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
-Intello membrane
-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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Replies
> steamy showers without the vent fan ... Thoughts?
If you can get away with it, wire the vent fan to a switch and a parallel humidistat.
> lack of vapor permeability ... and low-permeability rigid insulation?
Walls that can't dry well in either direction are, in some ways, less robust than ones with better drying. Consider using vapor permeable polyiso to retain some outward drying.
Slight positive building pressure in Summer and slight negative in Winter has a large effect on wall moisture in marginal assemblies. Perhaps a good backup plan.
Monitor wall moisture before taking post construction corrective action. Replacing spouses is expensive.
> If you can get away with it...
Funny, I tried installing a little combination humidistat / timer bathroom fan controller in our old bathroom. It lasted a couple days before she made me disable the humidistat function -- apparently "cold air" circulating when one is showering is unhealthy.
> Consider using vapor permeable polyiso...
Is this even a thing? Foil faced is pretty much a very good vapor barrier, but the numbers I've seen for other facings are showing less than 1.5 perms, which seems pretty vapor-closed.
Good point on positive / negative pressures and monitoring moisture -- might try building some sensors into the wall assembly -- but of course it'd be best to make it less risky from the start!
> "cold air" circulating when one is showering is unhealthy.
Note that if the bathroom door is sealed well enough, the bathroom fan will create desirable negative pressure without much flow. Ie, the room will remain very humid but little will move into the walls and ceiling.
> which seems pretty vapor-closed.
Agreed, best I've seen about about 1 perm for 3 inches. On the other hand, 1 perm is about 20x more diffusion drying than foil faced.
If it's a poured concrete or CMU foundation with more than a foot of exterior exposure and good drainage away from the house you don't really need a capillary break on the foundation sill. With less than a foot of exposure or grading that draws surface water toward the foundation it's risky.
If it's a fieldstone or quarried stone or rubble the capillary draw is low, and it doesn't need as much exposure.
If the exterior foam is extending down to the level of the band joist, insulating the interior side of the band joist and the top of the foundation sill with carefully sculpted R15 rock wool batt is fine. The foundation wall itself can be insulated with either R15 board foam (if CMU or poured concrete) or 2" of HFO blown closed cell foam (R14, which is close enough). If the basement has no history of flooding it's fine to use 2.5-3" of polyiso on the foundation walls, as long as the cut bottom edge is not in contact with the slab. If there is history of minor flooding, stopping the foam above the high tide mark, or going with 4" of EPS instead of polyiso is prudent.
Thanks Dana,
The basement wall is CMU (brick on the exterior face), there's at least a foot of exposure all around, and the grading is mostly reasonable. The basement itself stays mostly dry, except for one corner that occasionally sees some water come through the wall near the floor during heavy rain. Sounds like I have a pretty easy basement to deal with and your recommendations hold.
Would I still want Intello or similar to act as an additional air barrier to the interior of the rim joist bays, or would the exterior membrane and continuous insulation suffice for that?
Your walls sound pretty safe in general. The plaster walls should be fine, even with the many coats of paint, as there will be little moisture transport in either direction. Tile showers can be more of an issue in walls like this, but in my experience, most tile shower wall failures are related to poor interior tile detailing than a double vapor retarder. The cellulose provides a fair amount of buffering and, unless it's a ginormous party shower, the wall area is probably small enough that you can get some lateral drying through the cellulose and studs. Bulk water intrusion from either direction would be bad, but bulk water intrusion is always bad. Be fastidious with your window installation and other flashings and you should be fine.
> partially tiled,
> some lateral drying through the cellulose and studs
+1 on this - drill high and low holes in studs to enhance moisture movement to non-tiled areas and then to the interior.
If you want to get radical, understand that in Summer, with a well air sealed exterior, interior side holes greatly enhance inward drying and reduce wall moisture. So one could somehow have interior wall ventilating holes that are then sealed* in Winter. Perhaps another backup plan if you measure a problem.
* Many exterior air barrier homes have interior side holes at wall outlets that leak all Winter. But in Winter, an air sealed interior side is better.
> drill high and low holes in studs to enhance moisture movement to non-tiled areas and then to the interior.
Interesting idea -- and actually the side wall of the shower already has a form of that. The existing exterior wall was framed with flat 2x4s, and we added 2x2's shimmed out slightly from those to get the wall thickness more reasonable and in-plane -- so there are small vertical gaps between all the stud spaces on the long side wall of the shower.