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 ([email protected]=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…
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Replies
Hire a consutant is my best advice. Have you paid for a Pro membership here? Do you subscribe to JLC and Fine Homebuilding? Do you already do construction?
You are biting off a major project. DIY is possible but with just free advice... you will get what you pay for. Your success will depend on your studying and at least paying for the items I listed above. I do this for a living and going green even for me is difficult and risky that all will turn out as intended with no mold or wetting or ovedoing or underdoing or goofy doing.
I am about to do the Pro membership.. and do subscribe to FH...I already do construction and this is my 3rd house, but first as old as this...I've got a restoration contractor guiding me as well as a historical architect.. but what I've found is that I need an EXPERT opinion on some of the questions.. Even the pros I am dealing with have different opinions on the merits of airsealing and thickness of wall insulation.. Hence that is why I posted on this blog.. so far with only your response. Will a Pro membership provide access to that expert guidance?
Gary, you are asking about your entire build in one post. I know three contractors that could do your project. To completely advise you via some GBA chit chat is most likely to lead to a plethora of advice, not all the same.
If you search here and join there are discussions to do with all that you ask. Maybe some posts will follow here too.
Join the site, the cost is worth one of the bazillion pages.
Gary,
Q. "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?"
A. Put both layers of rigid foam on the exterior side of your wall sheathing. Creating a foam sandwich with wood between two layers of foam is not a good idea.
Q. "Should I use 2X4 or 2X6 studs with the exterior foam?"
A. It's up to you. Do the math. You need to run some energy modeling software to calculate the effect of the proposed specifications, and you need to estimate the cost of the two options.
Q. "Should I use 2 one-inch layers of foam or only one?"
A. Two layers of rigid foam with staggered seams are always better than one layer of foam. In your climate zone (climate zone 5), you need a minimum of R-5 of rigid foam for 2x4 walls, or a minimum of R-7.5 of rigid foam for 2x6 walls. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Q. "I think that I should add a layer of 1/2" plywood or OSB to provide structural rigidity. ... What order is best for installation of these items?"
A. The plywood needs to be installed on the outside of the existing sheathing. Then you install a layer of housewrap (optional), and then your rigid foam.
Q. "At what wall thickness are the plywood boxes required for the windows?"
A. The usual recommendation: You need plywood boxes if your foam is 2 inches thick or thicker. More information here: Nailing Window Flanges Through Foam and also here: How to Install Rigid Foam Sheathing.
Q. "What would you recommend for foam panel/cold roof insulation on the exterior?"
A. In your climate zone, the code calls for a minimum of R-38 ceiling insulation. This can be acheived with 6 inches of polyisocyanurate. More information here: How to Build an Insulated Cathedral Ceiling.
Q. "Is there a need for housewrap on top of the foam?"
A. Read this: Where Does the Housewrap Go?
Q. "There obviously is no fiberglass in this ceiling so what should the foam thickness/R-value be?"
A. The answer for the ceiling above your ell is the same answer I gave before: In your climate zone, the code calls for a minimum of R-38 ceiling insulation. This can be achieved with 6 inches of polyisocyanurate.
A final comment: Your project sounds like it would benefit from the professional services of an architect as well as an experienced general contractor. You are asking so many questions that there is a good chance that your are biting off more than you can chew.
Finally, before posting more questions here, I suggest you read the articles on this page: How To Do Everything.
Thanks to ALL who replied. Education , experienced advisors, and a good plan is the best means to success. .Your comments and the additional info I've found since my original post has answered most of the questions...I'm working with a historical architect as well as historical renovation contractor.. but their advice on air sealing and tightness of the house is not as clear cut and in sync as that expressed here...I prefer to take the old house up to 21st century standards...and not just conventional code requirements...Thanks again