How Twin-Wall Polycarbonate Works in a Greenhouse
Hold a twin-wall panel up to the light and you see the trick immediately: two sheets, a grid of air channels between them, and physics doing the rest.
Two walls and a grid of air channels
Look at a twin-wall panel edge-on and it resembles a very fine cardboard: an outer sheet, an inner sheet, and vertical ribs connecting them every few millimeters. The ribs do two jobs. They hold the sheets apart to create the air chamber, and they act as a continuous truss, which is why a panel a few millimeters thick resists bending far better than a flat sheet of the same weight.
Every Greenlyhouse greenhouse is glazed in 6 mm twin-wall polycarbonate as standard: the deeper chamber and stiffer ribs are what hold warmth and take impact.
The air chamber is the insulation
Still air is one of the best common insulators, and the chamber holds a captive layer of it across the entire glazed surface. Heat leaving the greenhouse must cross the inner sheet, the air layer and the outer sheet, and the air layer slows it dramatically compared with any single pane.
The felt result: the greenhouse stays warm further into the evening, cools slower overnight, and holds a wider margin over outdoor lows in the cold months. It is the property that turns a greenhouse from a sunny-day room into a season extender.
Diffused light, not spotlight light
Twin-wall polycarbonate passes about 85% of sunlight, and it scatters that light as it passes through the two sheets and the rib structure. Instead of sharp beams and sharp shadows, plants receive soft, even light from many angles: fewer scorched leaves under the roof at noon, more light reaching lower leaves and the back corners.
Glass passes a few points more light, directionally; the trade-offs are laid out honestly in polycarbonate vs glass.
UV protection: for the plants and the panel
The outer face of every panel carries a factory-applied UV-protective layer, and it protects two things. It blocks UV from the growing space, and it protects the polycarbonate itself, because unprotected polycarbonate yellows and turns brittle under years of sun. This coating is why panel orientation matters during assembly: the coated side faces out, and our panels are marked accordingly.
Condensation: what is normal, what is not
On cool mornings you may see mist or droplets inside the air channels. This is normal: air in the channels cools overnight and its moisture condenses, then clears as the day warms. It does not harm the panel or meaningfully affect light.
Heavy condensation dripping inside the greenhouse is a different signal: it means warm, humid air is meeting cold surfaces, and it is your cue to ventilate. Persistent interior humidity invites fungal disease, which is a growing problem, not a panel problem. The fix is airflow, covered in the greenhouse ventilation guide.
Care and lifespan
Maintenance is modest: rinse the panels with water and a soft cloth or sponge once or twice a year, because a season of dust and algae steals light exactly when winter has none to spare. Avoid abrasive scrubbing and solvents. Keep the channel ends seated in their frame profiles, and reseat any panel that has crept before winter.
Treated this way, UV-protected twin-wall stays clear and serviceable for many years, each panel is individually replaceable, and our greenhouses carry a 10-year warranty.
Against a single sheet: why twin-wall is the default
A single sheet of anything, glass, acrylic or thin polycarbonate, can only be a rain barrier that passes light. It cannot insulate, because there is no trapped air; it concentrates rather than diffuses; and in thin plastics it flexes without the rib truss that gives twin-wall its stiffness. Twin-wall is what you get when the glazing is designed for growing rather than just covering. Every Greenlyhouse model is glazed with it, edge-held in continuous channels in the galvanized steel frame.
Your questions, in short
The channel ends sit inside the frame profiles, which keeps most contamination out. Small amounts of channel condensation are normal and clear on their own as the day warms.
Polycarbonate is highly impact resistant: it flexes under hail and springs back where glass would shatter. Extreme hail can mark a panel, and any single panel can be replaced without touching the rest.
No. About 85% of sunlight passes through, and the diffusion spreads it more evenly through the canopy. Growth under twin-wall is excellent; just keep the panels clean in winter.
The face with the UV-protective coating, which is marked on the panels. Assembly instructions call it out at the right step.
