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Community and Q&A

Moisture barrier between crawl space and seasonal living space

Grant Robinson | Posted in Energy Efficiency and Durability on

Our 50 year old 3 season cottage (Kincardine Ontario) on the shore of Lake Huron is uninsulated and closed from mid October to mid April. In the summer time it is pretty much wide open except when the rain and wind are blowing sideways.
It is built on a sand dune and until 3 years ago it had no crawl space whatsoever. The 1×4 t&G pine floor sits atop 2×8 floor joists which were laid on sill plates bolted directly to the strip footings. There was only 3 inches air gap between bottom of joist to sand. Although the footings are at surface level, owing to the free draining sand there is no vertical movement on account of frost effect. Three years ago the cottage was raised and 4 rows of blocks put on the fittings to yield a 32″ crawl space. About a dozen punky joists were found and sistered. Budget spent we stopped at that.
I now want to address the damp rising from below. As there is no insulation, no central heat, I think it safe to say there is no conditioned space; however, it is to be acknowledged there can be thermal difference between the top of the floor and crawl space on a hot summer day. There are 2 vents on the front and 2 east on the back of the building. On windy days you can feel the wind on your hand so I know that air is getting sucked in on one side and blowing out the other. The main goal is to stop rising dampness as opposed to providing any insulation value. Through the various options I’ve read from numerous searches I like the idea of placing 2’x8′ foil backed foam boards directly onto the underside of the joists, sealed with tape and foamed tight to the sill plates around the perimeter. My assumption is that with the air/moisture seal at that level the crawl space can be left to vent via the vents.
Or am I all wet? Should we be placing poly on the sand, up the walls and sealing it to the sill plate or rim board somehow? But if I seal it on sand and up walls I assume we close off the vents too?
(The in-laws are questioning the necessity as after 50 years they really don’t see it a big deal. Probably because the place is so porous whatever rises up through the floor vents itself out anyway).
Any words of wisdom would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    1. At a minimum, you need to install a layer of 6-mil polyethylene (or, if you prefer, a thicker type of vapor barrier that holds up better to physical abuse) on top of the sand floor. The polyethylene should cover the sand, but need not go up the walls.

    2. If you weigh down the polyethylene with a few bricks or stones, you're done. Taping the seams of the polyethylene, or taping the polyethylene to the walls, is possible but not necessary.

    3. Your suggestion to install rigid foam on the underside of the joists isn't a bad idea, but may not be necessary. If you want to do it, use foil-faced polyisocyanurate, and tape the seams.

    -- Martin Holladay

  2. Grant Robinson | | #2

    Thanks for response. The 6 mil poly solution certainly costs out cheaper and now that we finally have visibility on all the joists from below I am reluctant to cover them all up again. Further, your advice that taping/sealing to the wall is not essential makes poly a very easy install though I can't help but wonder if moisture might not migrate and rise up around the perimeter of the plastic at the wall?
    Your response also day-lighted that it is not necessary to go all way up the block wall with the poly. The one outstanding question I have now is should we run the poly up to match the ground grade on the outside of the wall? While the wall is only 4 blocks high, to assure drainage away from building we placed fill material half way up on the outside and sloped it away from the building. Is there need/benefit to placing the poly on the inside to match the ground grade on the outside? (Or is it negligible influence relative to the 700 sq ft of sand floor on the inside?)

  3. GBA Editor
    Martin Holladay | | #3

    Don't overthink this. Most of the moisture is evaporating from the sandy floor. If you cover 95% of the floor with polyethylene, you stop 95% of the evaporation.

    Run the poly up the wall (or halfway up) if you want, but don't worry too much about it either way.

    -- Martin Holladay

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