What is a window seal for?
Short answer: without a seal, the window gap where the hose comes out lets hot outside air flow straight back into the room — your portable AC works against itself, cools less and uses more electricity. A custom acrylic plate closes that gap completely: hot air leaves through the hose and doesn't come back in. It's the difference between a portable unit that "just about copes" and one that genuinely cools the room.
Custom acrylic plate vs. fabric kit
There are two types of seal on the market. The zip-up fabric kit is cheap and universal, but fits poorly on casement windows and old frames — exactly the most common kind in Lisbon's buildings. The rigid acrylic plate is made to the millimetre for your window.
| Criterion | Fabric kit (zipper) | Custom acrylic plate |
|---|---|---|
| Sealing | Okay; leaves gaps on casement windows | Total — cut to the millimetre |
| Old Lisbon frames | Hard to fit | Fits any window |
| Durability | Fabric wears out after one season | 4 mm acrylic, lasts years |
| Light / privacy | Blocks the light | Clear (lets light in) or opaque for the bedroom |
| Looks | Improvised | Clean, discreet — great for short-term rentals |
| Price | €10–30, available immediately | ≈€60–75 made to order at an acrylic shop |
| Lead time | Buy it and take it home the same day | Made to order: up to 3–4 weeks in summer |
Rent from us and the window kit is included
- Rent a portable air conditioner from us and the window kit comes with the unit — nothing extra to buy
- We bring it and fit it on delivery: the hose is out of the window in about 5 minutes
- No works, no drilling, no landlord permission — it all comes out again afterwards
- Works on sliding, casement and tilt-and-turn windows, including Lisbon's old frames
- Rental from €40/day, €210/week or €350/month, with a €100 refundable deposit
Already own a unit? Then you don't need us for this: a fabric kit costs €10–30 at any DIY shop or online, and a custom acrylic plate can be ordered from any laser-cutting shop in Lisbon (take the measurements below). We no longer sell the custom plate ourselves — the price we could produce it at didn't work out in your favour.
How to measure your window (30 seconds)
Whether you buy a fabric kit or have a plate cut to size, you need these three measurements. Take them with a tape measure before ordering:
| Measurement | How to take it |
|---|---|
| Opening width | Side to side of the gap where the plate will fit (cm) |
| Opening height | Top to bottom of the same gap (cm) |
| Hose diameter | Measure the end of the AC hose (usually 13–15 cm) |
Always order the plate 0.5 cm smaller than the opening you measured (so it slots in) and the hole 0.5 cm wider than the hose. A laser-cutting shop gives you a polished edge with no sharp corners.
How it installs
The plate sits in the window opening during the warm months. With velcro it sticks to the frame without drilling (ideal for rented flats); with thumb screws it sits even more firmly. The hose slots into the hole and stays put. At the end of summer you take it out and store it — no works, no marks, no landlord permission needed.
AC Rental Lisbon's note
We see it in hundreds of Lisbon flats: the portable unit is good, but the hose hangs out of a half-open window and half the cool air is lost. For a single summer, the €10–30 fabric kit does the job. If the unit lives in the same spot every year, the rigid plate pays off. And if you're renting from us, the kit is already included — nothing to decide.
FAQ
What's the difference between the acrylic plate and the fabric kit?
The fabric kit is universal and cheap, but leaves gaps on casement windows and old frames. The acrylic plate is cut to the exact size of your window, seals completely, lasts years and looks better.
Will it fit my window?
A made-to-measure plate always fits, because it's cut to your dimensions — sliding, casement and tilt-and-turn windows all work. Universal fabric kits fit sliding windows well but leave gaps on many of Lisbon's older frames.
Do I need to drill the wall or frame?
No. The velcro fixing sticks to the frame without drilling, and the thumb screws don't need any works either. You can remove everything at the end of summer.
Does the plate let light through?
The clear acrylic version lets natural light in. If you'd rather block the light (a bedroom or short-term rental), there's an opaque version in white or black.
How much does a window kit cost?
A universal fabric kit costs €10–30 at DIY shops and online, and you take it home the same day. A 4 mm acrylic plate cut to size with a hole for the hose runs about €60–75 at a laser-cutting shop in Lisbon, with summer lead times reaching 3–4 weeks because it's made to order.
Do you sell the custom acrylic plate?
Not any more. The price we could produce it at didn't justify what we'd have to charge, so we'd rather tell you where to order one than sell you an expensive solution. Rent an air conditioner from us and the window kit is still included, at no cost.
I already own a portable air conditioner. How do I seal the window?
Measure the width and height of the opening and the hose diameter (see above), then pick either a universal fabric kit or a plate cut to size. Both work with any portable air conditioner, whatever the brand.