Pipes and AC line sets
Where the AC lines run through the wall, the hole is almost always bigger than the lines. A gap about the width of your finger is enough for a mouse.
Where to look
- The hole where the AC line set enters the wall, outside and inside.
- Under the plastic cover or chase that hides the lines on the outside wall.
- Where water, gas and drain pipes pass through walls and floors.
- Under sinks, behind the dishwasher, and around the water heater and furnace.
The rule of thumb: if a gap is about the width of your finger, a mouse can use it.
Why mice use it, and the common mistake
Why mice use it
The lines run straight from the outside into a warm wall, and the gap around them is usually hidden under a cover. Mice follow pipes and lines like roads.
Common DIY mistake
Packing the hole with steel wool or foam. Steel wool rusts and falls apart, and foam gets chewed through. Either way, the gap opens right back up.
The lasting fix
Close it with stainless steel or galvanized hardware cloth, or exterior-grade metal, sealed in with exterior-grade adhesives and sealants.
The lasting fix, step by step
Open the cover
Remove the line set cover, if there is one, so you can see the hole around the lines.
Clear out the old fill
Pull out crumbling foam, old steel wool and debris. Be gentle with the lines and their insulation.
Pack metal around the lines
Fit stainless steel or galvanized hardware cloth snugly around the lines so the opening is fully closed with metal.
Seal it in
Seal over the metal with exterior-grade adhesives and sealants, outside and inside.
Do the inside too
Check where the lines come through inside the house, often in a closet, garage or attic, and seal that side the same way.
Put the cover back
Refit the cover. It hides the work but doesn't keep mice out on its own.
In a condo, lines and pipes often pass through shared walls and chases. Seal what's inside your unit, and ask your building manager about the outside.
Materials
- Stainless steel or galvanized hardware cloth
- Exterior-grade adhesives and sealants
- Gloves, flashlight, putty knife
Safety notes
- Don't kink, crush or cut the AC lines or their insulation. If they're damaged, call an HVAC technician.
- Turn off power to the AC unit at the disconnect before working close to it.
- Leave gas lines alone beyond sealing the wall around them. If you smell gas, leave and call your gas utility.
Before you seal the last gap
- Make sure nothing is still living inside. If mice are already in, set indoor snap traps along the walls as you seal, never as a substitute for sealing.
- Keep food, pet food and birdseed in sealed containers.
On a real house
Before photos from homes we’ve inspected, with the gap marked in highlighter.
BeforeAC line set: the cover was packed with foam. It has crumbled away, and the gap around the lines is open. - Before · clip
Clip: the opening around the AC lines, with the old foam crumbled away.
Want us to look at your house?Send photos from your walk-around and we'll send back a custom plan: each gap marked, what to seal it with, and in what order.
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