1835 Farmhouse Exterior Wall Assembly
I am currently renovating an old 1835 farmhouse in zone 5a. This is a timber framed plank house. There are no exterior stud walls. There is the original about 2″ thick structural sheathing/planks that are nailed into the outside of sill and extends and secures to the top plate. Currently, there is plaster installed directly on the face of the interior side of the sheathing. And currently on the exterior, there is the original horizontal plank siding, and then there is vinyl siding that was done probably 30 years ago.
My current plan is to rip both the old and vinyl siding off, demolish the interior plaster, keep the original sheathing since it is in great shape still, and build new interior double 2×4 walls each filled with R-13 mineral wool plus 1″ comfortboard between 2×4 stud walls. This brings the total interior total value to R-30. I then plan to install 3″ of XPS rigid insulation on the exterior (R-15). This would bring the total wall R-value to 45 with 15 (33%) of it on the exterior. I will be installing drywall on the inside with a Class III vapor retarder. I’ve looked around for quite some time now and haven’t seen anyone in a similar situation with the 2″+ thick original sheathing on the outside of house.
Any thoughts or suggestions on this?
Please note I need to fur out my inside walls to match up to the 8×8 posts that are in the corners/outside walls of the house for layout purposes, we are fine with the small loss of floor space.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I would do Larson truss on exterior and leave interior plaster.
I have worked on a lot of renovations to old homes but don't recall seeing that system. It sounds similar to plank-framed construction. In any case, it's just thick board sheathing so nothing is really different from any other house of that era. The boards will have slightly lower vapor permeance and slightly higher R-value than normal ±1" board sheathing but not enough to matter.
If you're planning to use a low-permeance exterior sheathing such as any of the foams, be sure to follow the rules for ratios to limit condensation: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing.
As for control layers, here's a good place to start: https://www.greenbuildingadvisor.com/article/walls-that-work.
Thank Michael, after doing some more research I've realized it is "plank framing". I recently updated my question with some more details on what I plan on doing. Thanks for the reply!