🗓 Updated August 8, 2026 · EcoBack editorial team
The window seal is the most-bought accessory around portable air conditioners — and the one where most purchases go wrong: the wrong type for the window, a length that is too short, tape that lets go in the sun. Here is the decision in three steps: type, length, mounting.
Measure the sash (the part that moves), not the frame — that is the mistake people make.
Why (almost) nothing cools without a seal
A portable AC blows several hundred cubic metres of warm room air out through the hose every hour. That air is missing from the room — negative pressure builds, and it pulls outside air in through every opening. If the window is simply tilted open, the 35-degree air comes straight back: the unit cools against itself, runs non-stop and still manages only two or three degrees. That is why the seal is not an "accessory" but half the cooling capacity — the most effective single purchase in the whole setup.
The three types — and who each one is for
Velcro fabric seal (HOOMEE-style, many near-identical brands) — a fabric sleeve with a zip that wraps the whole tilted sash, attached with velcro strips. Fits tilt, tilt-and-turn and most casement windows, comes off without residue when you move out — which makes it the default for renters.
✓ Cheap (approx. €15–25) ✓ fits almost everywhere ✓ renter-friendly ✕ Window stays tilted = not lockable ✕ the tape is the weak point
See velcro seals →Rigid outlet panel (Eurom "Window Way Out"-style, or a cut-to-size hollow-chamber sheet) — a solid panel with a hose port that clamps into the tilt opening. Seals noticeably better than fabric, the window is nearly closed, and the hose gets a fixed connection.
✓ Best sealing ✓ nothing flaps ✓ clamps instead of glues ✕ Pricier (from ~€40–60) ✕ must match your window size
See outlet panels →Roof-window seal — the same fabric idea, but cut for pivot and top-hung roof windows (Velux & co.). Important here: the opening must point downward and rain must not run in along the hose — details in the roof-window guide.
✓ The only clean solution on a roof window ✕ Watch out in rain ✕ fitting is a little fiddlier
See roof-window versions →What length do you need?
The most common mispurchase is a seal that is too short. The rule: measure the moving sash (not the frame), and the seal must wrap its full perimeter — so 2 × (width + height). A typical 60 × 140 cm window therefore needs 4 m. Common off-the-shelf lengths are ~300, 400 and 560 cm; when in doubt take the next size up — extra length can be gathered, a gap cannot. The calculator at the top of this page does the maths for you and names the size that fits.
Glue or clamp? The honest weak point
In buyer reviews, fabric seals almost never fail at the fabric — they fail at the supplied adhesive tape. A white window frame in direct sun reaches 50–60 °C; the glue underneath softens, the tape lets go and leaves residue when peeled off. Three countermeasures:
- Degrease before sticking: wipe the frame with washing-up liquid, dry it, press the tape on firmly and let it rest unloaded for 24 h — this practically doubles the hold, for free.
- Clamp instead of glue where you can: outlet panels and some fabric variants clamp into the tilt opening — no glue, no residue, no mid-summer failure.
- Do not reach for heavy-duty universal tape: stronger third-party tape holds, but wrecks paint or the rubber gasket on removal — an expensive swap when you move out.
Frequently asked questions
Which window seal is best for tilt windows?
For occasional use, the universal velcro fabric seal (from around €15–25): it wraps the whole tilted sash and comes off without residue — which is why it is the default for renters. If the unit runs at the same window all summer, a rigid outlet panel seals better and keeps the window nearly closed, but costs more and has to match your window size.
What length does the window seal need to be?
Measure the moving sash (not the frame) and calculate 2 × (width + height) — that is the perimeter the seal has to wrap. A typical 60 × 140 cm window needs 4 metres. Common off-the-shelf lengths are around 300, 400 and 560 cm; when in doubt take the next size up — extra length can be gathered, a gap cannot.
Why does the window seal keep coming off?
The weak point is almost never the fabric but the supplied adhesive tape: in direct sun the frame reaches 50 to 60 degrees Celsius, the glue softens, lets go and leaves residue. Degrease the frame with washing-up liquid before sticking, press firmly and let it rest for 24 hours — or choose a variant that clamps instead of glues.
Does a window seal actually make a difference?
Yes — it is the most effective single purchase in the whole setup. A portable AC pushes room air outside and creates negative pressure: without a seal, hot outside air streams straight back in through the open window and the unit cools against itself. With a tight seal the same unit reaches the target temperature faster and cycles less afterwards.