whoops – last post didn’t include the details!

I’ve read a lot on this site on this issue and want have a few lingering questions. Thanks for all the info here already!

Background: In Boulder, CO, just remodeled a house. In the basement we put cork on 60-year old slab (we are assuming it is simply slab on grade with no insulation or moisture barrier underneath). Contractor put Whisper Wool (preferred product on this site– on slab beneath cork flooring. Whisper Wool should prevent mold. Well, we got mold. LOTS of mold that accumulated over the last 6-12 months–all over the floor under the cork (it’s a space of about 1000 square feet and it all had mold (not every inch, but major patches all over in every room). No sealant or anything was applied to the slab in construction. We also have a large Evaporative Cooler on the roof for the whole house (so it is 2 floors above the basement).
My questions are:
1) It seems like this is either condensation (my hunch) or seepage from the foundation. (We have checked our grades and our gutters — neither seem to be the cause of excess ground water). How do we know if we have seepage or condensation? Is there some type of pro I can hire to tell me this or get a second opinion? There is a lot of changing groundwater in our town after a big flood in 2013.
2) To address condensation, it sounds like we should put 1 or 2 inches of XPS or EPS foam insulation on top of the slab plus then plywood (read this in Martin’s post about this)….But, we have very little headroom already down there – so I would probably do Delta FL.. Do we do plywood over the Delta FL before the cork?
3) Do we also need a plastic vapor barrier on the slab?
4) Do you think the swamp cooler is part of the problem?We ducted the house for AC so we could switch…
5) In case it is seepage, Martin, in your article on this you recommend trying a sealer. If the problem is condensation this won’t help, right? But if it is seepage it will, right? If we find it is seepage — then a question — you listed these as options — Thoroseal, UGL Drylok, and Xypex. Are those ok for indoor air quality (we went to great pains to avoid things that emit harmful chemicals in the remodel)? I looked at their MSDS sheets, Thoroseal and Xypex looked best but it is hard to know for sure. Contractor recommending UZIN PE 460: which is an epoxy-based primer. It gets a European certification for low emissions called EC-1 Plus, but again I am no expert on what this means. Any views/thoughts on which of these to try–including the caveat of avoiding harming indoor air quality on a tight house?
6) Should we attempt to diagnose whether we have condensation or seepage before we proceed or just go ahead with all of the above?
7) Is there anything I am missing the could be the problem?
8) If these solutions don’t work or we find the problem is more significant, we will/may try adding drainage but obviously this is a much bigger project.
Any thoughts are appreciated!!

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  1. 2014green | | #1

    Also I forgot to say - there isn't a sump pump or perimeter drain in this part of the basement....Oh how I wish we had put that in when we built......

  2. 2014green | | #2

    Having read a little more of Martin's writing on basement walls, I'll just add that I know contractor used fiberglass batts in the basement walls (seems this is a bummer.....) and I am not sure there was a foam backing put against the concrete first. But I will be checking. At this time we aren't aware of mold in the walls behind the drywall, only on the floor, but now I think I'll be checking that too.

  3. Dana1 | | #3

    Average summertime outdoor dew points in Boulder are WELL below the subsoil temperatures, and wintertime dew points even lower. Unless you are actively humidifying the conditioned space air to above 50% RH none of the moisture is condensation from the room air. The deep subsoil temps in Boulder are in the 50s F, and even in summertime the average outdoor dew points are in the 40s, maybe 50F for the month of July:

    That's not enough to create a mold problem, unless the swamp cooler is keeping your indoor RH WAY north of 50% in summer. The dew point of 50% RH/75F air is about 55F, which is about the coolest your slab would be in summer.

    The far more likely scenario is ground moisture coming through the slab. I couldn't find a good spec for the thickness of the polypropylene facer on the whisper wool, or an ASTM E96 vapor permeance spec for the finished product, despite the advertising descriptor as a "moisture barrier". Even though polyproplyene doesn't wick, the wool certainly does, and unless the facer is thick enough to be a solid capillary break, the wool would be effective at spreading the moisture from where ever it could get in. The wool itself would support mold growth if kept wet.

    It's also not a broad sheet product that can be done without seams- how were the seams sealed?

    An inch of EPS and REAL vapor barrier (6-10mil polyethylene or EPDM membrane roofing material) Put the vapor barrier between the slab and the foam or between the foam and the subfloor, but don't put ANY mold food (wood, wool, cotton, etc) under either the insulation or the vapor barrier.

    You mention a basement, but then slab on grade. Which is it? A basement slab is below the grade of the surrounding landscaping, a slab on grade is at or usually slightly above the surface soils.

  4. 2014green | | #4

    Wow- many thanks. This is a basement- sorry I didn't understand the terminology!
    If it isn't condensation, then do we need the foam or just the vapor barrier before the subfloor?
    Any point in the sealants in the concrete?

    Also, we would really like to solve this for the long haul. Should we put in drainage and sump pump? We are even open to a new foundation. Crazy?

  5. 2014green | | #5

    Whoops I meant vapor barrier before the flooring- not subfloor- learning lots of new terms!

  6. GBA Editor
    Martin Holladay | | #6

    Using a basement as finished space is always risky, especially in an older house without polyethylene under the slab. The solution depends on how much money you are willing to spend, and how certain you need to be that the problem doesn't recur.

    This article discusses ways to address the types of problems you are facing: Fixing a Wet Basement.

    One approach is the incremental approach: "Let's try one or two things and see if our basement is better." That doesn't cost much money.

    Another approach is, "Our house was built incorrectly and I want to fix all of its problems, no matter what it costs." For example, you can lower your floor if you can afford the work.

    I suggest that you read my article and determine how far you are willing to go with this type of work.

    One other possibility is to abandon the below-ground room and build an above-ground addition.

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