Building a room within an unconditioned masonry garage
We have a 100 year old garage with three sides being double wythe masonry and the fourth framed with a door and two windows. There are two additional small windows, one on each of the side masonry walls. The floor is a more recently poured 4” slab. We’re in Cleveland, OH.
I am trying to carve out a conditioned workshop space to run my business out of.
I am aware of the issues with insulating masonry from the inside, and insulating the exterior is not practical in our situation. I wanted to ask the GBA community if building a “room within a room” made sense from a building science perspective. I plan to utilize as much of the space as possible.
Envisioned floor assembly: poly vapor barrier, rigid foam insulation, advantech plywood, and framing on top of that.
Envisioned wall assembly: sizeable air gap between the outside of the “room” and the masonry walls (possibly use it for garden tool storage), and replace the masonry-embedded sashes with louvered screens to allow for full ventilation of the interstitial space.
Walls for the three masonry sides of the room would be rigid insulation with taped seams, studs with unfaced mineral wool batts, and drywall. I’d tie this framing to the framed fourth wall of the garage, incorporating the two larger windows and door into the room.
Envisioned ceiling: mineral wool between ceiling joists with rigid foam applied to underside of ceiling joists, sealed to rigid insulation of walls, furring strips, and drywall.
I know this is not a perfect solution, but realistically we’re a decade out from being able to swing a new, properly designed, larger workshop.
So, (1) does this my proposed scheme make sense, and (2) is there a better way?
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Replies
I think you may be making too much of the issues related to insulating the interior of a masonry building. Those issues arise (simplified) when a previously heated and uninsulated building gets insulated. Suddenly, the walls that had been able to dry to the inside cannot do that anymore, and moisture loading from both inside and outside collects inside the walls and freezes, damaging the masonry and/or mortar.
Your garage has been unheated for 100 years and has survived hundreds of freeze/thaw cycles with its current construction and location. If your shop doesn't generate a lot of moisture, the masonry will be no worse off when you insulate it and heat the insulated space. It will still be exposed to the exterior wetting and freeze/thaw cycling with little or no change to its inputs from the inside.
You would probably be safe installing low permeability foam panels (XPS would be my choice) to the walls and taping the seams. Frame stud walls against the foam and fill with faced fluffy insulation. If you're a bit more conservative, use unfaced insulation and install a smart membrane on the interior, sealed and treated as an air barrier.
Your roof insulation will work, though a vented attic is often a more resilient and cheaper solution if you have the headroom and ability to install soffit vents. Even gable end vents might be enough depending on your interior moisture loading.
The main point is that the building has seemingly done just fine in its current configuration. If you don't add significant interior thermal and moisture loading, it should still be fine.
Thanks for the reply. I have a knack for overthinking things, so that sounds about right.
The interior of the walls were painted by a previous homeowner and are spalling (minimally) in places as a result. I was going to parge those spots. Should the the remaining paint be stripped to prevent further spalling, or should I be okay as is just putting up the XPS?