Portable air conditioner with a skylight or roof window

Attic flats and top-floor rooms are the hottest in a heatwave — and they're often the ones with only a skylight (roof window / Velux-style) instead of a normal wall window. That's exactly where people get stuck setting up a portable air conditioner: the standard kit doesn't fit, and the physics work against you. The good news: it's doable without drilling. Here's the honest, practical answer.

Why a skylight is harder than a normal window

Three things make roof windows the tricky case:

• Hot air rises. An exhaust hose run up to a roof window pushes hot air uphill, and the hose radiates heat back into the room the whole way. The longer that run, the more cooling you lose — keep it short and, if you can, insulated.
• The opening is angled and often out of reach. A flat seal strip made for a horizontal sliding window can't follow the slope; it leaves gaps that let the hot air you just expelled leak straight back in.
• Rain. A roof window is exposed. If the hose points up or sits flush, water tracks down it into the room. The outside end must angle downward.

Three ways to seal a skylight — compared

Method 1: universal cloth seal (what most people should buy)

Look for a window seal kit that explicitly lists casement, crank-out and skylight windows — these have an adjustable zipper and Velcro that follow a non-horizontal opening, unlike the flat sliding-window strip. Fit it around the roof-window frame, feed the hose through the zippered port, and zip it closed. No drilling, fully reversible.

See universal window seal kits on Amazon.de →

Method 2: cut panel (best seal, a bit of DIY)

Measure the opening, have an acrylic or lightweight XPS board cut to size, cut a hole for your hose diameter (usually 13–15 cm), and clamp it into the roof-window opening. Seal the edges with self-adhesive EPDM foam tape — it compresses to the frame and peels off cleanly. This is the option that sheds rain best because you can slope the panel outward.

Method 3: roof-window outlet adapter

Purpose-built roof-window and skylight vents route the exhaust while the window stays mostly closed. Tidiest if the AC lives under the same skylight all summer. Check it matches your window brand and hose diameter before buying.

Common mistakes to avoid

• Using the flat sliding-window kit. It can't follow the slope of a roof window — the gaps let hot air back in and the AC never keeps up.
• Pointing the hose up or flush. On a roof window the outside end must angle down, or rain runs into the room along the hose.
• A long uphill hose. Hot air rising already works against you; every extra metre and bend costs cooling. Keep the run short and, ideally, insulated — see our note on extending and insulating the exhaust hose.
• Forgetting the room seals nowhere else. A perfectly sealed skylight still loses if the room's normal windows or door leak warm air in.

The honest alternative for attic rooms

If a skylight is your only opening and the room bakes under the roof, a monoblock venting uphill is fighting an uphill battle — literally. A split unit that doesn't need core drilling puts the hot part outside with only a thin line through the window, which sidesteps both the rising-heat and the bulky-hose problems. For choosing the unit itself, see our portable AC buying guide and check the BTU you need — attic rooms usually need a size up.

We haven't lab-tested these products ourselves; we summarise publicly available tests and user reports.

Frequently asked questions

Can you vent a portable air conditioner through a skylight or roof window?
Yes, but not with the flat kit in the box. Use a universal cloth window seal made for casement, crank-out and skylight windows, a cut panel with a hose hole, or a purpose-built roof-window outlet. Angle the hose downward on the outside so rain runs away from the room.

Why is a skylight harder than a normal window?
Hot air rises, so an exhaust hose run up to a roof window fights gravity and the hose radiates heat back into the room along the way. The opening is also angled and often out of easy reach, and rain can track down the hose — so the seal has to shed water, not just block air.

Is it allowed in a rented flat?
A cloth seal kit or clamped panel is non-permanent and leaves no damage, so no landlord permission is needed in a typical rental — unlike drilling a fixed roof vent.