What is the best way to insulate gaps in an 1880s timber wall?

Dealing with brick nogging in an 1880s frame is usually a headache beacuse those bricks are often providing lateral stability to the whole structure. If the electrician knocked out chunks or drilled thru them to run wire, you first need to make sure the remaining bricks are actually secure. If they're loose, they can shift over time and start bowing your exterior siding or cracking your new drywall. Forget about using rigid foam board for this. Trying to scribe and cut foam to fit those irregular gaps around horizontal wiring is a massive waste of time, and you'll end up with gaps anyway. More importantly, old timber frames need to breathe. Rigid foam acts as a vapor barrier, and if you trap moisture between that board and your original wood siding, you're going to end up with rot. You want a material that allows for some moisture migration.

**Rockwool** is your best bet. It’s fire-rated, which is great for old houses, and it’s easy to tear into specific shapes to tuck around the wiring. It handles the irregular voids much better than anything else. Just tuck it in firmly so it stays in place, but don't compress it so much that it loses its loft. To really stop the drafts, you shold focus on air sealing the brick gaps themselves before you put the insulation in. Use a bit of mortar or a high-performance sealant to plug the holes where you feel the air coming through. The rockwool provides the thermal break, but it won't stop a literal breeze. Since you're finishing with a mix of plaster and drywall, make sure your transitions are taped well or the air will just leak out the edges of your patches.

I'm just starting to learn about this stuff, but the breathing part makes me wonder—if I go with the rockwool, is there a risk of it getting damp if the brick holds onto moisture? I've seen some people talk about using spray foam in small cans for gaps, but I'm guessing that woud cause the same rot issues as the rigid board?

Also, since the wires are running horizontal, do I need to worry about the rockwool putting pressure on them? I don't want to accidentally pull a connection loose while I'm trying to stuff it into those wierd corners.

Canned spray foam is okay for tiny cracks where the wire enters the brick, but don't go overboard. If you fill the whole cavity with it, you're back to that moisture-trapping nightmare. I ussually stick to rockwool for the bulk and just use a dab of fire-rated sealant where the wires pass trough the studs.

Since those wires run horizontal, you've created a path for fire to travel sideways. Use some fire-rated caulk at every stud penetration. It'll stop the draft and keep you up to code. As for the damp brick, as long as that exterior siding isn't bone-dry sealed, the rockwool handles seasonal humidity shifts better than fiberglass or foam.