How do I seal this hole?

Expanding foam all day long

Better yet, anti-rodent expanding foam
- Is that an actual thing? I diy'd some of that by shoving a bunch of steel-wool into the foam while it was wet. Seems to have worked.

@SirNuke All of the comments saying spray foam are technically correct - that is the easiest way but it will look like orange crap. Some people mentioned steel wool - avoid that because it rusts and will end up looking like orange crap. Find some "excluder mesh", which an inexpensive product sold to the pest control industry - it is a nylon mesh with reinforcing fibers...think Brillo pad stiffness. Stuff the gap to fill it then use silicone sealant to finish it off.

I would say the expanding foam would still be preferred. Just don't go crazy with it. Then before it's fully cured, cut it back flush. Wait for it to dry then caulk to cover/seal.
- @parrot@kbin.social
  @SirNuke
  My actual job is to inspect homes. It is an industry joke that Joe-diy-guy grabs a can of great-stuff expando foam and “fixes” it…leaving a jacked up mess that rats just chew through to get in again. There are better ways of securing entry points.
That's a good suggestion, I was looking at stainless steel wool and wasn't finding a lot of great options.
- @SirNuke Another option if you don’t want to source excluder mesh is to get some “backer rod” - a painter’s product sold as “caulk saver” at the home improvement palace. You use it to fill large gaps which you then seal over with caulk or silicone.
- @SirNuke The other reason to not use steel wool is that it can be a fire hazard. A low voltage shorted across steel wool will result in fire (I carry a 9-volt and steel wool in my camping kit as emergency fire-starter).

For context, this is leading to my AC unit. While hanging a light above my workbench, I noticed daylight coming in from the wall where there shouldn't be any. It appears a previous owner had pulled back the insulation and forgot to put it back - shudder to think how much money that's costed me over the last two years. Would like a hardier seal than insulation to stop water and mice, but not sure what is required.

Really depends on access. Can goy access it from the backside? Quick and easy, but a little messy, expanding foam.
- Okay, I know that's a typo, but I'm cracking up at the idea of you asking about unauthorized access by Jewish people.
- It's super awkward but I can get to it from inside the house (between insulation and hole). Going to have to move my workbench but so be it.
  Any direction on the type/brand of foam I should use? I have a can of fireblock to seal ethernet runs from my basement to main floor, though it's probably seized up by now.

Spray foam on the inside. Silicone on the outside.

Spray foam is generally what’s recommended. Be conservative with it though, it expands a lot and can push wires and cause other problems if you use too much.

Be conservative is not the Expanding Foam Insulation motto.
- True! And you forget to be conservative as soon as you start spraying haha

My first thought is great stuff but honestly I have only used it in a crafting context and I am sure someone else will have a better idea or be able to explain why my gut reaction is not going to work. Also could look ugly if the foam expands out of the hole though it can be cut off to be even once cured.

Sealant foam was also my thought.
Great Stuff (or one of its analogs) is exactly what is needed here.

https://www.bobvila.com/articles/expanding-foam-insulation/

Great Stuff Fireblock for a quick solution. Not the cheapest but will keep the pests out and might still be technically within code.

Duct seal.

That's pretty universally what I have seen used here. This stuff for those unfamiliar.

Spray expanding foam would be easiest and you can trim it after the fact for a cleaner look.

With a hole sealer