Foam Insulation for a Post-and-Beam 1771 Saltbox
I’m restoring a saltbox built in CT in 1771. This house was built using traditional posts and beams, with 1 inch thick oak random width VERTICAL planks for the exterior sheathing. That was topped by clapboard siding. The plank sheathing was installed with no attention to air movement – there are vertical gaps between most planks, but in some areas there are gaps as wide as 6 inches! So I am looking for guidance on the best approach to insulating and siding this house. Windows appear to be the 2nd generation in the house, and the vertical planks have rough openings up to 4 inches per side too wide for the current windows, thus windows are only held in by the exterior trim nailed into the plank sheathing.
I have been planning all along to use foam panels to insulate and air seal the house, since it is incredibly poor from an air infiltration perspective. I have been planning on adding a stud wall to the interior between the posts to provide 16 inch on center framing for electrical, plumbing, fiberglass cavity insulation, window mounting and interior finishes. I also need the stud plates to repair the bottoms of the vertical planks which have rotted to heights above the sills in some areas.
Thus, here are my questions:
1. Am I better off putting a layer of foam on BOTH sides of the plank sheathing and air sealing on the inside panel with foam to the posts and beams before installing the stud wall? Or is it easier and just as good to put BOTH layers of foam on the outside? Again, I want to make this house warm and tight which was not a priority in 1771.
2. should I use 2X4 or 2X6 studs with the exterior foam? Is it worth it for the extra R-value in a deeper cavity when there will be foam on the exterior? If using 2 layers of foam (2@R6.5=R13) PLUS R-13 Fiberglass( 3-1/2″) that is R-26 – isn’t that enough for CT winters when most heat is lost through the ceiling? I also need to consider the stud wall thickness since we do want the posts and beams to be partially exposed…And if the tapered posts are not at least 6″ deep at the base then we may be forced to use 2X4’s…
3. Given that I plan on using the fiberglass in the wall cavity, should I use 2 one-inch layers of foam or only one? (again is the cost of 2 layers worth it in the walls?)
4. Given the situation with the 1″ thick plank sheathing, I think that I should add a layer of 1/2″ plywood or OSB to provide structural rigidity (and reduce air movement) to the building – even though I plan to trim clean and reattach the bottom of the planks to the stud plate or additional studs installed on the flat, I feel that the plywood is the best to provide rigidity as well as a good base. What order is best for installation of these items? Planks then 2 layers of foam then plywood? That’s 3-1/2′ thick BEFORE penetrating the studs…and will make installing plywood on the whole house difficult since air driven nails are not available in longer lengths…Or should I skip the plywood since the reattached planks should be strong enough? Then I could use 5 inch TImberlock screws through the 3-3/4″ plank/foam/furring.. With the current condition of the planks, the house still appears straight, no sagging or racking. So perhaps the post/beams are rigid enough and adding plywood is not necessary..What would you suggest?
5. At what wall thickness are the plywood boxes required for the windows? Your walls in the article were 5-1/4″ excluding the studs. It doesn’t seem like much work or cost for the added protection.
6. I have log rafters for the roof on 4 foot centers. What would you recommend for foam panel/cold roof insulation on the exterior since I plan on re-roofing anyway and can also add fiberglass between the log rafters?
7. Is there a need for housewrap on top of the foam or since the air barrier is already there, is this overkill? Is there a problem if it IS installed on top of the foam panels/plywood?I am also retrofitting a mountain cabin which had no sheathing (only plastic & Vinyl siding) and already applied foam panels onto studs, followed by OSB sheathing then housewrap.. Siding has not been installed yet so it can come off if a problem.
8. Lastly, the “ell” on the back of the house is 1 story with a cathedral ceiling. Same log rafter arrangement. To leave the rafters exposed and provide a wainscot ceiling effect, I was planning on removing everything down to the rafters (shingles, plywood, 1/2″ horizontal planks) and then installing T&G 1″ beadboards ,then add foam layers, furring, plywood/OSB nail base and then shingles…Does this sound correct? What about venting or does the rain plane cover that? There obviously is no fiberglass in this ceiling so what should the foam thickness/R-value be?
Sorry for so many questions, but I can only do this once and prefer it to be the CORRECT way…
Thank You in advance…