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Most resilient or maintainable air-sealing approach?

I'm in Austin TX, where the soils are expansive clay. This means that as the soils expand and contract with different moisture levels the foundation and walls move a fair amount. This is particularly true for wooden pier and beam construction but also for slab based construction.

I realize that one option is to attempt to control movement by "watering" the foundations and an advance version of that would be to monitor the water content and attempt to maintain it at a near constant level. This is costly, both in materials and in water (but might be advisable).

Regardless, all around town one constantly sees cracked dry-wall, suggesting that the air-tight drywall approach might not be appropriate in an area with much shifting. Of course taping sheathing, foam and taping house wrap etc also appears likely to suffer from movement, as does spray foam. I wonder if the liquid applied air-barriers are significantly more resilient to movement and shifting, given that they have some flexibility?

I wonder whether this question applies to new construction more generally, given the tendency of the construction lumber to dry out over the first few years?

A second, related (perhaps better) question would be: what air tightness approach is most maintainable? On that front the air-tight drywall seems better (although it is a finish and requires spackle, paint etc, it is at least accessible for maintenance). Are any air-tightness approaches more maintainable than air-tight dry-wall? Certainly spray foam seems unlikely to be maintainable, as do any approach that is "buried" in the wall.

Asked by James Howison
Posted Mon, 12/09/2013 - 13:48

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5 Answers

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1.
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James,
I'm not an expert in expansive clay soils. But if you expect your house to move so much that the drywall is likely to crack and the tape you install on your plywood sheathing seams will be ripped apart, here's my advice: water your foundation.

If your house moves that much, you will have all kinds of problems -- more than just air leaks. I suggest that you talk to a soils engineer or a structural engineer and choose a foundation system that doesn't move.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/09/2013 - 15:48
Edited Mon, 12/09/2013 - 15:51.

2.
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Hi Martin,

It's not drywall per se that cracks, just joints (although sometimes some cracked drywall around doors, I think). And I don't know about sheathing and tapes, regular movement might not be enough to affect them, hard to see under all that siding ;)

In the spirit of including a diversity of areas in the discussion, this is a problem that everyone has. Here in Austin (and in other places with similar soils) there's only one real way to avoid movement like this, which is to sink piles all the way to the bedrock. That's expensive, but some of the high quality builders here in Austin do do it (e.g., Matt Risinger http://risinger.blogspot.com/ ), even when that's 30-40 feet (when the customer will pay, which is not always). In many places you simply can't go that deep, so you are going to be dealing with movement.

The thing I've never grasped is how watering a foundation is meant to work reliably; I suspect that in drought summers and wet periods the overall moisture levels change quite a bit. This Old House did a revisit on their Austin project that found some movement, even with the watering system installed. http://www.thisoldhouse.com/toh/tv/video/0,,20616669,00.html

Anyway, advise noted; I wonder if there's any long-term data on resilience and maintainability of air-sealing systems. I continue to think that the drywall approach has the advantage of being "user-serviceable" as it were.

Answered by James Howison
Posted Mon, 12/09/2013 - 21:12

3.
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James,
I can imagine several possible solutions to the problem, but I am not a foundation expert or an expert in expansive clay soils. Obviously, you have to listen to local advice on the best approach to building foundations in your area.

Research is now underway to gather data on whether envelope airtightness readings are consistent and persistent over the years. It will be interesting to learn what the researchers discover.

If you want an air-sealing tape that is particularly flexible, I recommend Pro Clima Tescon No. 1.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 12/10/2013 - 08:56

4.
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I was at a friend's 2 year old house in Phoenix and was amazed at how nonchalant they were about cracks in the drywall. It seems that it is just a given that there will be movement. They said their builder comes back in at 1 year and does drywall repair, then it's up to the HO. Neighbor next door who was in framing stage had the same expectation. It seems it's just cultural in that area to have cracks. They call the soil caliche, and it has the consistency of flour. Maybe that is a misnomer, I thought caliche was hard. It must be expansive or have expansive clay mixed in whatever it's called.

Anyhow, here in Colorado we have expansive soil as well. We either overdig, backfill and compact with good soil or drill and pour caissons. I've gone much deeper than 40 feet to reach bedrock. The cost difference between the two methods is a tossup depending on how far you have to haul the junk soil and how far away good backfill is. More often than not overdig/backfill is less expensive, but pier/beam has it's place. Obviously, both methods are more expensive than good native soil, usually 2X-4X cost for excavation/drilling/foundation. I engineer everything, so no cost difference there.

Around here, cracks in drywall are a structural problem. I wouldn't build unless I had a good soil/excavation/foundation plan that addresses the soil issues. That also solves the air sealing issue---two birds with one stone! lol

Bill

Answered by Bill Costain
Posted Tue, 12/10/2013 - 16:35

5.
Helpful? 0

I expect there's a benefit to a monolithic foundation rather than separating the slab from stem walls / footings. If the whole building can ride the shifting soil like a boat, it might stay free from cracks. There could still be issues with plumbing coming out of the ground if the movement is dramatic enough.

Answered by TJ Elder
Posted Tue, 12/10/2013 - 20:10

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