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What’s best method of converting a “semi-vented” crawl space with a rat slab w/no vapor barrier into an unvented crawl space?

Stephen Mager | Posted in Green Building Techniques on

Wondering how best to handle the crawl space of a tri-level in upstate NY (built in late 70’s). The house is “semi-vented” (2 vents on the east wall half covered by mulch & soil on the exterior and 2 vents going into a garage on the south wall that have been blocked off with wood). Entrance to the crawl space is provided by 2 common doors from the lower level. The house is heated with hot water baseboard, and currently has fiberglass insulation between the joists… which plays out just like you’d think – the insulation is very wet throughout the crawl space. In some of the areas with the least amount of circulation, there is evidence of critter damage due to the moisture, no signs of mildew/mold on the joists/sub-flooring. The crawl space has a poured rat slab which is in good condition, but DOES NOT HAVE A VAPOR BARRIER.

With all this, should I convert it to an unvented crawlspace, insulating the side walls and rim joists and adding a means to permit air to move between the living space and crawlspace? Reading every Joseph Lstiburek article I could find, I believe I need to seal the rat slab (somehow). I really don’t want to use a traditional vapor barrier over the slab (crawlspace is used for storage, etc.), so I’m thinking I need an epoxy or other means to provide an adequate vapor barrier to convert to an unvented/conditioned crawlspace. Per NY code, for an unvented crawl space, “exposed earth is covered with a continuous Class I vapor retarder”.

Alternatively – I may just correct the ventilation (clearing the vents directed into the garage and partially covered by ground cover) and throw foil faced rigid insulation under the floor framing (ensuring it’s sealed), and keep it vented for simplicity.

Looking for insight on how to handle the rat slab if I were to convert to to an unvented / conditioned crawlspace, or for a qualified, “take the easy path and keep it vented with improved insulation under the joists instead of between them”…

Thanks for the help!

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  1. GBA Editor
    Martin Holladay | | #1

    The right way to proceed is to create an unvented crawl space. This article explains the steps you have to take: Building an Unvented Crawl Space.

    If you are correct that there is no polyethylene beneath the existing rat slab, you have two choices:

    1. Install a layer of polyethylene on top of the rat slab, weighted down by bricks (and perhaps covered with scraps of plywood or OSB or old pallets if the homeowners want to use it for storage).

    2. Install a layer of polyethylene on top of the rat slab, and pour a new 2-inch or 3-inch rat slab on top of the polyethylene.

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