Proper Outsulation in Climate zone 2A
Hello all,
First, Thanks for the massive resource that is this website.
Second, our location is Mobile, Alabama Climate zone 2A, but urban not coastal. Still hot and humid down south. House is 1960’s just under 1600 sq ft on a vented crawl space, vented hip roof, with a master addition in the early 2000’s.
We are doing a bathroom remodel. In the process of open up a wall behind the old vanity to redo some plumbing we found a surprisingly large hole in the exterior gypsum sheathing. As you can see from the pictures there is a brick wall running along the crawlspace and an exterior false wall was built to bring the old vinyl siding flush with the brick. Fiberglass was used to fill 4 of the stud bays to help keep plumbing from freezing, I guess, and to provide a rodent superhighway.
We needed to replace a few studs and fix the hole so we decided to resheath then entire wall with Zip. The wall in question is south facing as well.
All that build up, to ask my question. I would like to do some sort of outsulation as the previous contractor did, but without the rat super highway.
My idea of the new wall is (inside to out) Drywall, 2×4 wall with Rockwool, ZIP, then the outside 2×4 wall to fillout the brick gap, then Rockwool, then an airgap/rain screen (leaning towards cor-a-vent), then Hardie panel board and batten.
Are there any pitfalls with this? Anything I should do better?
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Replies
Your proposed assembly should work, you will have drying potential to both sides of the WRB. If it were my build (I’m in the Houston area so similar climate), I would stick with regular 1x4 furring strips but install them on the diagonal to accommodate your vertical siding to save some money over Coravent. I would also do foil/faced polyiso on the exterior instead of Rockwool. Better r-value, cheaper, greener, and you get a built-in radiant barrier which on the West side of the house in your hot climate is the ideal location (second only to the roof).
First question is, where would the polyiso go? In between the studs on the outer wall? Should there be an air gap between the zip and the poly since, I think, they are both vapor barriers of sorts? Or would it be better to put the poly directly on the ZIP and then fur out from there for the siding. I have a 4 3/4 in gap to make up from the ZIP to the outside of the brick.
So maybe I am misunderstanding, but to get R13 in polyiso I would need two inches In my area that would be over $450 in foam board. To cover this wall in rockwool I need three bags which is under $250. Now an EPS would be closer to $350, but still more expensive and require 4 inches to match the rockwool.
Obviously this does not take into account the radiant barrier, which I had not thought about. Maybe a 1/2 in layer of polyiso over the rockwool could provide a radiant barrier then a rainsceen then siding?
Polyiso would go exterior of the Zip, you don't really need a gap between the polyiso and Zip (Matt Risinger did not have a gap here, for example, on his own personal home build). I would go with a full 4" of polyiso because you can get them in 2" thickness and do two layers to offset the seams. But you could also fur it out with just the 2" too. 4" foam + 0.75" furring strip would fit the bill for your depth.
Atlas 2" (R13) foil-faced polyiso can be delivered to Mobile for $2.30/sq. ft. I'm sure you can get that even cheaper if you find a local supply house that sells polyiso.
I couldn't find 3" (R12.6) Rockwool Comforboard 80 for less than $2.73/sq. ft and it was at the Mobile Lowe's. Also it said there was over a 30-day wait time. Home Depot just said they were out of stock wouldn't give me a price. I have NEVER seen Rockwool cheaper than polyiso, although I'm sure it's possible in certain areas of the country.
I got the delivered polyiso quote by using a random Mobile address at this site: https://insulation4us.com/products/atlas-energyshield-ref-foil-faced-2-sides-polyiso-insulation?variant=40365054459953
Rockwool quote: https://www.lowes.com/pd/ROCKWOOL-Comfortboard-80-R-6-Stone-Wool-Batt-Insulation-with-Sound-Barrier-24-in-W-x-48-in-L/1000339775
So maybe I wasn't clear enough, I am planning to put the 2x4's back up in a similar fashion to the way they are in the first picture then fill the cavities with insulation. I already have ComfortBATT's not ComfortBoard, so that is why I was hoping to use those.
Obviously this is less optimal than full continuous insulation, but it seems much easier from an installation point for a beginner DIY'er. As long as it wont cause more serious issues, I would like to use the Rockwool Batts. If it is an issues, I can throw them in the attic and invest in foam.
It sounds like the 2x4’s you are describing go outside the Zip. I didn’t fully understand that. Seems like an unnecessary thermal bridge to me, but I suppose it will be fine. Especially if you already have the Rockwool, I don’t see a problem with this setup. It’s not ideal but I doubt it will cause issues either.
matthew25,
In theory I like the idea of diagonal furring, but in practice always run into the difficulty of finding backing for the bottom of the siding - and there is the problem of the cavities that terminate on a corner with no vent or drainage path.