Exterior siding system and interior stud bay strategy for 1885 house?
I am renovating my 1885 house (first time home owner!). I’m looking for recommendations on how to approach my exterior wall / siding system and interior wall system.
My goals:
– Have a weatherproof exterior, preferably with cedar shingles
– Insulate the internal walls within reason (but such that water that gets in can dry)
– Minimize rebuilding, build strategically on what’s in place (the house is in good shape, something’s clearly working)
– Have the above support my goal of running heating/cooling entirely on mini-split heat pumps
My questions:
1. What is a strategic approach to the exterior wall system that strategically builds off what’s already there and is cost effective?
— Can I keep the lap siding? Must I put on sheathing? Is going over it reasonable? If so, with what given that’s it not flush / flat and is old wood (not plywood)? Tar as the WRB or Tyvek? I’ve heard folks say it needs to be something sufficiently breathable for the original wood lap siding to breathe, but not so much such that you make the house too air permeable and inefficient given my heat pump goals. Would the non-flush, corrugated nature of the plane act as a reasonable rain screen if I put new cedar shingles on top of that (as clearly was done at one point)? Or something like a drainage wrap like hydrogap?
2. What should be pursued from the inside to allow appropriate breathing if water gets in, but also get reasonable insulation and air sealing?
— For e.g., I could use the spray foam with tar paper in the stud bays technique (see links below) to get the highest R-value, at a financial and environmental cost–is the pursuit of the highest R-value misguided and unnecessary?
Thank you!
– Luke
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Additional Context:
– Climate zone 4A
– I plan to have HVAC run completely off of mini-splits, no backup (formerly oil)
– The current exterior is a layer of Dolly Varden style lap siding directly on the studs that originally sat on tar, but is now torn up (the exterior plane made up of that siding is not flush / flat), tar paper on top of the lap, cedar shakes, tar paper, old concrete siding). There is no sheathing such as plywood, zip, etc. I suspect the lap siding provides structural support.
— More on Dolly Varden style siding: https://thecraftsmanblog.com/the-ultimate-guide-to-wood-siding/
– Crawl space
– I have original 2×4 studs, the bays are empty, lathing has been removed, plan to put drywall up
I’ve read these:
– https://www.greenbuildingadvisor.com/article/insulating-walls-in-an-old-house-with-no-sheathing
— My situation is similar to this ^, but I’m open to doing work on the exterior, not just the interior as was the case in this article
– https://www.greenbuildingadvisor.com/article/managing-water-and-insulating-walls-without-sheathing
– https://www.greenbuildingadvisor.com/article/insulating-walls-from-the-interior
– https://www.greenbuildingadvisor.com/article/insulating-old-walls
– https://www.youtube.com/watch?v=jhoQ_f9ZnzQ
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Replies
Luke,
There are lots of issues here. I'll hit a few highlights.
1. You wrote, "I’ve heard folks say it needs to be something sufficiently breathable for the original wood lap siding to breathe, but not so much such that you make the house too air permeable." The term "breathe" is ambiguous, and best avoided. If you want a building assembly to be able to dry in a certain direction, you probably want the assembly to be vapor-permeable (which is different from air leaky). Every wall assembly should be as airtight as possible. Whether or not the assembly is vapor-permeable or vapor-impermeable depends on you goals (and which direction, if any, you expect the assembly to be able to dry).
2. The way you will need to proceed depend on your goals and your budget. Moreover, some decisions would need a site visit to understand fully. One example: If your wall assembly currently lacks wall sheathing, installing wall sheathing would always be desirable. But it might not be affordable. Living in an old house may require compromises, especially if your budget is tight.
3. Whether or not you should keep the siding or install new siding depends in part on the condition of the existing siding. Other factors include the condition of window flashing (if any), and whether or not there is evidence of water entry due to wind-driven rain. The amount of water entry depends not only on flashing condition but also on the width of your roof overhangs.
Thank you!
I) Re: sheathing - is it unreasonable to treat the existing siding boards / shiplap on studs as sheathing (if they're in good shape) and put a WRB on top of it? I suspect they provide shear strength as a sheathing would. I'm unclear if there are major downsides to going this route. If I took it off and put up plywood (instead of zip), would I really gain much?
II) My hypothesis is that if I were to treat the original wood siding boards as sheathing, to be safe, I should still probably maintain a gap behind them in the wall bays as described in your article before putting in insulation in the bays: https://www.greenbuildingadvisor.com/article/insulating-walls-in-an-old-house-with-no-sheathing. Then I could also, in theory, add a rain screen between this "faux-sheathing" and any siding installed on top of it. The resultant wall assembly, from outside to in, would be:
- a) new siding
- b) (possibly optional) rain screen
- c) WRB
- d) old lap siding on studs acting as my "faux-sheathing"
-- inside --
- e) studs
- f) stud bay air gap
- g) insulation
- h) drywall
Is there anything majorly unreasonable or risky about this that I'm missing?
(assume that the walls are already open and I'm going to add new siding anyway in place of the cement shingles on top of shakes (which sit on top of the old ship lap, aka "faux sheathing"))
Q. "Is it unreasonable to treat the existing siding boards / shiplap on studs as sheathing (if they're in good shape) and put a WRB on top of it? I suspect they provide shear strength as a sheathing would. I'm unclear if there are major downsides to going this route."
A. Plywood or OSB provides shear strength in a way that horizontal siding boards can't. (Because plywood and OSB come in large panels, they act similarly to diagonal board sheathing when well nailed.)
You wrote, "The exterior plane made up of that siding is not flush / flat." I'm not sure if you are simply referring to the profile of the siding, or some defects that are more serious. But I would hesitate to spend the money you are contemplating spending without good sheathing.
The problem with old houses is we all want modern insulation and that simply is not possible without adding modern water and air barriers to allow it to work without getting moldy. By the time you manage to add this modern stuff and get rid of the old lead paint the only thing you get to keep and reuse is some oddly shaped and spaced 2x4s.
My guess is if you wrote a full and honest budget and valued you time at $10 an hour the total would exceed 3 times the market value for the finished home.
I have a soft spot for old houses but not that soft.
Walta