Insulating Unreinforced Masonry Walls
I am in the process of remodeling a 1939 brick rambler in Salt Lake City (zone 5). We are working to tighten up the envelope and improve the insulation. The current masonry walls are a lathe and plaster > CMU > air gap > brick. I’ve read Martin’s articles and comments on insulating old brick, all of which are exceptionally helpful. I know Martin recommends ccSF and is hesitant to recommend anything else. That said, for a number of reasons (repair, cost), I am curious to hear this group’s thoughts on doing two layers of polyiso, taped and staggered seams, then internal 2×4 wall for mechanical (and sheer strength, it will be sheathed) with either bats or blown in cellulose. We will likely do drywall gaskets and/or aero barrier to get a solid air barrier.
Current brick has no moisture issue and Salt Lake is high dessert, so not a ton of moisture. Overhangs will be solid north and south, but no huge east and west.
This would make the project easier for trades, as the main floor would largely match the basement insulation assembly. Particularly with the CMU and the air gap, based on some of Lsturbiek’s comments, there should be enough exterior drying in our climate. It’s been a process trying come up with assemblies, but this sight has been a huge help.
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As long as you have good attention to detail and understand that the process is somewhat tedious, I see no problem with your proposal. Spray foam is best on uneven rubble walls but with finished brick it shouldn't be too hard to use polyiso. Ideally you would use fiber-faced polyiso so the brick could dry toward the interior but with that much insulation it's probably fine to use foil-faced polyiso.
I had faced polyiso insulation over masonry fail, turn into a moldy mess, this is not a robust assembly as it will prevent drying of any moisture that makes it between the masonry and the insulation. I don't know if unfaced insulation would hold up, I would not take a chance and go for closed cell SPF.
Thanks Michael for the response. That was generally what I thought, but I'm just a green building enthusiast, so its nice to hear this from someone actually in the practice. I definitely will need to work closely with the trades and my GC to make sure details are clear and the job is done right, particularly from an air sealing perspective.
One other question, what about keeping the lathe and plaster in the assembly? It seems like it would potentially give us an even flatter surface to work against and be yet another air and vapor retarder before the brick. It would also make demo way easier. That said, I don't want fast to get in the way in the way of a solid assembly.
I'm just bumping my last question for this brain trust. The new plan is to use 2 inches of closed cell foam straight over the plaster on the masonry, and then use bats interior of that and then drywall.
My main question is if this group see an issue with the plaster staying in the assembly? It seems like the ccSF should largely take care of moisture from inside the envelope, and the complete assembly can dry from the foam to the exterior. Because the plaster is essentially part of the wall assembly (its keyed into the interior masonry rather than being run over lathe), I'd really prefer not to pull any more plaster than I have to. It also helps to keep more of it out of the landfill. Thanks in advance for any guidance.
If the plaster is directly on the masonry and there is not air space behind it, there is no problem with keeping it.
It is pretty common here to insulate stone rubble basements around me with closed cell SPF and the parging (at least the parts that are not crumbling) is always left. These hold up just fine.
Thanks Akos. That was what I suspected, as it seems like the plaster has a decent capacity for moisture and can still dry to the exterior. While I’m happy to be saving bricks from the landfill, these old houses make for complicated assemblies. Consistent with you comment above, we are going to go with 2” of ccSPF and then likely batts in the remainder of the cavity. I had the 2x6 walls framed a half inch off the masonry to allow for some continuous inflation across the full masonry.