The Sandwich Zone

[Lynn Hepler]

Id 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.

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?

As an indicator for a safe proportion of exterior insulation to cavity insulation, see table R702.7.1: -7-wall-covering#IRC2018_Pt03_Ch07_SecR702.7. 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 75F 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.

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.

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.

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.

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.

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?

Lots has happened since I pressed pause on podcasting for my summer sabbatical. My time "off" definitely has NOT been filled with the restoration and relaxation I had hoped for...but such is life in the Sandwich Zone.

Soooo...this isn't what I thought I'd be airing today. As you'll hear my "sandwich" got a little sticky and I had to put my original plan on ice. But...that's how we roll in the Sandwich Zone, right? Gotta be able to pivot at the drop of a dime!

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