Spray foam insulation applied from the exterior?
Last Post 10 Jun 2010 06:30 PM by Dana1. 3 Replies.
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valorekhshovUser is Offline
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10 Jun 2010 03:59 PM
Hello! I am preparing to re-side my stick frame house, as a part of which I will be removing the existing water-damaged T1-11 siding (also doubles as sheathing), installing 1/2" CDX sheathing, Tyvek, 2" EPS, 2x1 furring strips and hanging 4'x8' fiber-cement panels. The exterior wall cavities are 4" and are filled with 60 year old fiberglass batts. Question: Since the wall cavity is going to be accessible from the outside, I am wondering whether it is technically possible to replace the fiberglass batts with 4" sprayed-on cellulose. I am leaning towards a type of a sprayed-on insulation to minimize air infiltration. What other types of insulation would work well in my situation? Thank you very much for your answers! v
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10 Jun 2010 05:33 PM
With only 4" (which might really be 3.5", if it it isn't "full dimension" 2x4) to work with, it might be worth spending some money on closed-cell polyurethane or isocyanurate, which will give you ~ 50-60% higher R-values, and will make a perfect air barrier on the interior side. The 2" of exterior EPS. With 3.5" of cellulose you'd be looking at ~ R13 (center-cavity), but with closed cell SPF you'd be looking at R21. The 2" EPS on the exterior adds another R8, so you'd be ~R30 with ccSPF, but only ~ R20 with spray cellulose.

Half-pound foam (open cell) would give you the perfect air seal, but at a slightly lower total R (and much lower thermal mass) than spray cellulose. Demilec's 0.7lb open cell foam would give you slightly better R than cellulose (~ R15.6, center cavity in a 3.5" depth) and give you the perfect air-seal.

But if you have the space to fatten up the wall, you can typically buy more R/$ with going thicker on the EPS. As long as there's no interior-side vapor barrier like poly, foil or vinyl wallpaper, you could then spray cellulose, staple 6-mil poly on the exterior of the CDX as an air & vapor barrier, and put 4-6" of EPS on the exterior to achieve high-R/low infiltration. (If vapor barrier at that location in the stackup is undesirable, painting the seams with primer and applying FSK or Huber Zip tape to air-seal it would work.) The housewrap serves a better function on the exterior side of the EPS where it becomes a drain-plane & air-barrier for your back-ventilated siding/rainscreen-gap.

Projects like this don't get done very often, so the opportunity for getting it "right" won't come around again for decades. Erring on the side of higher efficiency may look like a mistake in the very short term, but over the next 20-30 years, probably not.

What is your climate/location? In some US climates an R20 wall is enough to go all PassiveHouse in a very-tight house, but that wouldn't be the case in most. Where in the stackup you might want/need vapor retarders will also vary with climate, but in most instances with high enough R on the exterior putting all the wood inside the hygric boundary of the conditioned space works better than interior side vapor retarders. With enough EPS on the exterior the ability of the wood to dry toward the exerior can be hampered.

BTW: Take care that the house won't fall down when you strip the siding for insulating. In many many cases T1-11 is structural, and you'd be vulnerable to twist & racking forces during the process without installing temporary angle bracing.
valorekhshovUser is Offline
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10 Jun 2010 05:51 PM
Dana1,
Thank you very much for the detailed response and for the tip on watching out for structural issues as T1-11 comes down

The house is in Northern Virginia, which I understand is in both heating and cooling climate zone. I will need to research more on the PassiveHause specs...

Thanks again for an excellent response!

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10 Jun 2010 06:30 PM
In N. VA you can probably come close to hitting PassivHaus levels with ~ R30-R35 exterior walls, and an R20-25 insulated foundation, as long as the glazing is properly sized & placed (and low enough U-value). Splitting the R value between interior & exteiror with no vapor retarder would be fine there, as long as at least ~30-35% of the R is on the exterior of the wood. (3" of EPS would be ~R12.)
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