Condensation and dirt inside polycarbonate sheets: how to seal the ends correctly
A polycarbonate greenhouse or canopy looks perfect for the first few months, but after a year or two grey or greenish streaks, water droplets and dead insects appear inside the sheets. This dirt sits inside the channels of the sheet, so it cannot be washed off with a sponge or a pressure washer. The sheet loses its transparency, plants get less light and the structure looks neglected.
The cause is almost always the same: the ends of the multiwall polycarbonate were left open or were sealed the wrong way. This article explains why it happens and how to seal the sheet ends correctly so the problem does not come back.
Why condensation and dirt build up in the channels
Multiwall (twin-wall) polycarbonate consists of two or more skins joined by ribs, forming long channels. If the sheet ends are open, the channels collect:
- dust and pollen – blown in by the wind and settling on the channel walls;
- insects – the channels are a convenient hiding place, and they stay there;
- moisture – rain, snow and humid air. As temperatures change between day and night, the moisture condenses on the channel walls;
- algae and mould – moisture, dust and light together create ideal growing conditions.
The longer the ends stay open, the more dirt builds up. That is why polycarbonate installation guides clearly state: do not leave sheets with open ends – seal them during installation.
The right solution: solid tape at the top, breathable tape at the bottom
The ends are sealed with special tapes, and the top and bottom ends need different tapes:
- Top end – a solid (non-perforated) dustproof tape such as ALENOR SP dustproof adhesive tape for polycarbonate. It closes the channels completely, so water, dust and insects cannot get in from above.
- Bottom end – a perforated (breathable) tape with a dust filter. It keeps dust and insects out but lets moisture and condensation drain away.
- Arched structures (such as arched greenhouses) where both ends are at the bottom – perforated tape on both ends.
The most common mistake is sealing the bottom end airtight as well. Condensation then has nowhere to go and the channels turn into a damp "aquarium". The second mistake is leaving the top end open or covering it with perforated tape, which lets rainwater in.
Which tape width to choose
The tape should cover the whole sheet end and overlap both faces of the sheet. ALENOR SP comes in two widths:
- 25 mm × 50 m – usually for thinner sheets (4–10 mm);
- 38 mm × 50 m – for thicker sheets (16 mm and more).
One 50 m roll is enough for several greenhouses or a large canopy.
How to seal polycarbonate ends: step by step
- Orient the sheet correctly. The channels must run down the slope (top to bottom), and along the arch in arched structures, so condensation can drain.
- Peel back the protective film at the ends. About 80–100 mm from the edge, so the tape sticks directly to the polycarbonate. Remove the rest of the film only after installation.
- Clean and dry the end. There must be no dust, sawdust or moisture. If the channels are already dirty, rinse them and let the sheet dry completely.
- Apply the tape. Remove the release liner and apply the tape so it evenly covers both faces of the sheet. Press it firmly along its length, ideally with a pressure roller.
- Cover with a U-profile. The end profile protects the tape from damage and sunlight. ALENOR SP is thin, so the profile slides on easily.
- Drill drainage holes in the bottom profile. Manufacturers recommend 2–3 mm holes about every 300 mm so condensation can escape.
Common mistakes
- Ends left open "for now" – within a few weeks the channels already contain dust and insects.
- Bottom end sealed airtight – condensation stays inside.
- Ordinary adhesive or packing tape – not made for outdoor conditions and UV, it peels and crumbles and the channels are open again.
- Tape applied over the protective film – once the film is removed, the tape no longer holds.
- Sheets installed with channels running horizontally – moisture cannot drain out.
Frequently asked questions
Can dustproof tape be used on an existing greenhouse?
Yes. Remove the end profile if there is one, clean and dry the ends, apply the tape and refit the profile. If the channels are already dirty, rinse them first and let them dry.
Does ALENOR SP work on other materials?
Yes. It adheres well to polycarbonate, polypropylene, polystyrene, PVC, glass and steel, so it is also used to seal the ends and joints of other plastic sheets.
Where is this tape used?
Greenhouses and conservatories, canopies and entrance awnings, arched roofs, bus shelters, pool covers, kiosks and partitions – wherever multiwall polycarbonate is used.
How to order?
ALENOR SP dustproof tape and other mounting tapes can be ordered at alenor.lv with delivery to parcel lockers across Latvia, Lithuania and Estonia. Construction companies can ask for wholesale prices.
