Detailing exterior insulation on a CMU building
Been a longtime fan of GBA but am new to posting!
I’ve been slowly renovating an old mechanic’s garage into a small apartment and large sculpture studio. I’m in Climate zone 5 and the building is single wythe CMU, built in the 1950s.
Back when I bought the building I didn’t know much about building science and decided to insulate the interior first using 2.5” InSoFast EPS panels, R10. The panels are glued to the walls to form a continuous insulated layer on the inside of the building, and have drainage channels on the CMU-facing side.
As the inside is nearing completion I’ve turned my attention to the exterior. I’d like to add a continuous layer of either Rockwool Comfortboard or reclaimed EPS to the outside of the building and put siding up over that, to bring the walls up to R22.
Given the fact that I already have a continuous layer of EPS on the inside, albeit with moisture channels (in retrospect I would have done this differently, alas), what are my best options for the exterior? If I hang continuous foam on the exterior, do I need a drainage plane between the CMU and the foam? I’ll add furring strips over the foam to create an air space for the siding, but I’m concerned about the CMU’s ability to dry. It’s not particularly wet on a day to day basis, but neither the foundation nor the slab were insulated or constructed with a vapor barrier so I imagine there’s capillary action that will continue to bring low amounts of moisture into the block from the ground. Better to avoid foam on the outside?
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