I know this subject has been discussed at length and in depth but here goes.. would like opinions on the following wall sandwich ballon framed raise ranch with a 14'4" wall 2x6 on 24" centers 6" spray in cellulose 5/8" drywall paint on vapor barrier 5/8" plywood cdx sheathing not osb 2" xps Synthetic stucco

Question - why XPS vs EPS? EPS seems to have 4/5 the insulation value at 1/2 the price. Be sure to air seal really well, perhaps with two layers.

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John I said xps primarily because I do alot of radiant heat as a contractor and I guess I'm stuck in the xps is the best for below grade application. I would consider eps if the vapor transmission is as high or higher than xps. I guess I have a good grasp on the thermal performance of various walls but am concerned about moisture management.

ICFs seem to use EPS below grade sucessfully. Plus EPS has higher permeability.

I don't have the figures handy, but as I recall, for your climate, 2" of exterior foam is fine, less can be a moisture issue. Dana will have a link to the details.

Do not use less because of 1st surface of condensation? I use a lot of 25 and 40 psi xps under slabs in ag and commercial applications and I have understood xps to be less permeable than eps thus retaining more of its r value. I can see where eps would be better for exterior wall sheathing and also for vertical foundation insulation in some applications. I will consider eps .
Thanks Jon and sorry for the misspell.

Posted By jonr on 31 Oct 2011 12:13 PM
ICFs seem to use EPS below grade. Plus EPS has higher permeability.

I don't have the figures handy, but as I recall, for your climate, 2" of exterior foam is fine, less can be an issue. Dana will have a link to the details.

Regarding the very high suitability & excellent performance of EPS below grade:

http://www.epsmolders.org/PDF_FILES...ade103.pdf

EPS has a very high fraction of fully closed cells (higher than XPS), and while moisture passes relatively easily into the interstitial spaces between the beads (not the cells), which is typically ~5-7% of the total volume.  So while pond-soaked submerged for months it might lose 5-10% of it's total R, it's a reversible process. When XPS is submerged it takes on water more slowly, but it's absorbed into the  broken cells, and would likely require heating it up to get it back out.  The higher vapor permeabilty of EPS relative to XPS is undoubtedly an artifact of the interstitial spaces as well.

Regarding the min-R required on the exterior to in N-IL (US climate zone 5), it looks like 2" of EPS (~R8) makes it for a cellulose filled 2x6.

http://www.buildingscience.com/docu...quirements

With 2" of exterior EPS you'd be better off WITHOUT the vapor-barrier paint, since standard latex would be sufficient to limit the wintertime moisture accumulation in the sheathing, and it would give the assembly much better drying capacity.  Ventilating the cavity between the exterior foam and the stucco would also enhance drying toward the exterior,  and limit the summertime vapor drives of sun-on-wet-stucco toward the interior. Stucco wicks in and retains a lot of moisture from rain/dew etcetera, but releases it quickly when the sun hits it. If you made the interior side TOO vapor tight the odds of getting high moisture conditions inside the studwall cavity in summer goes up in an air conditioned building, even with the moderate vapor retardency of the exterior foam so slow it down. (Here again, having both top & bottom vents to the stucco cavity reduces this risk.)  Vapor retardent  latex reduces the inward-drying capacity by nearly an order of magnitude compared to standard latex paints, reducing the ability of the air-conditioning to keep the gypsum dry.

The actual R of EPS at 25F or lower is measurably higher than it's 75F ASTM C 518 R rating too.  Low density Type-I EPS is about R3.9/inch @ 75F (R7.8 for a 2" thickness) , but at 25F it'll be closer to R4.3/inch (for R8.5+ out of a 2" if installed the exterior).  This gives you a more than 10% of margin over the IRC minimums,  further reducing the number of hours per winter that the sheathing is below the dew point of the conditioned space air.  AT 2" the permeance of Type-I EPS will be between 1 & 3 perms- comparable to standard latex paint on the interior. Combined with a vented cavity the drying capacity in either direction will be roughly equal (and pretty good).  By comparison the 2" XPS & vapor retardent paint would give you about R10-  a bit better margin on the wintertime moisture drives, but would have a permeance of only ~0.6perms on the exterior, and less than 0.5perms on the interior- it can still dry seasonally, but not nearly as rapidly as with the more vapor-open solution.

Dana,
Thanks for the eps xps info I will continue to reconsider my use of xps. my biggest concern is preserving the integrity of the stucco in a cold climate so eps and no vapor barrier if I can get the county to sign off. Do you think all elevations need the ventilated wall ? How do you construct this? Does anybody think a house wrap is needed?

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If the IRC 2009 allows using only standard latex on the interior (and it does), local codes usually follow suit. Using XPS on the exterior instead of EPS that's just fine, as long as you don't unduly restrict the drying capacity toward the interior. With the lower permeance of the XPS the importance of the vent gap is also lower, but still worthwhile if you can.

BTW: I'd missed the "synthetic" in the stucco description (usually referred to as "EIFS"). Most of that stuff doesn't wick & retain water the way traditional goods do, but using a high permeance version is preferred. Using a commercial rainscreen mat between the foam and the EIFS, and leaving it open at/under the eaves, with period gaps in the EIFS near the bottom works. Google [EIFS rainscreen] for suitable products.

"Yes" on the housewrap question. Whether it goes between the foam & sheathing vs. foam & rainscreen mat depends on how you intend to install & flash the windows. See:

http://www.greenbuildingadvisor.com/blogs/dept/musings/where-does-housewrap-go

Will check those links out tonite. Which is a better product in my climate EIFS or standard stucco, its hard to find a reliable contractor for either. I may diy if I can get the help.

I'm personally more comfortable with cement based stucco on metal lath, but there can be problems with it if not done right. Getting it wrong is mostly mis-handling the moisture issues, but there are structural issues as well. If you go hard-stucco, go with 16" on center framing for the rigidity and structural capacity- you're hanging ~3/4" -1" of cement & sand on that wall, and if it shifts, it cracks. Some practitioners advocate not butting up the sheets of sheathing to each other to accomodate dimensional changes from humidity cycles, but with top $ bottom venting of a 1/4-1/2" gap and 2" XPS/EPS and a more vapor interior side that probably isn't necessary- it won't see the same moisture peaks that it would without the foam. Fastener spacings of the lath to the structure have to be pretty tight to handle the weight of the lath, and IIRC the structural sheathing has to be thicker stock to handle both the stress of the fasteners and remain sufficiently rigid to avoid cracking over time, etc. It's a very durable & trough siding method though- done right it can last for centuries, and can withstand quite a hurricane-beating, etc.

EIFS is a lot lighter weight and more flexible- it's less prone to cracking but also easier to damage. It has a somewhat checkered history on the overall moisture management issues (particularly on low-permeance versions), but those more often than not boil down to sloppy installation and inattention to details. But perfection, especially perfection over time is a lot to ask of a system. Read up on the particulars before diving in- there's a lot to consider with either, and the construction details count.

Dana do you think I could stay with 2x6 on 24" if I used 3/4 plywood sheathing, house wrap, EPS, metal lath, conventional stucco and no vapor barrier on the inside other than paint and primer? can I skip the rain screen mat? I have a full sun southern exposure as well as a full shade north elevation if it matters

I"m not well enough versed with stucco to make the call as to whether heavier sheathing alone would be sufficiently rigid for hard plaster. Some guys get pretty leery about thicker foam too, but I'm not sure whether that's from lack of familiarity with it or whether they've seen fastener failures & cracking when hanging it on such a long moment-arm or what.

No matter what exterior cladding you use, if you have at least 2" of foam you don't need (and shouldn't use) anything more than standard latex paints for an interior vapor retarder in your climate. (Making it more vapor tight to the interior than necessary only increases risk of moisture issues.)

If you're going to skip the rainscreen, use XPS, since it resists moisture drives from the exterior better than EPS.

Even the north elevations can get some morning sun during the summer soltstice, and are not fully immune from summertime moisture drives, but clearly the east & south elevations are of more concern. (The east because of the direct sun & morning dew, the south due to higher intensity sun over longer periods.)

You may want to read this a few times carefully:

http://www.buildingscience.com/docu...nable-eifs

(More feedstock for the stucco-is-better mill.)

A partial answer to why the pros shy away from thicker foam for hard stucco is on page 2 of this document:

"Adhere wire mesh for stucco finishes over AP by nailing to the studs. Industry practice restricts the thickness of foam
sheathing under conventional stucco to 1 1/2" (38 mm) maximum due to the potential for nail deflection. "

At 1.5" XPS would meet the IRC's R7.5 min for being able to skip the interior vapor retarder.  Iso would be nominally R10 at that thickness, but derated for 25F average temp through the layer it would be more like R8.5.

Using screws rather than nails would be more secure in a thicker-foam situation, but fewer fasteners could be used too.

So from what I have read It would seem to me that a 2 by 6 on 24" centers, 3/4 plywood, Typar, 2" EPS, Galv mesh lath with screw fastners, stucco with vapor permeable paint seems to be most trouble free and easier if I DIY. I can't see how EIFS if done properly could be less expensive to install.

I like the look of the windows set against the plywood sheathing looks like the details with stucco should be eiser to accomplish than with EIPS unless I am missing something. Now the next issue is an unvented roof assy but I will start a new post.

I think a solution to putting heavy stucco over thick foam without nail deflection is to use some type of brick ledge.

Jon, What about a band board say a 2 by 8 rough sawn cedar that is fastened with 1/4" lag screws over the eps to the the foundation or sill. The brick ledge messes with my thermal bridging.

I don't know, maybe. Or some type of fiberglass L bracket that attaches into the outside of the foundation.

worth looking into, thanks