Cooling a top-floor flat: what actually works for renters

To cool a top-floor flat as a renter, you need three things: real shade during the day, cross-ventilation at night, and a mobile air conditioner that needs no drilling, meaning a portable monoblock or a mobile split with a factory-sealed refrigerant line. Both are permission-free in Germany, and you usually don't need your landlord's consent either, because nothing about the roof or facade changes. What you can't realistically do is install a fixed split system, which needs your landlord's consent plus a certified installer. The rest is detail: why it gets so hot up there, and what to do about it without storing a 30-kilo machine all winter.

Why does a Dachgeschoss get so hot?

German listings sell the Dachgeschoss, the converted flat under the roof, with sloped ceilings and skylight charm. The physics behind the charm: your ceiling is the roof. It sits in direct sun from morning to evening, and between the tiles and your bedroom there are often just a few centimetres of insulation, in older conversions sometimes barely any. The heat doesn't sneak in through a window. It comes down across the whole surface above your head.

Sloped walls also store almost nothing. The thick brick of the floors below takes days to heat through, while a slope built from plasterboard and mineral wool is done in an afternoon. Skylights make it worse, because a window tilted toward the sun catches more direct radiation than a vertical one of the same size. And since warm air rises, everything the building collects during the day arrives at your floor. You live at the top of the chimney.

Cooling down at night is where roof flats lose again. Window area is small, and two windows facing each other are rare, so the night-time cross-draught that rescues flats two floors down barely works up here. Whatever gets in stays in.

What helps right now, for free?

• Close everything by day: shut the windows as soon as it's warmer outside than in. Obvious, and still the most common mistake.
• Shade the skylights: clamp-on heat-protection blinds or reflective film are renter-safe and leave no trace. External roller shutters work better, but fitting them is your landlord's call.
• Cross-ventilate at night and early morning: every window open, doors open, use every minute before the sun is back on the tiles.
• Cut internal heat: skip the oven on a 34-degree day, and mind dryers, old fridges and permanent standby.
• Put a fan in the open window at night so it pushes cool air in. A fan costs a tenth of an AC. It also doesn't cool the room. At all.

These steps shave off the worst. In an average summer they might carry you through. In a heatwave with tropical nights they won't, because the heat your roof stored all day has nowhere to go once the outside air stops cooling down.

Which air conditioners can renters actually use?

Permission-free means nothing about the building changes and nobody opens a refrigerant circuit. In practice that leaves two options: portable monoblocks with an exhaust hose, and mobile splits whose refrigerant line is sealed at the factory (the PortaSplit class). On a mobile split the compressor hangs outside on a bracket in the open window, so the noisy part leaves the room without a single hole in the wall.

Classic split systems and quick-connect kits are where it ends. Under the EU F-Gas Regulation only certified firms may work on the refrigerant circuit, and self-installation can draw fines running into five figures. The outdoor unit needs your landlord's consent on top. For a rented roof flat that combination is, realistically, a no.

How do you vent a portable AC through a skylight?

A monoblock lives or dies by its exhaust. For pivot and top-hung skylights there are fabric seals with zips that stretch around the open sash, and the hose passes through the opening. Check which window type you have before ordering, because a seal made for vertical windows won't fit a roof window. Keep the hose short and as straight as possible. It radiates heat back into the room while it works.

If you've bought a unit through a partner link and don't fancy the setup, KlimaLegal installs it for a one-off €149. KlimaLegal never sells the device itself. The retailer does, and the retailer carries the warranty.

Why seasonal rental fits a roof flat

Roof flats rarely come with a cellar, and the storage room, if one exists, is already full. A monoblock weighs around 30 kilos and needs a home from October to May. Anyone who's hauled one up a narrow Altbau staircase in Kreuzberg to the fifth floor knows that "portable" is a generous word.

That storage problem is exactly what seasonal rental removes. With the KlimaLegal Sommer-Abo (Summer Rental) the unit arrives for the season, 1 June to 30 September, and gets collected again in autumn: pickup runs from mid-September, and you agree the date in August. Standard costs €89 per month with a 4-month minimum term, plus a one-off €189 for delivery, setup and collection. That's €545 for the whole 2027 season, with a quiet monoblock sized to your room and the window seal already fitted. Premium "Leise" is €199 per month plus a one-off €299 for delivery and professional installation by two fitters, €1,095 for the season. You get a mobile split with the compressor outside on the window bracket, quiet enough for a bedroom, and it's always installed by KlimaLegal, never by you. There's no deposit. If a unit breaks, a replacement arrives within 3 working days.

What does the electricity roughly cost?

An honest ballpark, explicitly an estimate: a mid-size monoblock draws roughly one kilowatt. At around 40 cents per kilowatt-hour and four to six hours of runtime on a hot day, that's about €1.60 to €2.40 per day. A summer with 30 to 40 genuine AC days lands at roughly €50 to €100 in electricity. A mobile split is usually a bit more efficient but stays in the same range. The expensive scenario is a unit fighting an unshaded skylight, so shade first, cool second.

One timing note: there are 30 Sommer-Abo slots for 2027, allocated before the season, and skylight blinds and window seals tend to sell out in the first hot week of the year. The day to sort out your roof flat is a cool one.