Maintaining Inward Drying Potential with Exterior Insulation Retrofit
Zone 5A, and yes I’ve seen the tables in the oft-mentioned ABTG report (https://www.appliedbuildingtech.com/system/files/abtg_rr_1701-01_moisture_control_guidelines.pdf) and Martin’s recent article (www.greenbuildingadvisor.com/article/persistent-worries-about-exterior-rigid-foam). Partial interior and exterior retrofit of a 1950’s house, looking at continuous exterior insulation — unfortunately mineral wool boards seem to be expensive and unobtanium at the moment, and wood fiber even more so, which basically leaves me with polyiso (XPS out on environmental grounds, EPS on fire / safety).
So, from the studs out, the wall stackup looks like:
-Existing 1x pine board sheathing
-New air barrier (Solitex Adhero or Majvest SA?)
-3″ of polyiso in 2 layers, taped seams. Outer layer detailed as WRB (or is it better to be “belt, suspenders, and clean underwear” and use another membrane product here?)
-The usual 3/4″ rainscreen furring and wood siding of some sort
– Windows installed as “outies” integrated with the outer-surface-of-foam WRB.
On the areas that are fully gutted for interior remodel, I think I can maintain sufficient interior drying potential as recommended when using relatively impermeable exterior insulation. From the sheathing in:
-2×4 stud cavity with dense-pack cellulose or mineral wool batts
-5/8″ drywall or veneer plaster system
-An appropriately vapor open interior finish (paint or tinted plaster)
However, there are a few areas that I’m having trouble understanding the best approach for:
Areas where interior will remain — Wall stackup is:
-2×4 stud cavity with dense-pack cellulose (inconsistent quality)
-Rocklath and 3-coat plaster
-60 years worth of various paints (some oil-based)
Will either the lack of good air sealing on the interior layer, or the lack of vapor permeability due to the old paints, cause issues when paired with a new exterior air barrier and low-permeability rigid insulation?
New bathroom — Walls will be partially tiled, and unfortunately the tub / shower is partially on an exterior wall and will be fully tiled. So, wall stackup will include tile backer board and tile, and I think these walls should be as vapor impermeable as possible to prevent outward drive from the shower. (In the “occupant behavior” category, my wife also enjoys steamy showers without the vent fan being on.) However, if I have a vapor barrier and a big moisture source on the interior, and impermeable exterior insulation, it sounds like a recipe for problems. Thoughts on the best approach here?
Sill plate / rim joist area — As far as I can tell, there is no capillary break, and the sill plate is untreated, but in generally good condition. I’ve seen details that involve cut&cobble or spray foam on the interior and a small layer of mineral wool board on the exterior (instead of the rigid foam on the rest of the wall), and I’ve seen details involving mineral wool batts or other permeable insulation on the interior . I’ve also seen references indicating that the only “right” way to do it is jack up the whole house and insert a capillary break. Thoughts here? Thanks!
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