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Community and Q&A

Insulation & vapor barrier in crawlspace foundation with water tanks

MudBay | Posted in Energy Efficiency and Durability on

Hi everyone,
I recently bought a 1.5 story unfinished cabin in the temperate rainforest of Southeast Alaska (Climate Zone 7; 55 inches precip/year; typical temps: 50-70 in summer, 0-38 in winter). The walls and roof are very well insulated, but the 1st floor is completely uninsulated. The house sits on a concrete perimeter foundation with a 4 foot crawlspace, over well-drained, hard gravel ground. The concrete walls of the foundation are insulated with rigid foamboard insulation, and the floor of the crawspace is the bare gravel ground, which actually stays very dry. Most of the 1st floor is just plywood subfloor, although I recently installed solid hemlock T&G in the kitchen.

I’m planning to insulate the floor with rigid foamboard insulation, but in my research, I’ve found a number of articles recommending a vapor barrier on the floor of the crawlspace, to seal out moisture from the ground. This makes sense, but my situation is a little more complicated, because this is a dry cabin (no well or municipal water), and I intend to set up a rain catchment and graywater system for future indoor plumbing (I’ll keep my outhouse but install plumbing for a Paloma shower, kitchen sink, and laundry). I have 4 water tanks in the crawlspace with 1200 gallon capacity, and once my catchment system and plumbing are set up, I’ll be storing water in those tanks under the house. This makes me question whether I should install a vapor barrier on the floor? I’m concerned about sealing moisture from the water tanks into the crawlspace. There are two passive vents that vent to the outside. Should I consider installing a vapor barrier at the top of the crawlspace (under the 1st story floor) instead? If so, would I install that above or below the insulation? Should I cover the perimeter walls, too?

Thank you for your input!

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  1. GBA Editor
    Martin Holladay | | #1

    I assume that these are sealed water tanks, right? They aren't open to the atmosphere. If my guess is correct, and they are sealed, then they won't be contributing moisture to your crawl space unless they leak. Hopefully they will only leak once every 40 years or so.

    I would seal up the crawl space vents -- you don't want anything to freeze down there on those zero degree nights -- and install a layer of 6 mil polyethylene on the crawl space floor. If a water tank ever springs a leak, you can cut a slit in the poly and let the water drain, and then tape the slit.

    Here is a link to an article that explains how to detail your crawl space: Building an Unvented Crawl Space.

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