Wall assembly options for SF Bay Area Victorian
Hi all. We’re doing a deep remodel of a Victorian from the late 1800s. We live in the Climate Zone 3 (East Bay of the SF Bay area). I need to make a decision about our wall assembly and the different options are vexing me. I would appreciate any/all advice from this community.
What exists today is original redwood siding, redwood framing, and sheetrock. The previous owner neglected it and there is some rot, but generally it’s intact. The roof has overhangs and moisture doesn’t seem to have compromised the siding much. I suspect there’s so much air passing through that everything dries out quickly.
My architect and their waterproofing consultant suggested closed cell foam applied directly to the interior face of the siding as insulation and vapor barrier. When I found Martin’s piece “Insulating Walls in an Old House With No Sheathing” [https://www.greenbuildingadvisor.com/article/insulating-walls-in-an-old-house-with-no-sheathing] I grew concerned and started researching other options.
Martin suggests we leave a 3/4″ air space behind the redwood siding, followed by either closed cell (if we want a vapor barrier), or rock wool (if we don’t). I’m told this might compromise our Title 24 efficiency rating, so we might also need interior rigid foam. This home needs plywood sheer walls as well, so the full assembly would be redwood siding, air gap, insulation, plywood, rigid foam, sheetrock. My question here: does this make sense at all? Which combination of materials? Do we want a vapor barrier at all? All things being equal I’d rather avoid closed cell b/c of the greenhouse gasses.
A contractor suggested stripping the redwood siding and installing a modern wall assembly, with new pine siding, house wrap, exterior sheathing, and something like rock wool in the cavity. You can imagine this adds cost, including carefully removing and reinstalling the fiddly Victorian brackets and decorative fascia boards. I’m told this would add around 50k to the project cost.
In case it’s useful, the total sqft is 4300 across two units. Our projected load calcs with the originally designed closed-cell assembly are 30k btuh heating, 38.8 btuh cooling. We plan to heat & cool with heat pumps, and we’re mechanically ventilating with an HRV. There is an Aeroseal installer in the area, so if we keep the exterior siding we could try achieving decent tightness that way.
This is my first time asking a question here. I did my best to search past questions first, but my apologies if I overlooked a previous answer. Thanks in advance.
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T24 has changed a lot in the last few years, and being this is is a "historical" house, I highly recommend Ann Edminster, Architect from the Bay area, and one of the pioneers of sustainable and green building in the US. If she can't do it, she'll recommend someone else locally.
See: https://annedminster.com/ or 650-355-9150
Thanks Armando! I will look into this.
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One thing to keep in mind is your wall cavities are almost certainly a full 4" if you're dealing with an early 1900's house that was probably built with roughsawn redwood.
I don't have any extra advice other than if you take off the old redwood siding, remove it carefully and salvage it. I demoed an old exterior wall from a house in the South Bay and made sure to the save the redwood clapboard because it looks gorgeous after you clean it up.
Thanks! I personally will not salvage the old siding because it's absolutely crawling with lead, but maybe somebody would. I suspect whoever demos the siding would hazmat it.
Sorry for the downer reply, but our last remodel gave our son lead poisoning so I will forever shout from the rooftops for folks to be lead safe as penance for my sins. :(
I would read through this:
https://www.greenbuildingadvisor.com/article/managing-water-and-insulating-walls-without-sheathing
Look at post #2.
Try to find out what your code allows for renovations and aim for that. Going overboard on insulation in you are in warm climate might not be worth it. What would be considered code min (2x6 +R5 rigid) up north, would never have an ROI there.
As for the load, if these are reasonable sized semi/townhouse, the post insulation upgrade number of 30k btuh heating, 38.8 btuh cooling sound very questionable. This is the heat load for a ~1800sqft insulated 2x4 house in much colder zone 5, I can't see it being correct with your near 40F outdoor design temperature. The 38k cooling sounds even more silly unless you have a wall of south/west facing glass.
Thanks, I missed this article when I searched yesterday. It sounds like there's a viable path to keeping the existing siding. Sounds like I need to bounce ideas off the T24 certifier because our stud bays are actually just 4" nominal (not 4" actual). After accounting for the 1/2" gap and the water-repellent rigid there won't be room for much else in the bay.
Thanks also for the comment on load calcs. I'll double check those. They came from Monterey Energy Group which I think is a pretty respected outfit. Is it possible they seem off to you because this is a 3-story home including living space in the attic? The per-floor loads in their report are:
Basement (1638sqft): 12.9k htg, 12.6k cooling
Main level (1618sqft): 10.9k htg, 17.8k cooling
Attic (1040sqft): 6.1k htg, 8.3k cooling
The design temps in the report are 38° / 88°.
Hi Brian, I am curious about what you ended up deciding on. I am in a similar situation with an old house remodel in the Bay Area where the wood siding is attached directly to the frame, with a thin deteriorating layer of tar paper. Did you create that airgap and then add some WRB in the form of rigid foam like the Martin article suggested?