Polycarbonate vs Glass Greenhouse: Which to Buy

Glass transmits more light. Twinwall polycarbonate keeps more heat, survives more abuse, and costs far less to buy and to support.

For a hobby or backyard greenhouse used to grow things, twinwall polycarbonate wins on almost every practical measure. For a garden building you want to look at as much as work in, glass is the better answer and always has been. Neither is a compromise; they are optimised for different jobs.

Glass wins the raw transmission number, and in the depths of winter at high latitude that matters. But the number people quote misses what actually happens to the light after it gets in.

Glass transmits a direct beam. That beam lights the top of the plant brilliantly and leaves the lower leaves in shadow, and on a hot clear day it can scorch foliage where it lands. Twinwall polycarbonate diffuses light - it scatters it as it passes through, so it arrives from many angles at once. Diffused light penetrates deeper into the canopy, lights the underside of leaves that a direct beam never reaches, and does not burn. Commercial growers pay extra for diffusing films for exactly this reason.

So the honest comparison is not 90 percent against 80 percent. It is a smaller quantity of better-distributed light against a larger quantity of worse-distributed light. In practice, for most crops in most of the country, the twinwall is at least as productive.

What is genuinely true is that twinwall is translucent, not transparent. You cannot see through it clearly. If looking through the walls is part of what you want from a greenhouse, that is a real loss and no amount of technical argument changes it.

Twinwall polycarbonate is two skins joined by internal ribs, with a sealed air gap between them. That air gap is a genuine thermal break - the same principle as double glazing in a house. Single-pane glass has no such break; it is one layer of a fairly conductive material.

The practical effect: a twinwall house holds overnight temperature noticeably better, needs meaningfully less heater output to hold a given target, and swings less violently between day and night. Stepping from 4 mm to 6 mm adds another increment of insulation and, just as importantly, a stiffer panel.

If you intend to grow through winter, this single property outweighs everything else on the comparison table. It is decided once, at purchase, and it sets your fuel bill for the life of the greenhouse.

Impact. Polycarbonate is the material used for machine guards and riot shields. A branch, a stray ball or a hailstorm that would put a glass pane through the tomatoes bounces off twinwall. If you have children, trees, or hail, this is not a marginal consideration.

Safety. A broken greenhouse pane is a large piece of glass at head height above a soil floor. Toughened glass reduces the severity but not the event. Polycarbonate does not produce shards.

UV and yellowing. This is polycarbonate's genuine weakness and the thing to check on any quote. Unprotected polycarbonate yellows and turns brittle under sun within a few years. Proper sheets carry a co-extruded UV-protective layer on the exterior face, and they must be installed with that face outward - the sheets are marked. Ours are UV-protected twinwall, and installing them the right way up is the one assembly step where getting it wrong costs you years of panel life.

Glass does not yellow. That is a real advantage. A well-kept glass house looks the same after decades. Polycarbonate is a wearing part that will eventually be replaced, though replacing a panel is cheap, quick, and something you can do alone.

Condensation in the flutes. Twinwall channels can trap moisture and dust if the cut ends are left open. Seal the top edge with solid tape and the bottom edge with vented tape so the channels can drain and breathe. Skipping this is why some twinwall greenhouses look cloudy after a few seasons and others do not.

The frame is the one component you cannot replace. Glazing can be swapped panel by panel; a failed frame means a new greenhouse.

• Hot-dip galvanized steel. Strong, stiff, and it spans further for less money than anything else at this price. Galvanizing bonds zinc through the wall of the tube rather than coating the surface, and the protection lasts as long as that layer stays intact - which is why cut edges and drilled holes get sealed during assembly. Our frames are 1 in. square hot-dip galvanized tube.
• Lighter extruded metal sections. Self-passivating, corrosion-resistant, and common on premium hobby houses. They cost materially more for the same span, which is the trade you are making. Our buying guide sets out that tradeoff in full rather than pretending it does not exist.
• Wood, usually cedar or redwood. Naturally rot-resistant, insulates better than any metal, and looks superb. It also needs periodic maintenance that neither metal does.
• PVC and resin. Cheap and light. Fine for small structures in sheltered sites, not for span or snow load.

Whatever the material, the frame has to be rated for your local snow load and design wind speed, and it has to be anchored. A rating is a statement about a properly fixed structure.