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We have a detached garage that's made of hollow clay bricks (structural terra cotta) covered in stucco, both inside and out. I'd like to insulate and heat it for use in the winter. The problem is that the terra cotta bricks are incredibly brittle and the stucco makes it difficult to see what I'm drilling into -- I'm wary of putting up framing to hold regular insulation batts. What can I use instead to insulate these walls? Can I just glue up 2" XPS panels (with batts in the rafters)?

We're in St. Paul, Minnesota, so the winters are cold and summers are hot. I plan to install a mini-split for heating and cooling; I don't need it to be warm in the winter, just tolerable, so I'm hoping to use the mini-split's minimum heat settings.

Here are some pictures to demonstrate what I'm working with. This is one of the three stuccoed walls:

Full wall

This is the top of one of the walls, showing the top of the structural terra cotta:

Top of wall

And finally here's the interior of one of the terra cotta walls, where a previous owner removed some material (note that the brick behind the terra cotta block is a chimney, not part of the wall):

Terra cotta detail

My plan so far is to glue up 2" foam board insulation over the stucco, then put mineral wool batts in the rafters. I'm hoping that plus a mini-split will get me through the first winter or two, and after the garage floor is replaced I can frame in some walls and add more insulation.

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Providing an answer in case someone in a similar situation comes across this question:

I couldn't find any definitive answers on this approach so I had some insulation contractors come out to look. They did recommend gluing up foam panels as I had planned for the walls, but having good insulation in the ceiling was more important. To take care of that all of the contractors recommended 3 inches of closed-cell spray foam in a "hot roof" (adhered directly to the decking) arrangement -- any other approach would require ventilation and would yield inferior insulation quality.

To summarize, the answer here was to consult professionals.

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I had a house built like yours.

Blow-in is a bad idea... Terra cotta is brittle and allows moisture to pass. Therefore the monster holes would destroy the wall and the blow-in would get saturated.

Expansion foam is a costly idea... Sure, little holes can be drilled, but did you consider he cost of foam? Wow!

I built a a normal 2x4 framed wall in front of the terra cotta walls and installed normal insulation. It made my house smaller and I left the stairwell cold instead of shrinking its width, but it did work.

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