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Now that outsulation is “out,” what next?

Sal_123 | Posted in Energy Efficiency and Durability on

Zone 5, Northeast US, two story residential home. Planning for exterior “true” cement stucco and areas of stone and brick veneer. After many permutations and consults, I have concluded, NO on outsulation. Two engineers, one architect, one stucco contractor, one mason and a few general contractors all said the same thing. The same thing that a while back was told to me here on GBA by Martin, “it is not proven and risky, be very careful”. Admittedly this is not the most forward thinking, green minded area of the country, my framer balked at advanced framing techniques and everybody looked at me like I had 3 heads when I suggested 2.5″ exterior rigid insulation. But that said, what’s the next step?
I want to achieve the same goals, a well insulated, high R-value wall that dries accordingly, inward and/or outward, 100 year design that is limited by a 2″x6″ stick framing wall.
That said, what to stuff in the bays is a debate, spray foam, (full wall vs flash and batt), blown cellulose, blown fiberglass, are all being considered. What should I consider to specifically disrupt thermal bridging? Is lining the studs (under the sheetrock) with 2″ wide strips of 1″ rigid XPS a fair idea, effective and worth the effort, to block transmission from the cold exterior to the sheetrock panels?
Any input on design is appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    To meet IRC code minimum in zone 5 with 2x6 framing and no exterior or interior insulating sheathing requires a minimum cavity R of R20, and results in a "whole wall" R value of about R15, maybe R16+ with advanced framing. That's code minimum, taking into account the extra air films of the stucco cavity, the R-value of the stucco, the sheathing and the interior wall gypsum, etc.

    Even if you take it to R30+ cavity fill with closed cell foam it won't do better than R18 whole-wall-R, due to the thermal bridging of the studs. Insulating the sides of the studs with a high R/inch product will not affect the whole-wall R appreciably. Only if you can tolerate thicker wall can you improve the thermal performance in any significant way.

    The most important aspect of long term performance is the air tightness of the assembly. Caulking the framing to the sheathing at every stud bay, and cualking under the bottom plate, and between any doubled up studs or plates of not going the AF route) making the sheathing the primary air barrier works. If using spray foam as cavity fill the foam will seal the cavities, but not under the bottom plate or between doubled up studs/plates.

    Assuming you can tolerate 3/8" of additional thickness using fan-fold XPS siding underlayment on the exterior adds R1.6-R1.8 to the whole wall R, and the typical -08-2 perm facers (look up the product specs) protects the sheathing from high moisture drives from the stucco. With the thin exterior XPS the thermal performance of the wall using half pound open cell foam or any blown fiber will meet or exceed a full cavity fill of closed cell foam, at a much lower cost and lower enviromental impact.

    If using vapor permeable open cell foam or fiber cavity fill it's necessary to lower the vapor retardency of the interior side as well. The cheap way to do that is with "vapor barrier latex" primer, which delivers about 0.5 perm vapor retardency. The better way to go is 2 mil nylon, which is a "smart" vapor retarder with variable permeance. It slows the accumulation of moisture in the sheathing when the sheathing is cold all winter, but when that moisture is being released by the warmer temps of spring raising the humidity in the wall cavity, the more humid air causes the nylon to be more vapor open, which allows the assembly to dry toward the interior 3-5x faster than with half-perm latex. Thin sheet nylon has been used for decades to moderate the moisture content of food packaging, and now has more than a decade of experience in construction. The installed cost is between 10-15 US cents in my area, which can be thought of as cheap insurance.

    If even thicker walls are possible, using a laminated foam/OSB sheathing such as Huber ZIP-R or it's cousins works:

    With these products you would probably only be looking at those with the OSB on the exterior (to keep your stucco guys happy), and it would need to be at least R7.5 to be able to safely skip the 2 mil-nylon on the interior using blown fiber or open cell foam in the cavites.

  2. JC72 | | #2

    +1 for R-12 ZIP-R combined with dense pack cellulose.

    I'm of course assuming the required structural bracing will be installed.

  3. Expert Member
    Dana Dorsett | | #3

    Chris- You may have not refreshed the page as I was editing in the ZIP-R comments at the end. It definitely CAN work, but only if the engineers & architects do their homework and design it to meet the load requirements.

  4. GBA Editor
    Martin Holladay | | #4

    You haven't told us whether this is new construction or a retrofit job. I'm assuming that this is a retrofit, which is why you are limited to a wall thickness of 5.5 inches (a 2x6 wall). If it were new construction, you could consider a double-stud wall.

    Dana gave you a thorough answer. If you can't have a wall that is thicker than 5.5 inches, you end up with a wall that barely meets code and has thermal bridging through the studs.

    If you can make the wall a little thicker, one option is interior rigid foam. I wouldn't worry about inward solar vapor drive, as long as (a) you use OSB or plywood sheathing on the exterior (not fiberboard), and (b) you have a ventilated air gap between your sheathing and your cladding. For more on interior rigid foam, see Walls With Interior Rigid Foam.

    As Dana pointed out, one of the nailbase-like options (for example, Zip-R) is definitely worth considering. More here: Nailbase Panels for Walls.

    Finally, if I were a consultant on your project, I would ask you to seriously consider whether you needed to stick to your cladding plan -- stucco plus stone veneer plus brick veneer. It seems to me that (a) these claddings are preventing you from installing exterior rigid foam, and (b) you'll end up with what I call a "three-siding" house, which (to my eyes) is an aesthetic disaster. But of course, aesthetics are a matter of personal taste. Good luck with your project.

  5. Sal_123 | | #5

    New construction Martin. The house is up and framed, 2x6 walls, 5/8 plywood sheathing. I was in denial to some extent, hoping to get away with the outsulation concept and apply 2.5" exterior rigid XPS, even bought a truck load of recycled material off a roof in CT in like-new condition. Have stored if for 8 months under tarps. The one engineer who I trust, has always given me good advice, despite my GBA and references, basically said he would not feel comfortable and strongly recommends against it. That was followed by a parade of others saying don't do it. That said, I have plenty of areas to use the XPS, but the structure is up and I am considering my options, looking for good advice and insight.
    Interior rigid foam is new to me. I look forward to perusing the reference, thank you!
    As for the cladding, think Tuscan farm house, certain walls with cement stucco, others overgrout stone rubble, some windows framed in brick, barrel tile roof, extended wood soffits and exposed timber rafters. Below is a link to generic example.
    You are correct, this design is keeping me from my desired outsulation, but the idea of a common exterior cladding, asphalt roof, typical soffits and fascia makes me yawn. I guess beauty is in the eye of the beholder.
    Thanks Dana for your reply. I am very aware of the importance of airtightness, I have used 3" 3M construction tape at all exterior plywood seams then run a bead of silicone from the interior on all horizontal seams. Was holding off on caulking sheathing to framing as I am considering a closed cell polyurethane flash and fiberglass batt. Exterior Tyvek over plywood sheathing and a 15 lb felt under a corrugated plane, like an MTI product, is the plan to protect the ply from high moisture drives from the stucco, but I like the 3/8 fan-fold idea better.
    Will investigate the interior rigid foam and see if there is a role.
    Thanks again!

  6. Dana1 | | #6

    Closed cell polyurethane is wasted when applied between studs, and will hamper the capacity of the sheathing to dry toward the interior. From where you are right now 3/8" fan-fold XPS pm the exterior detailed as the WRB, with half pound open cell polyurethane in the cavities with 2-mil nylon as the interior side vapor retarder works. If you opt for interior side foam, unfaced EPS is the best option from a drying capacity point of view, since even Type-II (1.5lbs per cubic foot nominal density) is still only a class-III vapor retarder at 2" of thickness.

    For the fan-fold would be Owens Corning Propink, which has a facers rated1-perm minimum (the boundary of Class II and Class III vapor retardency, so it's still in Class-III territory), which protects the sheathing from peak exterior moisture drives while still allowing some drying into the rainscreen cavity, and adds R1.5 to the whole-wall performance (about a 9% improvement on a 2x6 o.c. foam wall alone.)

    Pactiv GreenGuard XP38 is also good choice, at 1.3 perms nominal:

    These products typically come in 4' x 50', and if the horizontal seams are first Z-flashed with the with the appropriate tapes then taped-over they become the weather resistant barrier.


  7. Sal_123 | | #7

    Thanks again Dana, so as it stands, exterior to interior:
    3/8" fan-fold XPS detailed as the WRB = R 1.5
    15lb felt
    5/8" Plywood Sheathing
    5.5” half pound open cell polyurethane in the cavities x R 3.6/inch = R 21.6
    1” interior foam, unfaced EPS x 4.0 = R 4.0
    Sheetrock __________________
    R value 27, much lower than I wanted. I am thinking increase unfaced interior 1” EPS (R-4) to 1.5” (R-6). Any other options you can suggest given the limitations? Unfortunate closed cell can't work in the cavities.

  8. Expert Member
    Dana Dorsett | | #8

    You're only calculating the center-cavity R, which is almost irrelevant. The number that matters is the whole-wall R with all thermal bridging and all layers factored in. You could get a much bigger center cavity number using closed cell foam, but even with advanced framing and a full 5.5" of closed cell foam (can't really get that since it's too hard to trim) that only adds about R2-R3 to the whole-wall number. (Also, don't cheat yourself- the 1" EPS is R4.2, not 4.0. :-) )

    With the additional air films of the rainscreen, 5/8" sheathing and 1/2" gypsum the basic R20 24" o.c. 2x6 walls wall comes in at about R16- R17, despite the R20 cavity fill. (With 0.7lb open cell fom foam it buys you another ~R1, if that's worth it to you.)

    Add the R1.5 for the fan-fold, and R4.2 for 1" of Type-II EPS and you're at R22-R23 whole-wall, despite the R27-ish center-cavity number.

    If you went with closed cell foam in the wall it would still only be R25-R26-ish, only a ~10% improvement, at a very high cost (in both dollars and environmental burden.)

    There is less polymer in 5.5" of half-pound foam than there is in 1.5" of 2lb foam, and open cell foam is blown with water instead of HFCs. There are places where the extra cost of closed cell foam is arguably "worth it", where its waterproofness, higher R/inch, and low vapor permeance is actually buying you something. But between stud framing where it's thermal performance is robbed by the framing and the drying capacity is impeded isn't among those places.

  9. Chaubenee | | #9

    Sal, seeing that you are framed up and only have the 2x6 wall, there IS another way to increase wall thickness and gain another R3.6/inch in the open cell. You can "Mooney Strap" your walls on the interior horizontally 16" oc with 2x2 material effectively adding 1.5" more for insulation PLUS you will effectively reduce your bridging which increases the effective R value. The downside is that you lose 1.5 of floor space along each wall. Seeing as you plan on adding one inch of xps interior foam, it seems to me that you are making deep extension jambs on all of the windows and doors, anyway. Dana, is this a solution for Sal?

  10. Chaubenee | | #10

    Ps- I would also dense pack cellulose into the cavities if I were you after caulking all the stud bays. Use the oc spray foam where first floor meets second at that plate and noise ends, corners, and above mud sill. Resilient. Go with Mooney straps.

  11. Dana1 | | #11

    Mooney walls & dense pack work, as does damp sprayed cellulose in a sheathing-caulked Mooney wall.

    Dense pack may be more expensive than half-pound foam, damp spray should be comparable or cheaper. Cellulose offers somewhat higher reslilence due to it's moisture buffering capacity sharing the moisture burden with the structural wood.

    See also the discussion around Case 3 (a 2x3 Mooney wall), beginning at the bottom of p.34 (p.38 in PDF pagination):

  12. Sal_123 | | #12

    Thanks for your input and comments Joe! My plan after this discussion and Dana's advice was to install 1.5" EPS on the interior wall before the sheetrock and 3/8" XPS fan-fold to stave off high moisture drive from the stucco on the exterior. The interior 1.5" EPS adds an additional continuous layer of R- 6.3 insulation and further thermal bridging disruption. In this scenario, would I need a Mooney wall? If I was adding an additional, say 3" to 4", then the bracing would be needed to hold the sheetrock 3.5" inches away from the wall. At 1.5", even if I bump it to 2" (for a trade off of losing 2" on exterior wall side with a gain of R-8.4), I think I should be OK with appropriately spaced, longer fasteners sans the bracing, you guys agree?
    Had not considered dense pack cellulose at r-4 per inch due to cost. I liked the flash and batt concept since the flash of open cell will caulk and seal, acting as a good barrier to help me achieve good air tightness while the fiberglass batts at around R-3.1 per inch are easy and cheaper. What do you guys think?

  13. Expert Member
    Dana Dorsett | | #13

    You can still long-screw the gypsum through 1.5" foam (be sure to seal up any misses with caulk, not a thin spackle-over). Plan ahead on how you are going to mount and air seal the electrical boxes, since foam + gypsum will be ~2" proud of the stud edges.

    Caulking the plywood sheathing to the framing is a lot cheaper than a flash-fill of open cell foam. Dense-packed cellulose isn't really R4/inch- it's really R3.6-R3.7/inch, but so is damp-sprayed cellulose, which is cheaper. A full cavity fill of open cell foam is usually cheaper than dense-pack, but usually more expensive than damp-sprayed.

    Even crummy R19s are good for R3.27/inch when compressed to 5.5" in a 2x6 stud bay, and compressed to 3.5" (with a 2" open cell flash) would end up at R3.7/inch. But getting a perfect void-free fit with batt takes meticulous attention even when the surfaces are pretty flat (milled studs & plywood) let alone pressed into open cell foam. If you're going to do batts, caulk the sheathing to the framing inside every stud bay and seal all seams & penetrations, and use R23 rock wool, fitted with care.

    The wallboard isn't structural sheathing- the 5/8" plywood is, so no bracing is needed.

  14. Chaubenee | | #14

    If you are really planning on blowing open cell foam, then just do the mooney wall with 2x2s which will give you 7" of nominal fill, better thermal bridging and SKIP the interior eps foam. You will save a ton of money and you should get R26 and super air sealing. That is how I would do the job if I wasn't doing dense pack or wet blown cellulose. Either way I would do mooney wall. It gives you a great solid place to nail Sheetrock and a thick wall of insulation. No, it isn't R30, but the reduction in bridging is terrific and that counts for something. Just check with your foam installer to make sure he is good with it.

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