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Risky wall assembly design with stucco/stone over 2.5″ XPS outsulation

Sal_123 | Posted in Energy Efficiency and Durability on

Building in Zone 5, NE US want to adhere to the concept of outsulation and break up thermal bridging of 2×6 stick framing (sheathing is 5/8″ plywood) and add 12.5 R-value to the wall. Final exterior product planned to be a combination of stucco and stone veneer (lick and stick).
Had enthusiastically wanted to use 2.5″ XPS, taped seams, over Tyvek applied directly over plywood followed by a layer of asphalt #15 felt over the Tyvek, followed by the taped XPS. The details to support the weight of stone veneer is the issue. The areas to get stucco (EIFS) would be fine atop the XPS, would apply a weep panel atop the XPS under the EIFS to vent the stucco layer but the weight of the stone veneer is an issue.
Was taking a belt and suspenders approach by starting with an angle iron acting as a brick ledge. Not that stone veneer is meant to transduce its weight to the brick ledge, but figured it can’t hurt to combat creep. Then to support the stone veneer weight was thinking first insert a layer to vent the stone and then secure furring strips AND the overlying metal lathe directly to the 2×6 studs with the appropriate caliber fasteners and frequency installation, according to the weight of the exterior stone.
Finally proceed over the metal lathe with a scratch coat and stone.
I have consulted with the structural engineer we used on other parts of the structure who obviously warned me to proceed with extreme caution and referred me to my architect citing wall assembly is not his area of expertise (albeit he was very helpful). My architect and the contractors I am working with all seem stuck on why I would wrap the entire house in 2.5″ of XPS. The architect has suggested I get another engineer involved. Is it me or does nobody want to go on record to design a 2.5″ XPS outsulated wall with exterior stone veneer? (for obvious reasons right?)
My engineer did suggest cutting the XPS layer to 1.5″ to help mitigate the structural risk. Given that thickness would suffice in Zone 5 to keep my plywood sheathing above the dew-point (from my understanding) than I am ready to say uncle and decrease it to 1.5″.
Any good suggestions/advice is appreciated and I offer my regards in advance.

btw: in the event of 1.5″ layer of foil wrapped poly-iso, given its low perm, would it obviate the need for a vent layer below the stone and/or stucco limiting drying to exterior only?

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  1. GBA Editor
    Martin Holladay | | #1

    It's a great idea that you plan to wrap your building with 2.5 inches of rigid foam (although EPS would have been a more environmentally friendly choice than XPS). So, unlike your clueless architect and contractor, you don't have to convince me of the value of exterior rigid foam.

    I don't wonder why you want to wrap your building with rigid foam. But I do wonder why you would want to clad your building with "lick-and-stick" stone veneer. It's a risky wall cladding.

    Your suggested approach -- installing furring strips with adequate fasteners through the foam to the underlying studs, followed by some type of substrate to support your foam veneer -- will work, and it's the proper way to proceed. If your engineer can't figure out the required fasteners, find an engineer who can. There should be enough information in my article on the topic (“Fastening Furring Strips to a Foam-Sheathed Wall”) to get your newly-hired engineer started on the required calculations.

  2. Expert Member
    Dana Dorsett | | #2

    It's not just polyiso- 1.5" of ANY rigid foam on the exterior the sheathing would have to dry primarily toward the interior. Even 1.5" of Type-II EPS would come in at a bit over 1.5 perms (2 perms absolute max).

    But 1.5" of EPS would not provide sufficient R value for dew point control at the sheathing in Zone 5, and would not meet code. At 1.5" polyiso would still provide JUST enough dew point control at the sheathing even after derating for climate & stackup, but more is clearly better/safer, and greener overall than XPS. At 1.5" XPS would initially provide sufficient dew point control, but in 50 years probably not, depending on just how fast the HFC blowing agents leaked (doing it's climate damage). For the long term a minimum of 2" would be required if XPS or EPS.

    From a long term energy use/cost point of view it's rational to put 3" of polyiso on the exterior of a 2x6 fiber-insulated wall, raising the whole-wall R to about R30 after thermal bridging. If the rest of the house is similarly up to snuff that would make it possible to hit Net Zero Energy with a PV array that actually fits on the house. At the rate the installed cost of PV is falling (looking at 30-40 year trend lines, or "learning curve", in economist-speak) this is relevant. Long before the lifecycle of the house is up, site-sourced PV will be the cheapest form of energy available. In any new design roof orientation and thermal performance that make Net Zero possible is worthwhile to consider.

    How soon will that cost cross-over occur? Estimates range from 6-8 years on the optimistic side, 15-25 years from the sober (and often too conservative) analysts. The trend lines look like this:

    Since the year 2000 the installed capacity of PV world wide has been increasing 1.43x/year, year-on-year, doubling roughly every two years (though that rate has been creeping up). The long term learning curve on PV for decades has been that every time the installed capacity doubles, the installed cost drops between 20-25% (more rapidly in recent years). Last year PV hit roughly 1% share of the worlds total electricity source. At the recent 15 year trend of 1.43x year-on-year capacity increase, starting at 1% in 2015 it hits 100% by 2028, and the installed cost will have fallen to 20-25% of the 2015 cost. (~$3.50/watt was the US average for small scale PV in 2015, so it should be well under a buck-a-watt by 2028, if not sooner). With the developing electrification of the transportation sector PV will be disrupting all energy markets through the 2020s, not just electricity. The Saudis aren't crazy to be pumping oil as fast as they can while it still has a price in excess of their production cost.

    Will the 1.43x per year continue that long? Maybe not, (in the US it's expected to grow at a 1.6x multiplier in 2016) but it will have to slow by quite a lot to not become the cheapest source of energy in 20 years.

    While incremental efficiency improvements in both PV and electricity end-uses slowly shrink the amount of roof real estate necessary to hit Net Zero, it would require true breakthroughs on efficiency to get there with much less than an R30 whole-wall performance. The "extra" 25- 30 cents per square foot for bumping up from 2.5" to 3" on the foam gives it some margin, if Net Zero is a design goal.

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