Obviously it needs insulation—real insulation, not just a lick and a promise—and obviously this is a problem other people have tried to solve before me. I want to avoid building out a second wall and infilling the cavity, because that would be a lot of extra work, and also because the cob will have a whole lot of curvature and sculptural work, which any insulation method would need to follow. I want to avoid sprayfoam, because it’s nasty, and also, it doesn’t breathe. No matter how many assurances I receive to the contrary, I don’t really believe strawbale is appropriate in our soggy climate, and also, those big blocky shapes will not marry well to the shape I need them to cover. A couple inches of slipstraw are not going to be warm enough.
So I am considering a super-thick perlite plaster, say 6”-8” (15-20 cm). Perlite coated with clay slip probably doesn’t hold together well enough to do that without being packed into an exterior form, and if I had to do that, I’d pretty much be back to the “second wall + cavity infill” thing. I hear interesting things about perlite or perlite-slip stabilized with waterglass (also spelled water glass), but I don’t know anyone who has tried that, and usually when I read about someone who does try it, it is for an application that will then be fired, which this would not be.
Anyone out there know anything about this? First, does waterglass stabilize a thick perlite plaster enough for it to adhere to itself and the wall while curing? Can it form or follow shapes? Does it set up hard enough to take some weathering and/or a final exterior plaster? As I understand it, perlite is rather strongly absorptive of moisture, and so although it would be protected from direct rainfall, I’d have some concern about it taking up ambient water vapor (of which we have plenty; this really is a soggy climate!) and either becoming heavy, softening a clay slip binder, or worse yet, softening the cob wall beneath. Conversely, waterglass might set up so waterproof as to inhibit the breathability of the walls. I have no idea how those two contradictory properties might interact. Do you?
Appreciate your help!
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