EIFS vs ‘outsulation’
Building Science Corp says about EIFS (http://www.buildingscience.com/documents/reports/rr-0406-face-sealed-drainable-eifs):
Exterior insulation and finishing systems (EIFS) are inherently defective and unfit of use as an exterior cladding system where moisture sensitive components are used without a provision for drainage or in locations and assemblies without adequate drying.
Yet, the Building Science ‘perfect wall’ uses exterior rigid insulation over sheathing, without any specific requirement for a drainage plane between the rigid insulation and sheathing. BSC also doesn’t specify a rigid insulation of a specific permeance, so this could include polyiso with foil facing, which is effectively not permeable.
My question is, why is the ‘perfect wall’ any different than EIFS? I realize there is a strong recommendation to have a rain screen or air gap behind the cladding, but what about moisture that is able to make it behind the rigid insulation?
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Replies
Roger,
The famous EIFS failures in North Carolina occurred in houses with interior polyethylene. Moreover, the synthetic stucco used in traditional EIFS was not installed over a rainscreen or drainage mat. (Newer EIFS systems now include drainage mats.)
The "perfect wall" advocated by the BSC includes a rainscreen between the siding and the foam. This rainscreen effectively allows wet siding to dry quickly. Moreover, BSC advises against interior polyethylene.
Thanks for your reply Martin!
The EIFS drainage mats are typically installed between the rigid foam and the sheathing, which addresses the problem of trapping moisture between the OSB and foamboard. In fact, this is the crux of the question: Could we be repeating the EIFS debacle by not providing drainage between low-perm rigid foam and the sheathing?
Roger,
Building scientists have studied the question extensively, and have conducted multi-year tests with test huts and model homes with walls filled with data monitoring equipment. The question has also been investigated using sophisticated hygrothermal modeling programs like WUFI.
As long as foam-sheathed walls do not include interior poly, and as long as the foam is thick enough to keep the sheathing above the dew point during the winter, these walls perform very well. In fact they are much less subject to moisture accumulation in the sheathing than walls without exterior foam.
Martin, are the findings of these multi-year tests generally available? Any reference links would be much appreciated. The WUFI models/reports are nice, but I really would like to read up on real-world tests that have been conducted. Thanks again!
I was involved in a project that performed a WUFI analysis of an insulated stud wall assembly with an ~8-10" exterior EIFS package in climate zone 6. (If I recall correctly) the building forensics consultants, who made their money on commercial EIFS litigations, advised based on their WUFI analyses that with thick EIFS packages not only should there be no poly vapor barrier but the stud wall cavity should NOT be insulated in order to provide adequate drying potential to the interior.
This was my understanding of their position at that time. Subsequently, reading some information on the subject on this website I wonder if the building forensic experts were playing it too safe and with so much insulation to the exterior of the sheathing the sheathing would never be cold enough to be at much risk. (In the field at least, maybe around window installation with more likelihood of exfiltration moisture accumulation could pose problems.)
J, that wall assembly sounds like it would be used in a very cold climate. I'm particularly interested in wall assemblies that are using ~1-1.5" of exterior insulation, which appears to be common in conjunction with standard wall framing practices, and is primarily meant to address thermal bridging. I'm also specifically interested in non-lab, real-world test results.
Quick update: I did find this study (http://www.buildingsciencevancouver.com/index.html), but I couldn't find any specific results.
Another study, this one relates to REMOTE: http://www.cchrc.org/docs/reports/MTL-2final.pdf, but it does provide interesting information about drying potential for sheathing behind foam.
J. Chestunt's point is very well illustrated: the wall with the insulation in the studs appeared to keep the moisture levels considerably higher.
Is it possible to extrapolate the findings in this study to other climates? For example, a more moderate climate, one that only requires 1" of exterior insulation to avoid condensation in the walls, but does (as is typical) include insulation in the walls, even cellulose. Would this moderate climate wall exhibit a similar humidity problem?
Roger,
Yesterday I e-mailed John Straube, asking for suggestions on the best monitoring study to show the safety of properly specified rigid foam installed on the exterior of OSB or plywood wall sheathing.
His response: "Wow. So the millions of homes built over the last 30 years are not enough? Surely the question is dependent on climate, since in Zone 5 and higher this has been done.
"I don't have access to my laptop at the moment and not sure which paper to dig up. I guess I would reference the Ontario wall-drying study which actually tested wet lumber drying. Or the Vancouver test hut which is on line. Or the various test houses we did for the vapor barrier code change.
"Since it is not really a question that a serious building science researcher would have (unless there is a vapor barrier sheathing with low R-value) I doubt there is a study. For example, where is the study that shows the nail tips don't corrode when installing OSB? Where is the study showing OSB does not rot walls when used instead of plywood?
This smacks of ‘show me Obama's birth certificate.’ Really!!???”
[This morning, John Straube send me a follow-up e-mail:]
"I worry that we pander to people who should know better. A study you can reference should not be sufficient to answer the question. IS NOT sufficient. It is part of the answer for many questions, but this is so basic, answering it directly accepts the questioner's premise that such a study is needed or relevant: in fact it reinforces this mistaken view."
Two papers on moisture in walls with exterior rigid foam
John Straube has suggested two papers -- not particularly recent, but relevant -- that address the issue under discussion. I have posted the pdf files here:
Studies of moisture in walls with exterior foam