1974 construction retrofit
A friend is looking to replace siding and windows on his 1974 house in climate 4 marine. The main floor and upstairs walls are 2×4 and the basement walls are 2×6. There is timber framing inside that I suspect bridges to the ext sheathing w/o any insulation. Where he has removed some of the vinyl siding, he found fiberboard sheathing (R1.32 stamped on it). I don’t know what is under that. (The garage is just studs + fiberboard + siding). The gable walls have 3″ overhangs and the roofing is 2 years old.
The wind can be felt inside the house at any given time and there is bulk water intrusion at some of the windows and deck attachments.
That is a concrete sky bridge going to the front door – the Google photo doesn’t do it justice!
The first quote he got was far beyond his budget. I suggested he do all the work up to the top of the rake walls, ~13′ (and hire out the upper).
I suggested he remove all the sheathing (progressively, in sections), remove any insulation and air seal j-boxes, replace insulation, Zip + tape the walls. Once the entire house looks like a Huber advertisement, then add 2″ of EPS and reside the house.
Is Zip + tape suitable under the EPS or should there be some Tyvek too?
If there is some poly behind the sheetrock, should it stay or go?
How would he insulate the inside of the rim joist from the exterior?
Is a rain screen over the EPS a necessity?
What does he do about a lack of overhangs on a new roof?
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Replies
Mark,
Q. "Is Zip + tape suitable under the EPS or should there be some Tyvek too?"
A. Zip sheathing, installed according to the manufacturer's instructions, is a water-resistive barrier. You don't need Tyvek, as long as you don't mind the fact that all of the window flashing and door flashing will need to be integrated with the WRB -- in this case, the Zip sheathing. For more information, see All About Water-Resistive Barriers.
Instead of installing Zip sheathing plus EPS, it would be possible to install Zip R sheathing. For more information on Zip R, see Nailbase Panels for Walls.
Q. "If there is some poly behind the sheetrock, should it stay or go?"
A. I would remove the polyethylene once the stud bays are exposed.
Q. "How would he insulate the inside of the rim joist from the exterior?"
A. One method is described in this article: How to Install Cellulose Insulation. (Scroll down until you see this heading: "What about the rim joist area?")
Q. "Is a rain screen over the EPS a necessity?"
A. It is for most types of siding. You might get away without furring strips, however, if you install vinyl siding or corrugated steel panels.
Q. "What does he do about a lack of overhangs on a new roof?"
A. Extend the roof. It's tricky, but possible.
-- Martin Holladay
Great info. Thank you.
Notes:
ACME and Premier are proud of their nailbase. Polyiso is an unfaithful friend in the winter. For flashing the window heads, I might just integrate some tyvek into the seam of the next row of sheathing. It could just come out of the wall and down to the window. No reliance on tape or fluid applied wrb.
I was hung up on how to dam the joist bay. Great solution. We could practice on some mock-ups.
Maybe Chuck Miller has a roof stretching video.
"Polyiso is an unfaithful friend in the winter. "
How so?
At zone 4 marine wintertime outdoor temperatures polyiso doesn't have to be derated at all if it's the entire insulation layer in a wall application,, and will deliver R5+ /inch even if it's the exterior 20% of the R value. Inch for inch polyiso will always beat EPS in that climate, even if it's just one skinny inch on the exterior of a 2x6 /R23 rock wool wall. The 99% outside design temps in that climate zone are typically ~+20F or higher, and the January outdoor mean temps of ~+40F. That puts the mean temp through the outer inch of foam north of 25F, and the average wintertime mean temp through the foam 45F or higher. Even the very worst performing sample in this set did better than R5/inch at a mean temp through the foam of 45F, most were running ~R5.5 or higher:
https://www.greenbuildingadvisor.com/sites/default/files/Polyiso%20penalty%20-%20BSC%201.jpg
Assume a 2" layer would be warmer still with average performance better than R11 in your climate, whereas even graphite loaded Type-II EPS would still be well shy of R10 performance over the heating season.
"Assume a 2" layer would be warmer still with average performance better than R11 in your climate"
Glad to know that. I thought the R fell off earlier than it does.
Q. "What does he do about a lack of overhangs on a new roof?"
A. Extend the roof. It's tricky, but possible.
Martin,
Would it make more sense to insulate the roof first before insulating the walls?
(Cut off eves, seal walls to roof, stack on 3 layers of polyiso + 1 layer EPS, install new rake eves and lookouts for gable eves, new roof.)
https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing
Mark,
The approach you describe is called the "chainsaw retrofit" approach. GBA has many articles on the topic:
The History of the Chainsaw Retrofit
A Real Chainsaw Retrofit
Chainsaw retrofit: a time-lapse video on YouTube
In the following article, Marc Rosenbaum describes a similar technique for a new construction project:
Airtight Wall and Roof Sheathing
-- Martin Holladay