Zip R12 over vertical plank sheathing on my 1720 colonial
Unfortunately, I don’t come across many examples of homes in blogs that are quite like mine and I’m hoping someone might be able to throw in their two cents and tell me if I’m heading in the right direction, or if I’m absolutely crazy.
I own a unique 1720 (year, not sqft) 2-story colonial (climate zone 5 – MA) that is going through an unplanned restoration after some gusty winds decided to send some large pine trees down onto my roof. The house is a timber frame structure (zero wall studs) sitting on top of the 300 yr old stone foundation (basement), which places the sill merely inches above grade. The frame is enclosed in vertical wide plank (18-29″ wide) 5/4 sheathing. The exterior walls consist of this plank sheathing, 30 lb tar paper, 1″ foil (both sides) polyiso rigid foam, strapping and cedar shingles. There is no stud framing at all on the interior side of the sheathing, meaning the only wall insulation is the 1″ polyiso that was installed in the late 70’s, in addition to 5″ of loose-fill mineral wool in the attic. Without any heating or cooling, the indoor temperature of the house is typically 10-20 degrees cooler when 55 degrees or warmer outside (max 79 degrees indoor temp recorded when 96 degrees outside), and a low indoor temperature recording of 31 degrees when the temperature was below freezing down to 0 degrees (monitored over two 24 months). The polyiso was not taped, only held on by strapping, and in many areas, was in full contact with the ground. Remarkably, the exterior side of the sheathing looks about as good as it does on the interior side, no rot whatsoever. When the house was occupied, I went through approx. 1,250 gallons of oil (Oct-Mar). The interior side of the sheathing only had 3/8″ lath and 1/2″ of horsehair attached directly to the sheathing.
Now, I am ditching the oil/baseboard heat in place of a mini-split system and need to rethink the wall system. My plan is to keep the original plank sheathing in place as-is and to cover it again with real 30 lb tar paper lapped, simply because it worked for the past four decades and I don’t see any reason why I should go with anything else. On top of that, I plan to install Zip R-12 sheathing (J roll lap tape the seams, liquid flash exposed penetrations, bent aluminum flashing at sill), purely for ease of installation with the foam attached. On top of that, 1×4 vertical strapping underneath the EWP clapboard siding. Rough window/door openings will be box framed with Advantech 3/4″ (sill at 5 degree slope) and completely liquid flashed, no stretchy tape. Inside of the plank sheathing, I am going to infill the timber frame with 2×6 studs, spaced 24″ oc, and fill the stud bays with Rockwool R23 Comfortbatts. The 2x’s will actually sit .5-1″ away from sheathing. Lastly, the 2x studs will be covered with 1/2 sheetrock. The roof is a cold roof and will have Rockwool R-23 Comfortbatts in the floor joist cavities, 3/4 plywood on top of it, and another 7″ of Rockwool R30 Comfortbatts sitting on top of the plywood flooring, for +/- R50-ish. For some reason, I just don’t like the idea of spray foam.
I’m a college educated pencil pusher, not a home builder, so most of my plans/ideas come from watching hours of videos (I sense eyes rolling) from Fine Homebuilding, Matt Risinger, etc. and reading articles from Martin Holladay. I apologize for the length of my first post, but I’m hoping someone with more experience than me could toss in their two cents (negative feedback preferred!) and let me know if I should put down my hammer and retreat back to my calculator. Thank you for taking the time to read this post.
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