Exterior wall insulation in 1855 house no sheathing or paper
Restoring a 1855 Greek revival house in the gold country of CA. No insulation in the exterior 2″x3″ walls. No sheathing, no building paper, no headers no real foundation, square nails pretty much intact except for all modern amenities. You can see light thru much of the lap siding and evidence of water penetration on the siding. No rot no mold and amazingly square and plumb. If insulate I worry that I will trap moisture on the exterior siding. It hasn’t rotted in 1855 years so I dont want to create a problem far worse than high energy bills. 1 insulate with batt or blown but put typar on the inside face of exterior walls. Worry this will create trapped moisture, no airspace 2 Insulate with 2″ rigid and leave a 1″ airspace Any suggestions. I think spray foam is out because of cost and possibly dmage to the siding
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I would consider polyiso foam panels, or new insulated stud walls inside. But, even that will reduce the air circulation in the original walls, and could lead to problems. I think foam sprayed in the old wall cavity would be the best option. I don't think typar or batts inside the old wall would be a good solution.
Do any state agencies have suggestions? I'm guessing this is common in your area and some research has been done on how to deal with it. Check with the agency responsible for energy efficiency.
You are right to worry about trapping moisture!
I think alot of people have gotten into trouble with this because they want to keep both the exterior siding details AND the interior details. I don't think you can have both.
If you can't keep water out of the walls due to the old siding, you need to allow it to dry out to the outside. You could add a thin layer of strapping to the inside wall and apply rigid foam board there, then drywall through the foam to the strapping. The strapping provides a break so the studs can fully dry, even on the back side. This detail obviously shrinks your internal rooms a few inches in every direction, including down from the ceiling. A new leak free roof would also be required if yours was of unknown vintage.
First off, this is relatively arid country, right? But the problem is that the entire year's rain quota falls in two months? So it's not Seattle-like weather you need to worry about, but you can't just ignore the rain either?
I wonder if you could take a strip of housewrap or felt, staple the top near the top of the stud cavity, then tuck the bottom end in between the clapboards near the bottom. Would be far from perfect, but should keep out 70-80% of the moisture that otherwise would get into the stud cavity.
Then fit in a piece of foamboard, but hold it about 1-1.5" away from the siding. (You might want to insert some pieces of lath to maintain this spacing.) Foam the edges of the board, I'd guess, unless you can get a "squeek" fit. Then you could possibly add a thin layer of FG (depending on how the foam is) before you SR.
Just a thought.