Rainscreen Over Existing Cedar Shingles
I have a 1912 house in Seattle (Marine Zone 4). In the late eighties a second story was added. The downstairs portion has cedar shingles covered with lead paint, 3/4 inch lap sheathing, mineral wool insulation in a 2×4 cavity, with intello db+ on the interior which is taped to reasonable airtightness, then furring strips, another inch of fiberglass insulation and drywall. The upstairs has more cedar shingles, tar paper, plywood sheathing, 2×6 walls with paper faced fiberglass and drywall. I have retroactively air sealed the upstairs with tescon tape and spray foam, but it is likely less airtight than the downstairs.
That’s likely more info than you need. My question is, is there any reason not to just run furring strips over the cedar shingles, using them as my rainscreen water barrier, and hang fiber cement over that?
I wish to do this in stages over the next few years, and I don’t want to deal with the mess from all the lead paint which flakes off easily when disturbed.
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The only problem I can see is getting the furring strips plumb and coplanar.
The advantages to stripping them are having access to the sheathing for repairs, having the ability to upgrade the seismic resistance of the structure, to know where you have backing for the furring, and being able to flash openings without adding jamb extensions. Whether those benefits outweigh the complications of removing the lead painted shingles is a judgement call.
I would get a quote for removing the shingles before assuming it's expensive.
Without stripping the shingles you can't tell how well-attached they are or the sheathing is. In a wall that old there's a good chance there are spots where the sheathing is missing or rotted, the nails have rusted out, the flashing is leaking, etc. Cement siding is really heavy and what you don't want is for it to just pull off.
With the way shingle walls are typically flashed I don't see how you can side over them and keep the integrity of the flashing. Old houses often weren't flashed all that well, they accepted a little bit of leakage and drying to the exterior. By disrupting the drainage plane you could exacerbate flashing problems.
How are you planning on treating the trim around windows and doors?
Normally I hate to see cedar shingles covered up or removed, but if they can't just be painted and it's what you really want to do and they're really that far gone, and you've checked and they are lead painted then...
Just take them off. Follow the RRP guidelines for working with lead which in this case could just mean some amount of plastic on the ground (10feet?) since you're just pulling them and not sanding them. But check for yourself as it's been a while since I had to do any lead painted remediation work. Following the lead paint rules isn't too big of a deal and usually just means plastic on the ground, tyvek suits, and good masks/respirators. It's not nearly as onerous or expensive as the asbestos abatement.
Besides all of that, I don't think there's too many contractors in Seattle who would be willing to side over existing siding right now. Too much hassle and too much other easy work available to want to take it on.