How to vent a portable AC through a sash window

A sash window slides vertically, one pane over the other. It is the friendliest window for a portable AC, which is why most units ship with a kit built exactly for it. Common in the UK and in older buildings across Europe, it gives you a clean horizontal gap to work with.

The basic method

Raise the lower sash, drop an adjustable panel into the gap, and close the sash down onto it. The panel telescopes to your window width and carries a 150 mm flange for the hose. Most portables include one; if not, a sash or universal window kit costs roughly €15 to €30.

For a tighter or taller fit, a custom acrylic insert with the hole pre-cut does the same job with less rattle. A dual-hose unit wants a wider panel or two holes, so check the kit before you buy.

The gap people forget

The panel seals the bottom gap, but the two sashes still overlap at the meeting rail in the middle, and that overlap often leaves a draught. Close it with self-adhesive foam strip or draught tape. It takes a minute and recovers cooling you would otherwise leak straight out.

Security and rain

Once the panel is in, the upper sash rests on it and the window cannot lock. Drop a sash stop or a cut dowel into the upper track so the window cannot be slid open from outside. Slope the hose down to the outside so rain at the panel sheds away rather than pooling.

Why this type is forgiving

The horizontal gap means a single-hose unit seals well with the standard kit, so the negative-pressure penalty that hurts portables in other window types is easy to avoid here. Dual-hose and split units work too, but on a sash window even a basic single-hose unit can perform close to its rating if you seal the meeting rail.

Want a unit matched to your room size and budget rather than just the window? Run the finder.