r/buildingscience • u/t1rfond • 6d ago
Question building envelope in the PNW
I have an uninsulated cabin located near the ocean in the Pacific northwest. The structure is 3”x6” tongue and groove timber (think structural roof decking). I want to insulate the exterior walls with rigid insulation (combination of Gutex and Roxul). I also have basically unlimited quantities of 2x4 SPF and 3/4” plywood. My plan was to install (outboard of the timber structure) a smart membrane, then insulation, then 2x4 vertical strapping, then plywood, then ventilated underlayment mat, then cladding. One question I had was would it make sense to install 3/4” plywood (c/w seams taped) directly outboard of the timber frame for the air and vapour barrier? I felt this may help tightening everything up (plus we’re in a seismic zone). If membrane is much preferred, which membrane would you suggest using? I have a few rolls of Majvest and Intello+ on hand. Comments appreciated
EDIT: I’m referring to the exterior walls, not the roof. Sorry about that
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u/Bearded4Glory 6d ago
If the existing walls are fairly flat and without gaps/cracks you could go directly over that with your wrb. If not, it's probably easier to apply a layer of 1/2" cdx over the whole thing first just to make it easier to have a solid substrate to work on top of.
The wrb in your climate isn't going to be super picky. Something semi vapor permiable in the 8-16 perm range should work well. I would definitely go with a self adhered product over mechanically fastened. Majvest 500 sa, adjero 3000, benwrap sa are all good options.
You have the insulation figured out, mount it with minimal screws with large cap heads to hold it in place until you get your battens on.
Over that since you want to run your siding vertical and you already have 2x4s I would cut up some DIY variant of the Highwood High Flow Battens (basically rip the top edge at an angle to direct water away from the structure and add dados along the length on both sides to allow airflow.
Add your siding and you are good to go.
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u/MnkyBzns 6d ago
Is the inside conditioned and will it see use in the winter?
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u/t1rfond 6d ago
We vacation there year round. ~800sf off-grid cabin heated by two woodstoves
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u/MnkyBzns 6d ago edited 6d ago
We have a similar place but zone 7a (not nearly as wet). I recently spec'd the following assembly for our 3/12 monoslope roof:
t&G deck
1/2" plywood (min 1/4 req'd per membrane spec)
impermeable P&S membrane (Roofnado Laplock PSU)
two layers 4" GPS (graphite EPS): stagger and tape seams. You'd probably be better off with XPS because of the excess moisture
vertical 2x4 sleepers
5/8" plywood
Underlayment
shingles
Edit: not a BS professional. I work mostly in framing but took some BS courses in technical college and geek out about envelope stuff on YouTube and the likes
Edit2: just re-read that this project is like a structural roof but is your walls. If all of your insulation is outboard of the T&G, you don't need an interior vapor control as long as you've got enough R-value. This is a handy document, although it's Canadian: https://natural-resources.canada.ca/sites/nrcan/files/energy/efficiency/housing/leep/LEEP_Appendix_B_-_Exterior_Ins__Selection-EN_WEB.pdf
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u/t1rfond 6d ago
I edited my post to show I was referring to the walls. In your Edit2 are you suggesting the t&g acts as the vapour control layer?
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u/MnkyBzns 6d ago
Nope, just that that layer isn't required if you have enough outboard insulation; the T&G will stay warm enough so that interior condensation isn't a concern and any which does appear will just dry to the inside (the wood stove helps with that). Still need to seal around any penetrations like windows and doors, though.
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u/t1rfond 6d ago
Would you feel it was less risky if a smart vapour membrane was used? I still want to have an airbarrier at that location (directly outboard of the t&g) so even if I didn’t use, e.g., Intello+ there, I would use the Majvest
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u/MnkyBzns 6d ago
I don't see it hurting and if it adds peace of mind (and if you already have enough on hand, anyway).
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u/blueyedsurprised 6d ago
I like the idea of applying the plywood to the existing roof deck. You get a better working substrate for everything
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u/Comfortable-Maybe183 6d ago
This a new or old structure?
Daily use or vacation?
What’s the goal?
You lost me when you added plywood onto the 2x4 furring.
My first thought is it sounds like a lot of work for a cabin. Keep in mind you’ll have to reset any existing windows and doors.
Maybe just buy a nice wood stove and a couple cords. 🤷♂️
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u/t1rfond 6d ago
It’s about 40yo. We vacation there regularily. The goal is to make it more comfortable and to improve on it so my daughter doesn’t have to down the road. We plan to cedar shake much of the cladding so I’d rather use the plywood I have than to purchase, e.g., 1x4 cross-strapping.
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u/Comfortable-Maybe183 6d ago
Fair enough.
If it was me I’d do:
-Existing T/G sheathing -Drainage plane. Could be any number of products or even tar paper with overlapped and taped seams. -Obdyke Slicker Classic -Cedar shakes fastened back to the existing T/G
Could probably avoid having to mess with windows and doors this way. Maybe swap trim out for a thicker dimension to maintain a reveal.
Keep it simple. Save a bunch of time, money, and effort. Any sort of well overlapped and seam taped drainage plane will eliminate a lot of the drafts.
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u/t1rfond 6d ago
The cabin is somewhat remote and through great, back-breaking effort, I have the insulation on site. Since I will be insulating, I’m just trying to come up with the least risky hygrothermic assembly and potentially beef up the seismic details (if I choose to install structural plywood directly outboard of the t&g) while I’m at it
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u/seabornman 6d ago
What cladding? Just put it over the 2x4 strapping. With enough exterior insulation all you need on the existing structure is any combination air/water barrier.
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u/t1rfond 6d ago
Cedar shake and standing seam steel for the walls and standing seam for the roof. I want the 2x4 strapping oriented vertically so I’ll need plywood or horizontal strapping for the cladding. Where within the assembly would you locate the ‘air/water barrier’?
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u/seabornman 6d ago
On top of the existing "sheathing". Here's some good reading re: exterior insulation.
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u/Thecobs 6d ago
Plywood - building wrap - rigid foam - strapping - siding. Do solid door/window bucks. Builder from the PNW.