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Overclad EPS foam on top of stucco-covered fiberboard?

I'm doing a deep energy retrofit of my New Mexico house (zone 5B) and it's time to plan out the walls. The existing exterior walls have stucco cladding with no deliberate drainage gap, over two layers of tar paper, over fiberboard. In my extremely dry climate, this wall seems to have proven robust in its 42 years of life so far, as I have found no evidence of moisture damage whenever I have ripped out drywall or chiseled away stucco.

Given the cost-prohibitive nature of removing all the stucco, I am considering overcladding the wall with 4 few inches of high-density, termite-treated EPS foam and cladding it with a new layer of stucco or an EIFS-like polymeric equivalent. My thinking is that I would re-use the tar paper as the wall's drainage plane and keep the windows and doors in line with it, essentially putting them in the middle of the final wall assembly.

Ih there anything obviously wrong with this idea? One possible issue that comes to mind is the possibility of sandwiching the existing stucco between two vapor barriers (tar paper and foam). EPS is not really that much of a vapor barrier, but 4 inches of it may be enough to function as one. But the material between them would be cementitious, and the 8 annual inches of rain in this climate sort of restrict the damage you can do with less-than-perfect water-management, as my current stucco-over-fiberboard wall illustrates! Thoughts?

Asked by Nathaniel G
Posted Sun, 06/15/2014 - 19:00
Edited Mon, 06/16/2014 - 00:47

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4 Answers

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1.
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Nathaniel,
Your plan will work. EIFS contractors do this kind of work all the time; I suggest that you call up an EIFS contractor.

If you are doing the work yourself, make sure that you understand flashing principles and water-managed EIFS details before proceeding.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 06/16/2014 - 06:04

2.
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Thanks Martin!

Answered by Nathaniel G
Posted Mon, 06/16/2014 - 11:16

3.
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Another question, actually.

I've read all about the problems of "barrier-type" EIFS. However, in my low-rain climate, with a thicker-than-normal layer of foam, and existing stucco and tar paper between the foam and the fiberboard sheathing, is it possible that I could get away with an imperfectly-applied barrier-type system rather than a water-managed one? I ask not because I would prefer it, but because I'll be hiring people to do the work, and if I can only find people who do barrier-type systems, I'd like to know if using that would doom the house.

Alternatively, is there any feasible way to apply traditional stucco over foam in an overclad job like this? Maybe a thinner coat than normal? Or maybe surface-bonding cement rather than traditional 3-coat stucco? Or is that just asking for trouble?

Answered by Nathaniel G
Posted Wed, 06/18/2014 - 16:37

4.
Helpful? 0

Nathaniel,
Q. "Is it possible that I could get away with an imperfectly-applied barrier-type system rather than a water-managed one?"

A. Why risk it? First of all, if you are hiring an EIFS contractor, I don't think you'll be able to find any contractors who want to install barrier EIFS. All of the EIFS manufacturers have switched to water-managed EIFS, because the liability for barrier EIFS is unacceptable.

Second, even i you could find a contractor willing to violate the recommendations of the EIFS manufacturer, you wouldn't want to hire him.

Q. "Is there any feasible way to apply traditional stucco over foam in an overclad job like this?"

A. This article explains how to install traditional cementitious stucco over rigid foam: To Install Stucco Right, Include an Air Gap.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 06/19/2014 - 05:53

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