The Best Crawl Space

[John B]

Perhaps the best crawl space is NO CRAWLSPACE
Isn't that what Joe L. has said ?

Assuming that a slab on grade is a good strategy.
Is it ALWAYS better to have a vapor barrier under the slab?
Is it possible that in certain climates that there may be an advantage to NOT have a vapor barrier below the slab?

[mrd999]

With a vapor barrier, the moisture conditions of the ground won't affect the moisture levels in the home. Without, moisture will migrate one way or the other depending on which has a higher moisture level.

If the moisture level of both the ground and the home are predictable and compatible, then it's possible the absence of a vapor barrier could be beneficial. For instance, in a cold climate with constant heating, indoor humidity may be uncomfortably low. Ground moisture might help to increase indoor humidity and improve occupant comfort.

All sources I've read recommend a vapor barrier for any climate, so perhaps there are too many variables to make such a situation workable.

[inspector pete]

In areas with frequent flooding, there's not much point to having a vapor barrier under the slab. We still use a slab floor sloped to a drain or a sump, but for this situation, you really don't want the insulation/vapor/moisture controls on the floor. The only choice in such houses is to insulate the floor frame (or outsulate it), use a good vapor management technique, and ventilate the crawl.

[John B]

that's 2 good explanations and possible advantages.
Are there more?

[NW Architect]

Sloping sites = crawl spaces
Ease of future utility repair or additions = crawl space

[John B]

Any more porous slab advantages?
You may need to think outside of your climate

[Riversong]

Assuming that a slab on grade is a good strategy.
Is it ALWAYS better to have a vapor barrier under the slab?

Except in the absurd case mentioned by Inspector Pete (of building in a flood zone), the only place where a VB would be useless and unnecessary is in the desert, building on sand.

Don't forget that a sub-slab vapor barrier (properly installed) is also a radon barrier, and radon is found everywhere unpredictably.

[John B]

Except in the absurd case mentioned by Inspector Pete (of building in a flood zone), the only place where a VB would be useless and unnecessary is in the desert, building on sand.

Don't forget that a sub-slab vapor barrier (properly installed) is also a radon barrier, and radon is found everywhere unpredictably.

My understanding is that only a few counties in Texas have radon issues.
Related to Geology... and rather predictable.
That may be some more fractured BS that I picked up in Texas?

I am looking for an advantage that a permeable slab might have in a hot/mixed humid climate
Say no to ALL vapor barriers? No?
Vapor control good... barrier bad?

[Shophound]

Hot/mixed humid climate? Would not even consider omitting a vapor barrier from a concrete slab. Especially if wood based flooring, or carpeting, or vinyl based flooring, or ceramic tile, or throw rugs, or just about anything else is planned to brighten up the dull gray of a concrete slab. There's also the matter of interior sole plates resting on a slab with no vapor barrier, and should any part of the HVAC system suck on the slab (such as an upflow furnace mounted in a closet with a void underneath serving as a return air plenum), I can't see that as being a good thing.

Vapor barriers in slab foundations = non optional in my book. Have you read Lstiburek's articles about California slabs where sand is placed between the vapor barrier and the concrete of the slab? Man o man. <shakes head>

[Riversong]

My understanding is that only a few counties in Texas have radon issues. Related to Geology... and rather predictable.

Most of Texas is in a low probability zone, but there's no way to pinpoint which building sites and which homes will have radon problems based on generalized maps (see attached).

Both the EPA and WHO recommend universal precautions on new construction - always use a condom under the slab and watch out for over-pressurization ;-)

Vapor control good... barrier bad?

I think we should stop calling the subslab membrane a vapor barrier, since it is primarily a capillary break. Small amounts of soil vapor are unlikely to be problematic, but capillary conduction of liquid water most certainly will. The theoretical vertical capillary limit for concrete is 6 miles. The stuff really sucks.

For the same reason, I never pour a concrete foundation wall atop concrete footings without a capillary break between them. [Great article on Capillarity—Small Sacrifices]

[John B]

I knew you guys would think I am nuts.
Yes I have read Lstiburek's article.
Have you ever heard Lstiburek list his failures?
I can not take credit for this "absurd" idea.
Let's just say someone well respected has suggested such a possibility.

[Riversong]

Have you read Lstiburek's articles about California slabs where sand is placed between the vapor barrier and the concrete of the slab? Man o man. <shakes head>

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[mrd999]

How about using this instead of a vapor barrier under the slab..? Though I suppose it's cheating, as there's still a vapor barrier present.

http://www.moxie-intl.com/moxie1800.htm

[John B]

With a vapor barrier, the moisture conditions of the ground won't affect the moisture levels in the home. Without, moisture will migrate one way or the other depending on which has a higher moisture level.

If the moisture level of both the ground and the home are predictable and compatible, then it's possible the absence of a vapor barrier could be beneficial. For instance, in a cold climate with constant heating, indoor humidity may be uncomfortably low. Ground moisture might help to increase indoor humidity and improve occupant comfort.

All sources I've read recommend a vapor barrier for any climate, so perhaps there are too many variables to make such a situation workable.

MRD,
I thought your explanation was very well said.. except I have an opposite perspective.
The Cooling season and excess humidity are my bears.
What if there was a way to reverse the flow?

A true BS Artist has offered me this intriguing nugget:
"Foundations can, in theory, dehumidify"

Please,
Try not to imagine the typical slab and typical site conditions.
Imagine the not-so-typical.
A permeable slab and a Site with soil that is extremely well drained.
An enclosure that by design has a dry (shielded from rain and run-off) earth perimeter.

[Riversong]

How about using this instead of a vapor barrier under the slab..?

If I'm going to go for the Moxie, I prefer this.............................................. ....