# Window Seal for Portable ACs: Which Type Fits? (2026) > Velcro fabric, rigid panel or roof-window version? Which window seal fits, what length you need — and why it's usually the tape that fails, not the fabric. Canonical (HTML, zitierfähig): https://getecoback.com/en/guide/window-seal-portable-ac.html Live data on this page (hourly, rendered in the browser, not included in this Markdown view): https://getecoback.com/api/heat 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. **Short answer:** For a **tilt or tilt-and-turn window**, take the universal **velcro fabric seal** (from ~€15–25, HOOMEE-style) — or a **rigid outlet panel** if the unit runs at the same window all summer. For a **roof window** you need the dedicated roof-window version. Length = **2 × (sash width + sash height)**, next size up when in doubt. And degrease the frame first — it is almost always the tape that fails, not the fabric. As an Amazon Associate, EcoBack earns from qualifying purchases. Product links are affiliate links — you pay the same price. We have not tested these products ourselves; we summarise public tests and buyer experiences. Amazon.de ships to most EU countries — site and checkout are available in English. ## 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 Universal classic **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 → For all-summer use **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 → For roof windows **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](https://getecoback.com/en/guide/portable-ac-skylight-roof-window.html). ✓ 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. **Straight to the right type:** Velcro seals →   Outlet panels → See also: [what a rented flat allows](https://getecoback.com/en/guide/portable-ac-rented-apartment.html) → **More guides** [Portable AC with tilt-and-turn windows: step by step →](https://getecoback.com/en/guide/portable-ac-tilt-and-turn-windows.html) [Portable AC at a skylight or roof window →](https://getecoback.com/en/guide/portable-ac-skylight-roof-window.html) [Extending the exhaust hose: the 2 m rule →](https://getecoback.com/en/guide/portable-ac-hose-extension.html) --- Maschinenlesbare Übersicht: https://getecoback.com/for-agents.html · Sizing-Datensatz (CC BY 4.0): https://getecoback.com/sizing-data.json MCP-Server für Assistenten: https://getecoback.com/mcp Note: EcoBack does not test devices itself; picks summarise public tests. 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