Thoughts on this Wall Assembly for Historic House
Hello GBA Forum:
My family and I recently moved into an 1857/1885 house on the St Croix River in Washington County, MN; so (climate predictions aside) zone 6A. We have replaced an old porch that was collapsing off the back of the kitchen with a post and beam addition that is now fully open to the kitchen itself. I will be insulating the floor of the addition, as well as the perimeter of its encapsulated crawlspace, but I am curious as how to properly fit the walls (both old and new) with insulation (cavity and exterior) and air barriers. I would classify myself as a marginally informed novice, but I have been coming up against some pretty entrenched building practices within the contracting and inspection community here that find my suggestions of ‘rain screening’ and ‘exterior insulation’ to be suspect and occasionally worthy of derision. Oh well, I’ll get over it, but it’s just hard finding knowledgeable people to bounce ideas off of. Tradespeople just ain’t available in this pandemic, so if people here have suggestions, I really would appreciate them.
What I have before me:
Single story 10′ x 13′ turn o’ the 20th century kitchen, gutted to its rough-cut/dimensional 2×4 stud walls and 7/8ths-1″ board sheathing, along with (and fully open to) a single story, new construction, 12’x17′ nominal 2×6 stud wall addition with OSB sheathing.
The plan:
Tape OSB seams; apply WRB all around and tape carefully; apply 1.5″ Rockwool Comfortboard-80 around entire kitchen/addition assembly; cover that with a 3/8″ CorAvent rain screen and bevelled clapboard siding; fill 2×6 and 2×4 stud cavities with appropriate Rockwool Comfort Batts; Cover stud wall interiors with a smart vapor barrier like MemBrain; apply drywall; and then perhaps toast the whole damn thing with a scotch and soda and wish it luck.
Look forward to hearing your thoughts!
Best regards,
David
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Replies
While you wait for experts to weigh in on your proposed wall assembly, take a look at this article; it includes resources you might make use of: Walls that Work
Thank you for your response, Kiley! Had a few fires to put out, but I'm back to this question. I will pick this article apart. Hope I can find a solution there. Thank again!
Is all the siding coming off? If yes, consider a self adhered WRB for the entire structure (OSB and board sheathing) and skip the sheathing tape.
Thank you, Wooba Gooba! Definitely a good idea to look into non-mechanically applied WRBs.
> but I have been coming up against some pretty entrenched building practices within the contracting and inspection community here that find my suggestions of ‘rain screening’ and ‘exterior insulation’ to be suspect and occasionally worthy of derision.
By and large, contractors don't do anything the law and/or local custom doesn't force them to do.
Ask them to explain their position. Anyone with a cogent argument should be able to explain why such things are bad rather than instead hand waving around why they're not necessary.
Thank you, Patrick. I have called a couple on their positions, the ones with whom I've become friendly. With others, I leave it alone; I think there are those that just find 'new' ideas an insult to their fathers who taught them the trade; kind of a 'pick your battles' sorta thing, I guess. Hope to get some good ideas from this forum.
Hoping not to put more than 1.25/1.5" of Comfortboard-80 on the exterior walls, but concerned about dew points and sorption on the OSB sheathing. This seems to be a well-understood topic on GBA.
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David, the only practical hole I see in your assembly is the 3/8" Coravent rain screen over Comfortboard. Neither of those products will hold a nail, so your siding would have to be installed with long nails that reach into the framing. I did that once many years ago, over rigid foam, and found it to be problematic--way too much movement, lots of cracked clapboards, wavy surfaces, etc.. I recommend using 1x or 5/4 wood battens instead.
While Rockwool and other mineral wool products perform well, they have relatively high embodied carbon; I prefer to use low-carbon or carbon-sequestering materials instead, when there is not a significant financial or other penalty. There are some moisture-related advantages to using cellulose insulation. But as long as your house and basement are dry, Rockwool should work fine.
Thank you, Michael! This is very helpful.
I will look further into cellulose options, thank you. If I may: Are you specifically referring to the 'dense pack' variety of cellulose? Curious as to what the 'moisture advantages' you refer to are. I've read, though cellulose stands up well to humidity, roof leaks can be a big problem. Have I read incorrectly? My initial concern was that with a Comfortboard thickness of 1.5" or less, being insufficient to keep my sheathing appropriately warm in zone 6A, I should use mineral wool batts on the inside along with a Smart vapor barrier for the combination's drying potential. The carbon factor with cellulose is very attractive, however. So I'll read more. Thank you, again.
I found 4" siding nails and screws online, and was hoping that a 1 3/8" to 1 5/8' sink into sheathing and framing would be sufficient for the Coravent's SturdiStrips, but I am rethinking that now. Although a 3/4" rain screen might be overkill here in Minnesota, I had been thinking about 3/8ths primarily to keep the walls from becoming too thick, thereby burying the windows, and potentially pulling the house's aesthetics away from their 19th century significance. I will now, however, look into ways to 'cheat-frame' the windows allowing for a thicker wall; hopefully, I will be able to avoid too contemporary a look.
So, as it looks as if I should be thickening the walls anyway, do you think I should just get the ComfortBoard up to Mr Lstiburek's 3" recommendation and forego the internal vapor barrier (Smart or otherwise)? Kinda leaning in that direction now (if I can find an historically sensitive way to furr out the windows), but as code here is poly under the drywall anyway I may well need to contend with some industry eye-rolling. Oh well.
Again - thank you, Michael.
All the best,
David