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Amercian clay and water (wall plans)

We're in Austin TX (Zone 2). The house has decent eves (3') but lots of high gables. Our climate is mostly dry, but with some humid periods and rain comes in large, sometimes driving, storms.

Our current wall profile (probably 50 years old) and, from inside -> out:

Concrete block (8") -> Stucco (very, very hard) -> Paint (lead based).

We have some newer sections with just concrete block, parged with Type S, unpainted. These were windows we made smaller.

Eventually we want this profile:

American Clay -> Sanded Primer -> Concrete block (8") -> ? -> board insulation (most likely Roxul 80, but possibly foam) -> 3/4" drainage plane -> mixed sidings (wood, metal, stucco).

So far inside we've concentrated on air-sealing (airtight drywall to attic, top-plates on wall bridged to ceiling (concrete -> wood -> drywall) with 2" fiberglass covered with latex-modified thin set, seems to have worked well).

We've been told that American Clay can suffer and come off the wall if it gets wet. Our advisor (at American Clay retail) asked us if the concrete blocks are vapor sealed on the outside. I'm unsure if she really meant water-proofed, or if full-on vapor sealing is necessary. From what I've read about the Clay online most problems have stemmed from what sounds more like bulk-water intrusion (that would be a problem for any surface) than condensation (from inside to out or outside to in) from either air-intrusion or from vapor diffusion.

Reading Vapor Retarders and Vapor Barriers again assures me that worrying about vapor diffusion in this wall and in Austin is not worthwhile (no high humidity sources in the house, no pool, no green house). Just get it air-tight and don't worry about it.

This left me wondering about what treatment will be needed at the ? in the cross-section above (i.e. outside the old paint, behind the new insulation). I expected to make it water-proofed, I expect the currently painted areas already are but was considering something like Ardex Ardicoat Plus especially over the new, parged but unpainted block areas and anywhere there are cracks (repair first) in the old stucco.

Does that sound about right? Any experience with American Clay that might change the general advice?

Asked by James Howison
Posted Nov 30, 2012 12:02 PM ET


2 Answers

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First of all, a disclaimer: I have never installed American Clay plaster, and I'm not familiar with the requirements for this product. If you have any doubt about your planned installation, you should address your questions to the experts at American Clay.

If you install exterior insulation, a rainscreen gap, and new cladding, I don't think you have to worry that your concrete blocks will get wet due to rain. Moreover, I don't think your concrete blocks are likely to get wet due to vapor diffusion from the interior.

The real weakness of your proposed assembly would be due to inward diffusion of water vapor during the summer -- in other words, the dread inward solar vapor drive. You can prevent this potential problem by choosing to install rigid foam on the exterior of your concrete blocks instead of Roxul.

To learn more about inward solar vapor drive, see When Sunshine Drives Moisture Into Walls.

Answered by Martin Holladay
Posted Dec 1, 2012 7:02 AM ET


Thanks Martin,

I'll definitely follow up with American Clay, but as you say I think we can rule out those two sources of water (rain infiltration and interior vapor) as problems (provided everything is built right :)

I'd discounted solar drive from the exterior as a problem as I'd understood that to only be relevant with a reservoir cladding (predominantly bricks). But re-reading I see that this can also be a problem for stucco and I assume that was your reason for mentioning it (wood siding too, but lesser extent). So in the stucco sections we could have vapor driven behind the siding. You are pointing out that it could come through the siding, through the permeable Roxul and then through the vapor permeable bricks.

So that suggests that a vapor barrier behind the Roxul (or as you say using foam sheathing) would be a good idea. So something like the Delta Dry or a paintable water and vapor barrier painted onto the walls before putting up the insulation?

Thanks for the reminder on the solar vapor drive.

Answered by James Howison
Posted Dec 1, 2012 12:19 PM ET

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