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Community and Q&A

Reducing heat loss and moisture in a stone basement

Rob Wotzak | Posted in Energy Efficiency and Durability on

I’m more concerned with moisture than I am with heat loss. I have a full basement in my 140-year-old house. The floor is crumbling concrete; the walls are limestone up to grade and brick above that. I’ve gone around the sills, doors, and windows with spray foam and taken care of much of the air infiltration. I still need to deal with getting gutters to drain farther from the foundation and I could stand to do some grading around the house. The yard is pretty much flat but it wouldn’t be too much trouble to drain gutters to nearby hills. The soil is coarse sand.

What’s the problem? The basement is dry all winter but starts to get so humid this time of year that I can’t stand to be down there for more than a few minutes; there’s serious mold growth – I can feel it when I breath. The more humid it feels, the more damp patches there generally are on the floor – but they’re never very wet and are not near the walls. My first impression is that the water table is rising and seeping through the old slab; or could it be condensation? I can also tell which parts of the walls have routinely been damp because of spalling limestone and brick – it’s not all over the wall, but in isolated patches here and there. Only once in five years has water visibly come in through the walls and this was due to a blocked up gutter and soil erosion near the foundation.

What’s the solution? I’m pretty comfortable dealing with the floor (EPS, poly, and a new slab on top?), but I’m worried about any of my options for the walls because I’m not sure if any of the interior insulation/wall systems I might use would trap moisture and increase the spalling. This makes me think that my first line of defense for the walls should be on the exterior. I’m prepared to dig down a foot or two (in most places) if some sort of water shedding system would be ideal (poly and XPS pitched away and buried?) and I realize that I should get the downspouts farther away from the foundation. But I would really like a solution that allows me to fully insulate and air-seal the whole space and still be confident that I’m not compromising the durability of my foundation.

What would you do?

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Replies

  1. Michael Chandler | | #1

    Rob

    First you need to be absolutely sure that the moisture is not coming from the air. We had a very similar issue with a house that had a high water table and we had put in a french drain and sump and still had excess humidity. I ran a dehumidifier into the sump for a while and dried it out and then sealed up all the outside air ventilation and covered the sump and it stayed dry. the issue was that the high water table was creating a cool wall situation that was causing outdoor air to drop it's moisture content as condensation on the walls that the homeowner was sure was water seeping in through the concrete.

    My recommendation to you is that you run a dehumidifier for a while with the ventilation to the outside shut down to get a better grasp of if and where you may have seepage from below.

    If it does turn out that you have a seepage problem then you can deal with that with a perimeter drain and sump. If it's a condensation problem then you can go ahead and insulate the walls and/or install a dehumidifier on a humidistat combined with air infiltration control should solve the problem.

    Assuming that your basement is below grade you may want to look at the WindChaser dehumidifier/AC that exhausts hot, moist air through a dryer vent hose rather than dumping water through a garden hose and circulating hot dry air back into the basement like most straight dehumidifiers. This is what I use for my storage bldg to keep my stuff from getting moldy. It has a built in humidistat and can vent through the ceiling if necessary. cost is about $350 to $400 (at MallWart - ugh) or http://www.nextag.com/WindChaser-Fedders-A6P09S2B-Air-553937183/prices-html?nxtg=110da0a240516-C2F6A00D4E001BC3

  2. GBA Editor
    Rob Wotzak | | #2

    Thanks Michael,
    I'll do a little more investigating to try to pinpoint the moisture source. If it does seem to be condensation and not seepage, I'm still worried about how best to insulate the walls. I'm not completely confident in my understanding of what conditions will or will not cause the limestone and old bricks to degrade. If I seal them up from the inside I'm afraid that I'll lose sleep because I'll no longer be able to monitor the condition of the wall. Can you help me wrap my head around the science of the situation?

  3. Michael Chandler | | #3

    Rob

    I agree that you want to maintain an inspection strip to monitor conditions and you need to have a pathway to remove any moisture that might accumulate between the wall and the insulation. In NC we would also need an inspection gap to monitor for termites, probably not an issue in Vermont.

    If you do find that the problem is condensation then closing up the air leaks and warming up the surface of the wall that is exposed to the air should solve your problem. I'd still keep a humidistatically controlled dehumidifier down there but I'd tack a J-channel to the walls about three inches off the floor and stack two layers of 3/4" EPS foam with offset joints on top of it. this gives you a place to quickly check for moisture issues but also gives you a warm interior surface that won't support condensation and keeps air away from the cold rock.

    If it's seepage from the ground water you might want to try an approach similar to a Frost protected shallow foundation. excavate a french drain about two to four feet away from your foundation and then scrape away the soil between the trench and the house to make a sloped shoulder leading to the drain, line this all w/ six mil poly and insulate the shoulder with two layers of XPS leading into a washed gravel bed with a perforated drain tile and filter fabric sock all draining to daylight or a reliable sump pump w/ alarm on a separate circuit. this helps keep the soil around the old stone foundation warm and dry and should help with the seepage.

    Our motto on foundation waterproofing is "install the best waterproofing you can afford and then DON"T LET IT GET WET".

    Surface and subsurface drainage to divert water from landscape and roof away from the foundation is the best bang for the buck by a long shot.

  4. GBA Editor
    Rob Wotzak | | #4

    Michael,
    I think had a pretty good idea of what to do but you filled in all the gaps and gave me the confidence to move forward. Regarding your comment on surface and subsurface drainage, I think your philosophy applies to many sitiuations - you have to understand the whole problem, but the simplest or most direct solution is often the best.
    Thanks again!

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