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How to treat wet sandy cottage crawlspace in zone 5A/6A?

user-36575 | Posted in Energy Efficiency and Durability on

Situation: cottage on the sandy shores of a large lake in Michigan on the border of climate zones 5A and 6A. The cottage is set back from the lake front perhaps 200 feet, but is probably less than 2’ above lake level. The footings are likely below lake level. To make things worse, there is a natural drainage swale running behind the cottage that some good neighbors have effectively dammed, so there is permanent standing water about 100’ behind the cottage.

So of course the cottage is built with a crawlspace, with a sand floor and concrete block walls.
Other details: well and septic, baseboard heat, no a/c or dehumidifier. There are about 20 air vents around the crawlspace perimeter, but most have been closed. The cottage is infrequently used at this point. Perhaps 15 years ago, moisture was an issue (shocking), and the cottage was raised perhaps 2’ so that there is now roughly 40” of crawlspace wall exposed. From the times that I’ve been into the crawl, I’d say the sand floor is damp but only occasionally has a small amount of standing surface water in a few spots.
The cottage is a special case for moisture because the air is unconditioned (humidity and temperature are not controlled as in a full-time house) and because the crawlspace is very wet and closely coupled with the rest of the house.

Conventional crawlspace wisdom says to seal up the crawlspace and bring it into the building’s conditioned space. Add a sump pump, and you should probably insulate the crawlspace walls while you’re at it. And then there’s code for conditioning unvented crawlspaces, requiring connecting the crawlspace to conditioned upstairs via transfer grille and then either adding an exhaust fan or a forced air register. Not that we ignore code, or promote ignoring code, but let’s ignore code for a moment.
The problem with this cottage is that there is in fact no “conditioned” air space, other than a bit of baseboard heat during the winter (thermostat set to 50F).

The specific question: is sealing the crawlspace, by itself, worth doing? If we sealed the crawlspace walls and floor with a 20mm plastic liner and did a complete job of sealing the vents, but didn’t add a sump pump (we can’t drain the lake), and didn’t add a 100 pint/day dehumidifier, would the liner improve things significantly? And by that I mean slow decay of the building?

Thanks for your thoughts,

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  1. Jon_R | | #1

    Raise the crawlspace level to the point where you don't get standing water. Then air seal, insulate and line with plastic. Then either:

    a) dehumidify the crawl space. This is best and won't cost much (because most moisture sources have been blocked). Or

    b) maintain lots of airflow to the outside. The crawlspace insulation and plastic will reduce moisture levels from what you have now.

  2. user-36575 | | #2

    Thanks Jon.
    I could potentially raise the crawlspace floor by adding sand and/or leveling out what's down there now. Is there really any harm that would come from a bit of standing water underneath a fully sealed liner? This is not an area that will see any foot traffic or storage.

    I kinda suspect that your option b) wouldn't be great because it's not drawing conditioned air down from above, it's just filtering unconditioned outdoor air thru the house. So you're probably right about the dehumidifier being a best option.

    I still would like to know if we can get away without a humidifier, or if we're wasting the liner without it.

  3. Jon_R | | #3

    I think you will find that while the plastic is a good vapor barrier, it isn't a reliable water barrier. But there may be other ways to raise the plastic that are easier than sand fill. Perlite?

    The liner is useful even without dehumidification. I don't know exactly how effective b) would be - you could try it with dehumidification as a backup plan.

  4. GBA Editor
    Martin Holladay | | #4

    I'm going to second Jon's suggestion of raising the crawlspace floor -- I would use crushed stone, but I suppose sand would work -- and then installing a heavy polyethylene vapor barrier. Those two steps may be enough to dramatically improve the crawlspace. You can monitor the situation after these changes are made -- you may not need a dehumidifier.

  5. user-36575 | | #5

    Thanks for feedback Jon and Martin.
    I will try to figure out exactly how high the water has gotten in the past and then see how large a task I am facing to raise the crawlspace floor.
    Will also have to look for some ideas for moving building materials (gravel, sand, perlite, other?) around in height constrained places.

  6. user-6623302 | | #6

    Pumped in concrete or flowable fill perhaps. Exterior drains? Were are the mechanicals/ water supply? Unless you need to condition all or some of the crawspace, I would go with drains and max ventilation. Insulate at the floor level.

  7. Expert Member
    Dana Dorsett | | #7

    Even at a 50F conditioned space temp it's worth insulating the crawlspace walls in your climate, preferable to insulating between the joists. (That's especially true when heating with electric baseboards.) Insulating between the joists lowers the temperature of the joist edges in the crawlspace, increasing the need for mechanical dehumidification, and increasing the likelihood of frozen plumbing. Insulating the crawlspace walls raises the temperature in the crawlspace, mitigating those risks.

    A couple inches of fire rated rigid polyiso glued & capscrewed to the CMU wall is a relatively easy DIY. If using polystyrene or unrated polyiso code would demand a thermal barrier against ignition between the foam & crawlspace, which adds to the time & materials expense. If using polyiso, be sure the floor vapor barrier laps up the side of the wall, between the wall foam & CMU.

    If air conditioning is ever being considered, a cold climate mini-split heat pump can cut the wintertime heating bills by more than half (and the occupied shoulder season heating bills by 2/3 or more, if using the space during the shoulder seasons.) Both Fujitsu & Mitsubishi can be set up to maintain ~50F even though that is below the the lowest heating setpoint in their normal heating mode. Fujitsu has "MINIMUM HEAT" mode that runs about 50F, Mitsubishi can be set as low as 50F using their "SMART SET" mode, which isn't obvious until/unless you've read & absorbed everything in the user manual. A cold climate heat pump is a considerable upcharge from a window-shaker AC, to be sure, but not a ridiculous up-charge from a warm-climate heat pump or cooling-only mini-split. Even a warmer-climate mini-split would still save money during the heating season, but may not have sufficient capacity at the 99% outside design temperature even for the load at 50F indoors, if sized for the cooling load, and may incur defrost ice build-up damage in the bottom pan of outdoor unit if used all winter. (Cold climate mini-splits have pan heaters to automatically avoid that problem.)

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