Your wall-mounted air conditioner must be prepared for winter so it cannot allow any cold air to enter your unit through the air conditioner opening and freeze the heating pipes below. Owners could be liable for any damage as even a tiny air leak admitting cold air  could lead to pipe freezing. Please refer to the example below. It shows an insulated wooden box whose joints are sealed with caulking, covering the air conditioner on the balcony side.

Interior of insulated box

Insulated box, rotated

Box cover in place

Close-up of caulking around box

It is essential that the connection joint between the box and the outside wall is also completely airtight. This can be achieved through caulking all four sides.

If a pliable seal is located between the wall and the box it is still necessary to apply aluminum foil tape, or other air tight tape, over the joint between the wall and the box. Again, all four sides need to be done.

If you’re handy you could do similar work yourself but you might prefer to hire a contractor.

– Doug Brandy, updated November 18, 2014