Vapor transmission in wall assembly
I’m trying to puzzle out a wall assembly for a cabin that I’m building, and struggling with a vapor transmission issue that I’d like some input on. First time poster – long time reader
Some info: I’m in the northeast (zone 6). I intend this to be a weekender, and being the conservationist and cheapskate that I am, I do not intend to heat it continuously. Best view and exposure are to the south, and though I have a pretty good tree canopy overhead, I anticipate that I’ll get reasonable winter solar exposure. I’m designing quite a bit of thermal mass (slab on grade and plastered straw bale), and quite a bit of south-facing glazing, so I’m hoping that I’ll stay above freezing through the unheated spells (though I’m sure that I will still freeze protect).
The bulk of the wall system is straw bale with lime plaster inside and out. I’m fairly comfortable with this in a go cold scenario. The lime plaster is vapor permeable, the straw has fairly robust vapor storage capacities (that’s vapor storage, not water), and I can dry to the inside or the out. The straw bale sits on a toe-up/knee wall: 6” for part of the house (about 40% of the perimeter), 24” for the rest (about 60% of the perimeter. This 24” knee wall is my struggle. Outside-in, that knee wall assembly is approximately 10” of rubble masonry, an airspace (optional), an insulated stud wall, lath and lime plaster. The knee wall sits on a concrete bond beam, fyi.
Given that this is rubble masonry, I cannot insulate from the outside – that would defeat the point of using the natural stone. My slab insulation marries to the insulated stud wall and straw bale sill for insulation continuity. Also given that this is rubble, I want to ensure that I’m protecting the stud wall assembly from air and water (water, not vapor) intrusion from the outside. Lastly, I want to make sure that I’m allowing the natural vapor drive (inside to outside) to occur so that I’m not forcing drying only to the inside. I think I’m to the rub.
My first pass at this was (inside-out) lime plaster on lath, on studs insulated with cellulose, 1-2″ rigid xps, airspace for weeps, rubble masonry. But I’m concerned that I’ve built a condensation surface at the xps. As it goes cold, the inside of the xps will certainly fall below the depoint, so I’ll have moisture (frost) building up in the insulation cavity. I like the cellulose for its vapor storage capacity, but I can’t image that what I’m describing is a good thing. Problem is, if I omit the xps, I solve the vapor problem, but I’m allowing thermal bridging, and more importantly, I’m not protecting the stud wall or cellulose from air or water.
I have considered spf in the stud cavity – if closed cell, I could keep the xps on the outside and the airspace, if open cell, I would not want the xps (lower permeability as you move outwards), so I suppose I would spray against the back side of the rubble wall. That seems somewhat questionable, as I do think I want to weep the back surface of that rubble wall. All that said, I’m very hesitant about using spray foam based on my own sketicism about off-gassing and things that I’ve read here and elsewhere.
Thoughts? Any help from your experience would be greatly apprciated. Always best to do it right the first time!
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