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With a masonry exterior home, is “outsulation” the wrong way to go?

Sal_123 | Posted in Energy Efficiency and Durability on

I am in the process of building a home in Zone 5, Northeast. The planned exterior is stone and stucco. Ideally I want to maximize the R-value of the wall, upwards of R-30 with an air tight, well sealed wall assembly. As it stands we have a traditional 2×6 wall on the prints. I have looked at various options, ICF, CMU, double 2×4 wall, etc.. Wanting to adhere to the “outsulation” concept, I find it challenging, expensive and questionable hanging a stone veneer wall 2 to 4 inches off the wall to accommodate 1 or 2 layers of taped, stepped rigid insulation. I’ve evolved to the opinion that with a CMU exterior wall, the wall is ready for direct application of stone or stucco. With a traditional wood wall, I have to apply 2 layers of 30lb felt, a corrugated drainage plane, metal lathe, scratch coat THEN apply stone or stucco. Since the exterior of the house is essentially ALL stone or stucco based, even though a CMU wall is more expensive than a 2×6 wall, I am thinking the savings on all that prep labor and materials getting a 2×6 wall ready for stone veneer or stucco might be a break even or close to it (?). Not sure what the tipping point is, want to start on a cost analysis to define which is less and which is a better quality wall. I like the elimination of the wood and better structural integrity of CMU. Want to get some input from this forum.
As for the insulation issue, with a CMU wall, I envision the home insulation starts INTERIOR to the CMU wall. say a 1/2″ taped rigid foam layer (to insulate the studs from the cold CMU) with 2×4 or 2×6 studs (wood or metal) every 24″ to accommodate the mechanics and insulation, then could fill the cavity or flash-and-batt/fill. The masonry component is allowed to dry to the exterior with the interior being isolated from the masonry. A plus is the elimination of organic materials from the wall. The wall thickness is substantial, I’d lose about 4-5″ of floor space all around the perimeter of the room but gain that in wall thickness, if you consider a true thick wall a prestigious item. It raises the issue with window placement and flashing. I like “innies” ie. windows sucken into the wall so you see the thick wall dimensions from the exterior adding value to the project.
I saw our local community college put up a new building and it was poured concrete walls, wrapped in an exterior peel and stick membrane over primer prepped walls, exterior to this, attachable prefinished facades were installed. All the insulation is interior to the concrete wall. Again, violating the “outsulation” philosophy.
What are your thoughts? Any constructive input/comment or other ideas are appreciated. thank you

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  1. GBA Editor
    Martin Holladay | | #1

    A few reactions:

    1. If you are interested in stone veneer, there are limits to rigid foam thickness. I'm guessing that most masons won't want to attempt such an installation with more than 2 inches of exterior rigid foam.

    2. The easiest way to proceed with your plan is to install EIFS. Call up a few EIFS contractors and ask them about the maximum foam thickness they offer.

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