I've noticed that fairly often the oversized hole that is created in the concrete to run the mini-split line set is left completely open and the installers just plop the AC unit over the hole without any attempt at sealing.

 

This leaves a nice compromise for bugs and critters to crawl around in the wall and get into your home, I'd think. Especially if the other end is not sealed as well.

 

What is the best way to seal that hole? From what I've researched online, in the US a lot of people use roof flashing to seal line sets, but I can't seem to find that for sale in Thailand. Perhaps some sort of self-self hardening foam? Any thoughts?

I'd use some of the aerosol PU foam, readily available and easy to use.

 

Get a tube that used to to have silicon sealant in it, clean it out and put in fluid concrete and then use the same silicone sealant gun to fill the hole! No worries about that critters will eat through that!!

 

 

Just silicone is my choice.

The guys that installed mine use silicone sealant. 

I used half a dozen wadded up, green, Big C carrier bags, now obsolete.

 

Otherwise check the laundry room for any 'orphan' socks as they can do the job as well.

Thais call anything that squirts 'silicone', even if it's acrylic. There are different silicones for different purposes, but most tradesmen use anything for anything. The acrylic caulks generally shrink and crack because they're usually mixed cheap filler products to bulk them out and they're not suitable for this. Silicone will work but you use loads of it to fill the gap, it's awkward and acetic cure silicones will stink for days. . .

 

The correct product is PU foam. You can get it from anywhere. The cheaper ones will work fine, less than a hundred Baht on Lazada.