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This is a wall in the extension to my house, which I estimate to have been added around the 1930s (house is 1909) using mostly large hollow bricks that look more like cinder blocks but are not of concrete and brick-like clay instead.

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I need to plaster the exposed brick (there is a reason why I couldn't frame and drywall) and I was thinking, since these elements are hollow, does it make sense to drill a small hole in each and fill it with insulation foam, to improve the house R value. The only reason against I see is that the foam may expand if moisture comes from the outside (the other side of where I'm plastering) and explode the brick.

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Those are terra cotta blocks. They are, indeed, hollow. You could drill (carefully! Terra Cotta is fragile when drilling) and fill each and every block with low expansion foam (has normal expansion foam could break the blocks).

But that would be a huge project. And you'd still have major thermal bridging issues along the edges of each brick. It'd be much better to apply 2"+ of foam to the inside. What is the reason that you can't go that route?

Another reason to try and gain some wall on the 'inside' is to make it easier for future owners...if you could add 2x framing that also will give a surface for mounding shelves, pictures, etc. You don't want to take a nail and hammer it into a terra cotta block as it will just fracture.

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    Beat me to the punch. Two additions: 1. I think filling the bricks with low expansion foam could still be risky since it still expands, just to a lesser degree and would recommend practicing on a few spare bricks first. 2. If applying XPS or EPS foam board to the inside isn't possible, also consider applying on the exterior of the home if also refinishing the outside of the home now or in the near future. – statueuphemism Dec 18 '15 at 20:01
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There is no way you touch those bricks. This is an exterior wall. You are weakening the bricks, you are allowing for more entry points for moisture and vapor (your insulation won't cover every hole like the bricks are), some of these will break, and so on. Please erase this idea from your mind as it is one of those things that you would do that would be very low benefit with very high cost/risk.

I renovate in France. They have similar construction. You either put poly sheets on the outside and stucco (done most often) or frame on the inside and insulate. Some/all of these bricks could be filled or partially filled too - you wouldn't know unless you start drilling into all of them. Also how would you ever know how much spray foam to put in each as each might have mortar or something else in it... Just for your sanity, check this idea off the list. If it makes you feel better I have had far worse ideas.

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