insulating existing brick wall without a WRB
I just had a contractor call me with a question about renovating the garage on his own home to be living space. I gave an answer and thought I would check here to see whether it was a good one. He wanted to know my thoughts on creating a “vapor barrier” for an existing brick wall which has the brick ties attached directly to the wood studs, with no sheathing of any kind and no WRB. We are in climate zone 5. The wall is 1 story high, faces west, and has a 12″ overhang. Only one tree on that side of the house.
Because the brick is attached directly to the studs with only 1/2″ of space between the brick and studs, it doesn’t seem possible to shimmy any sheathing in between them. So I figured our main goal was to protect the wood wall assembly from decay and mold issues. The assembly we landed on is to paint the interior side of the brick with a dry-lock. I figured this would protect the wood assembly inside from water coming through the brick. I told him not to put any siloxane on the exterior of the brick so it could dry to the exterior and not to use a closed cell spray foam so the interior part of the wall can still dry to the interior. I recommeded a dense pack cellulose insulation for its ability to minimize air leaks and its moisture buffer capacity.
While I wouldn’t ever design something this way from scratch, is this a decent approach? Any other thoughts on how he could handle the situation?
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>" I told him not to put any siloxane on the exterior of the brick so it could dry to the exterior and not to use a closed cell spray foam so the interior part of the wall can still dry to the interior."
Siloxane doesn't dramatically change the vapor permeance of the brick, but it limits the capillary draw, reducing the amount of rain/dew water that gets drawn in. It should definitely get masonry sealer on the exterior once per decade or so. The DryLock on the interior is fine.
Closed cell foam is still somewhat vapor permeable, about 1-1.5 perms @ 1" , 0.5-0.75 perms @ 2", but it's not very green, and reduces the overall drying capacity.
In a 2x4 no-sheathing wall where the studs are 1/2" nominally away from the brick, installing ~1" wide strips of 3/4" rigid foam to the brick on both sides of each stud (and top/bottom plates) with foam board construction adhesive and another strip midway between the studs as spacers for a 1" foil faced polyiso exterior side air barrier, creates a 3/4" cavity which is a GREAT capillary break. That leaves nominally 2.25-2.5" of stud cavity for fiber insulation. Compressing 3" rock wool "sound abatement" batts or R13 fiberglass into that space would deliver about R10, the foil facer facing the 3/4" cavity another R1, and the 1" polyiso about R6, for R17 total (center cavity). That's not enough to hit code-min, but it's not terrible. With a third of the total R as air impermeable rigid foam on the exterior of the fiber, there is no need for interior vapor retarders in a zone 5 climate.
If code minimums are the goal, installing edge strips of 1" polyiso on the interior side of the framing leaves 3.25" of cavity, and compressing R15 HD fiberglass or rock wool delivers about R14. With the R6 polyiso it already hits the R20 code minimum for zone 5, and has sufficient dew point control at the foam/fiber boundary to be able to use a Class-III vapor retarder (standard interior latex on wallboard) on the interior, though a "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) would be cheap insurance.
Thank you very much Dana!