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Wall assembly for old house with cedar shingles, foam sandwich risk? Zone 4a

lmosakow | Posted in General Questions on

I’m in climate zone 4A. The house is over a century old, wood-framed, has old board sheathing, is leaky and has true 2×4 studs.

I’d love feedback on this wall assembly with cedar shingles, it aims for good sealing and reducing thermal bridging without building out extension jambs for windows. (I’d like exterior foam for thermal bridging, but my contractor says I can’t do more than 0.5” of foam to accommodate already installed windows w/ a 0.25” rain screen.)

Wall assembly from outside in:

a) Cedar shingles (non-negotiable)
b) Rain screen (0.25”, via horizontal furring or cedar breather)
c) 0.5” exterior foam
— Either foil faced Tuff R (3.3R, very low perm, <0.03) or (unfaced XPS at 2.8 R, 1.5 perm)
d) Housewrap as my WRB
e) Sheathing
f) 1” closed cell spray foam (to practically get good air sealing on this old sheathing with some good R at the same time, ~6 per in)
g) Batt the rest (a flash and batte approach), I assume compressing R-15 3.5” batte to the remaining 3” will get me about R-13

Total R is just north of 20, which is what I’m told I should aim for.

Do I have a real foam sandwich risk? My assumption is that this assembly can at least dry inward, but I’m not confident. Is the foil-faced polyiso as exterior foam a bad idea? Would love feedback on whether this would work or if it’s risky, thank you!




These threads suggests 0.5” exterior foam for zone 4a is still worth doing; 0.5” polyiso with facer was recommended; and that zone 4a doesn’t have a min foam thickness, but >2.5R is probably safe to avoid condensation on the outside of the sheathing:

These comment thread suggests 1” closed cell is “north of 0.5 perms” that will still allow drying to the interior:

This thread suggests horizontal strapping on foam to secure the foam and create nailers for the shingles is sound:


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  1. iwatson | | #1

    I'd be wary of the potential for a foam sandwich, and I generally think of spray foam as an "insulation of last resort" - i.e. for those locations where it's pretty much impossible to use any other method.

    If you're stripping down the interior walls and can afford to lose some interior sqfootage, perhaps consider some version of the Bonfiglioli wall. This will provide the thermal break for the studs, and will allow you to fur out the stud bays to take 5.5" batts.

    1. lmosakow | | #2

      I hear you on the Bonfiglioli wall, my rooms are already small given the age of the house, so I've been reluctant to go that route and I still have the air sealing problem.

      I'd prefer not to go spray foam if I can avoid it, but it really may be the best route available for me for air sealing given the age of the house, which I've seen some folks argue here about these really old houses. I'm attempting to minimize its downsides by going only 1'' or so and HFO closed cell. Is there a better way?

  2. lmosakow | | #3

    I'd appreciate any perspectives. I'm new to houses, learning as I go, and am trying my best to get my old house retrofitted as best as possible within a number of constraints—and then get off oil to simply use air source heat pumps (mini splits). I wish I had found GBA 6 months ago, but alas, here I am.

    Is this wall assembly viable or will it likely cause me problems that I'll regret? Is there a better path without extending the studs inward or creating window extension jambs outward?

  3. freyr_design | | #4

    I would push back on the .5” ext insulation, there are a number of ways to accommodate windows at the sheathing layer. I would forgo spray foam and use a peel and stick wrb for air seal at sheathing layer. This will probably be cheaper than the ccSPF. Fill the cavity with whatever you like, but dense pack cellulose is nice, and you can use something like intello for interior vapor membrane.

    If you are dead set on only half inch exterior (which again I advise against) I would instead use something like rockwool , for its vapor open profile.

    Window extension jambs toward exterior in not particularly challenging.

    1. lmosakow | | #5

      Thanks! When you say you push back on 0.5'' ext insulation, are you saying don't do any exterior at all? Or go thicker than 0.5''?

      (And is your core concern that the 0.5'' is just too thin, which could lead to condensation on the sheathing? Or is your concern with something else?)

      1. freyr_design | | #6

        Sorry what I meant is you are going through a lot of work in this remodel, there is no reason to not do it right with thicker exterior insulation because the contractor does not want/know how to trim out around your openings. And yes, doing a half job with not enough exterior rigid foam makes me nervous from a condensation point of view, which is a non issue/much safer with something like rockwool. I would also do 1/2” rainscreen if it was me. Also the cost of spray foam can be used to offset the slightly higher cost on exterior.

  4. Expert Member
    Michael Maines | | #7

    Is your basement or crawlspace damp? Do you or will you have equipment to control the interior humidity year-round? Do you have good cellulose installers in your area?

    As an indicator for a safe proportion of exterior insulation to cavity insulation, see table R702.7.1: If you reverse-engineer the math, it says that in your climate zone you should have a bare minimum of 15% of the total R-value on the exterior to avoid a condensation risk. That's with a class 3 interior vapor retarder, which is needed to slow down how much air and vapor gets into the assembly. It's not stated but elsewhere I've read that it also assumes that your interior relative humidity is kept within a normal range.

    1/2" polyiso is roughly R-2.5 at 75°F but at lower temperatures, when you need the condensation resistance, it will be lower, roughly R-2. Whether that's enough to make a difference I can't say but I know I'd be more comfortable with either higher R-value on the exterior OR a more permeable type of insulation.

    Spray foam is an ok air-sealer but it's actually not great. A continuous, self-adhering membrane will perform better with far lower embodied carbon emissions and risk of improper cure.

    1. lmosakow | | #8

      Thanks, Michael. I will be leaning on heat pumps being always on to control humidity levels. My crawl space hovers around 40-50%, I could very likely further dehumidify or apply strategies to make it even dryer). I'm not sure about cellulose installers, good question.

      I hear you re: 15% of total R-value. At R-3.3 Super Tuff (w/ foil facer), I'd need total assembly to be no more than R-22. At R-2 performance in colder temperature, I'd need total assembly to be no more 13.33, so I'd have a problem since I need to go higher. My understanding from Dana from the link below is that with the foil facer and my rain screen (which would be 0.25 in, not 0.5in), it should perform a bit better than 3.3. At cold temperatures, he guessed about R2.75 for half inch. So R2.75, I'd need a wall assemble no more than 18.33 (2.75/15%). If I need to get 19 or more, then I'd have a problem during these low temperatures in theory.

      1. MartinHolladay | | #10

        Dana is right, and your calculations look good to me. I think that if your wall assembly has an R-value no higher than R-18.3, your wall should work.

        As other commenters have pointed out, it's worth pushing back on your contractor's plan to install 1/2-inch exterior rigid foam. Ideally, you want thicker exterior foam. And if you fill the studs with fluffy insulation, installing a variable-perm interior vapor retarder (a "smart" vapor retarder) will increase the safety of the wall assembly.

      2. Expert Member
        Michael Maines | | #11

        Your crawlspace is probably cooler than room temperature so those RH values should be ok. If you find that your conditioned space is consistently over 50%RH, I'd consider a dehumidifier, regardless of your wall assembly. Modern heat pumps can be a bit too efficient when it comes to dehumidification but there are a lot of variables.

        I generally defer to Dana when things get that nerdy. Suffice to say that you're cutting it close but it might be fine. I'm always more comfortable with vapor-permeable assemblies. If you used a relatively dense (15 psi or higher) unfaced 1/2" EPS it would be quite vapor-open and give you a consistent R-2 or so, but because it would allow outward drying it would be safe. But I'm not sure it's available without a facer. XPS (or the new, less-damaging version, NGX) would have similar performance but might be easier to find.

        I'm really not an expert in your climate zone so don't take my advice as gospel.

    2. lmosakow | | #9

      Rockwool exterior unfortunately doesn't seem to come in a 0.5'', do you know of other exterior foams that might work? What perm rating range for exterior foam constitutes "permeable" in this case?

  5. Expert Member
    Akos | | #12

    I would skip any foam or spray foam. With real 2x4s you are only 1.5" away from a 2x6 cavity. Strap out the inside with 2x2 and install high density 2x6 batts.

    For air seal, peel and stick WRB as recommended above. I've done this with old board sheathing and works great You'll still need something like cedar breather over that for the siding. If peel and stick is too much, you can always nail up a layer of sheathing over the whole thing and tape the seams of that.

    The assembly you are proposing would actually work. That combination of thin rigid plus a flash of spray foam is enough for condensation control in your climate. If you look at the R value of the whole assembly, this works out to about the same as my 2x6 suggestion above but whole lot more complicated and expensive to build.

    1. lmosakow | | #13

      Thanks! What material do you envision for the 2x2 strapping? Is that some material to reduce thermal bridging? To your point that if I have a true 4 in cavity, wouldn't I then just need a 1.5 in thick strapping to get me a 5.5 in cavity, which is typical of 2x6 in framing?

      1. Expert Member
        Akos | | #14

        2x2 is nominal designation, the actual lumber is 1.5"x1.5" so exactly what you need. This is regular construction lumber, nothing special.

        Instead of lumber, you could do something like a Bonfig wall:

        but I doubt it is worthwhile in zone 4 for the small R value bump.

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