Plaster or Drywall?
Our home is being designed as (from exterior to interior)…
1.vertically hung corrugated metal siding
2. coravent rainscreen
3. LP weather logic
4. 2×8 studs with rock wool insulation
5. intello smart vapor barrier
6. plaster or drywall
Originally we had hoped to go with plaster as it is more breathable and more mold resistant. Prices for plaster are coming back significantly higher and I’m wondering since it’s a one time cost, if it’s worth it? I’m hoping to avoid any trapped moisture behind the walls and back of the drywall. Without exterior insulation being located in central WI, I was thinking it would be important to allow drying but maybe it’s not as big of deal as I’m thinking?
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I have never heard of plaster being more mold-resistant than drywall, and if your wall is probably designed and built, there should be no mold anyway. Plaster is almost always installed on a gypsum panel very similar to that used for drywall, with similar airtightness ratings. In fact, the standard for "airtight" is gypsum wallboard.
If you're talking about applying plaster to wood or metal lathe, or some other substrate, that's a different matter, but even then the airtightness will be similar to that of skim-coating gypsum panels.
Thank you and no, it’s not over wood or metal. I was just told at one point plaster was much better for mold resistance (a health issue we’ve dealt with as a family so we were trying to avoid).
The plaster itself doesn't provide food value for mold but the paper facing on the substrate does. In any case, it's far better to create conditions that don't promote mold growth than to apply "mold-resistant" materials and hope they work.
Mold requires three things: a comfortable temperature, roughly similar to what people like; food, which can be dust or practically anything else; moisture. The only one we can really control is moisture.
In your case, where framing spans from interior to exterior, you have thermal bridges that could lower the interior temperature enough to result in condensation. Your 2x8s will perform at about R-8 which might be enough but I would keep a close eye on the interior relative humidity in cold weather. I would also be triple sure that window and door flashing, and other potential stormwater infiltration points, are done as well as possible.
This makes sense. By keeping an eye on it, do you mean when it gets cold keeping it below the 50% humidity? Or is there a number I’m watching for?
In a heating dominant climate like yours any moisture in your walls is going to be coming from the interior. You want the interior surface to be a vapor retarder or vapor barrier to keep moisture out of the wall. Unless your walls are incapable of drying to the outside, any moisture that gets into the wall will be driven out because the temperature difference between the inside and outside will create a strong vapor drive. That vapor drive is why you want the interior wall to have some sort of resistance to vapor flow.
I've never heard that plaster is more vapor-open than drywall, but even if it is it's not a property that you want. If you were in a humid, cooling-dominated climate like Florida the vapor drive would be from outside to inside and you would want the wall assembly to be vapor-closed on the exterior and vapor-open on the interior to block vapor from entering and allow it to exit.
Thank you. Reading your comment, I’m thinking the intello smart membrane would accomplish this and stop the vapor drive? Let me know if that’s not right.
That is the purpose of the smart membrane. It's also supposed to be more vapor open in the summer, when the drive is toward the interior, which allows the wall to dry inward in the summer. That's what makes it "smart."