A split needs a lined hole through the wall for pipes and cable. In a cavity wall the installer has to know what is in the cavity and whether a cavity barrier sits on that line. Full-fill, beads, foam, partial fill and an empty cavity are different making-good jobs, and fire barriers are not something to drill through and forget. This page is not a drill specification and not a DIY method. Use an F-gas certified company for the circuit, and a firm that will reinstate the wall. Outdoor siting in England is a Planning Portal question on top of the hole.
Beads, foam, wool or an empty cavity
UK cavity walls may be empty, partial-fill, or full of mineral wool, beads or injected foam. Each one behaves differently when a core is cut: beads spill, foam is a fire and fumes question for the people doing the work, mineral wool can be packed back, an empty cavity is a weathering and insulation gap. If you have a guarantee, a survey or a photo from when the wall was filled, send it. You do not need to know the drill size. You need to stop a fitter assuming every cavity is the same brick leaf they drilled last week. Say if you know the insulation type. Say if the wall is timber frame rather than masonry. Ask how they will reinstate insulation and the weather seal.
A sleeved hole that is not a rain path
The hole should be sleeved, the pipes insulated, the cavity not left as a rain path, and the inside and outside made good so the trunking is a cover rather than a bung. Access to joints stays possible. A neat cover over a soaked cavity is a latent damp patch. The cover hides the pipes. The sleeve, the insulation and the seal have to be right underneath. Leaks and rain still need a path you can inspect. Refrigerant connections remain F-gas work. The core is building work beside that. In England the outdoor unit still has to satisfy Planning Portal conditions, which a tidy hole does not answer.
Frequently asked questions
Can I core the cavity myself and let the installer pipe it?
No. The hole has to suit the pipes, the insulation, any cavity barrier and the weather seal. This page is not a drill guide. Leave the opening to the installer. Do not core the wall yourself.
Why does insulation type matter?
Beads, foam, mineral wool and an empty cavity all need a different make-good. A barrier in the cavity is a fire detail, not scrap to be left on the floor.
Does a neat trunking cover finish the job?
The cover hides the pipes. The sleeve, the insulation and the seal have to be right underneath. Leaks and rain still need a path you can inspect.
What if a cavity barrier sits on the pipe line?
Cavity barriers close the cavity against fire spread, commonly at party lines, around openings, and in timber-frame buildings. A core that punches a barrier has to be made good in a way that restores that job. An air conditioning quote that never asks about the wall construction has not priced that. This page will not describe how to drill, what diameter to use, or how to patch a barrier.
Is a flat wall the same job as a house cavity?
Flats and purpose-built blocks are stricter than a simple house wall. If you are in a flat, the freeholder and the fire strategy come before any hole.
Do I need to specify the drill diameter?
No. You do not need to know the drill size. This page is not a drill specification. Tell them the insulation type if you know it, and whether a cavity barrier is on that line.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.