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Community and Q&A

Is a rat slab necessary?

rodrob15 | Posted in General Questions on

I’m building a cabin in the NC mountains with a conditioned crawlspace. I’m trying to figure out if it makes sense to spend the extra money on a rat slab or not. In my situation, the benefits seem rather minimal. In terms of the crawlspace layers, I’ll definitely have a 4 inch base of crushed stone, then 4 inches of rigid insulation (R20), and then a premium 20 mil polyethylene vapor barrier.

I’m under the assumption that a rat slab mainly has the following benefits:
1. Keeps water from percolating up from the soil which prevents mold & water damage under the house (better indoor air quality)
2. Creates a cleaner underfloor environment & makes crawling under the house easier
3. Helps keep rodents and termites from getting into the house
4. Provides a drainage plane to the crawlspace drain.

For point (1), I’m thinking that a rat slab might not have much benefit at reducing percolation since my plan of 4 inches stone, rigid insulation, and 20 mil vapor barrier should already do a good job of that.

For point (2), I honestly don’t care that much about aesthetics and I also think that the 20 mil vapor barriers create a pretty durable & clean environment to move around in.

Point (3) and (4) definitely seem like they could be beneficial depending on building circumstances, but lets just assume that I’m building on free-draining soil (no crawlspace drain) and there’s no termite issues.

Please comment on if I’m missing any of the Rat Slab benefits. Here’s my basic questions. First, are there circumstances where a rat slab is purely overkill and doesn’t provide much benefit (like i’ve layed out above). Second, is the 20 mil poly sheet a decent substitute for a 6 mil poly sheet + rat slab. I’m under the assumption that the 20 mil poly sheet is fairly rigid and won’t tear easily. Also, termites don’t consume plastic, so that could seem like an effective barrier too.


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  1. GBA Editor
    Martin Holladay | | #1

    Most crawl spaces don't have a rat slab. It's optional, not required. For more information, see Building an Unvented Crawl Space.

  2. seabornman | | #2

    Other advantages - less damage during construction and a stable, clean surface to build on.

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