Every winter someone posts a photo of a greenhouse that went down under snow. It's almost never the panel that failed - more often it's the anchoring, or a door left open in a storm. Which is worth knowing before you buy.
Are polycarbonate greenhouses any good?
Yes. For year-round growing in a backyard, polycarbonate is the most practical glazing you can buy. Twin-wall polycarbonate diffuses light instead of concentrating it, shrugs off hail that would destroy glass, and costs a fraction as much per square foot.
What you're buying is a hollow panel: two thin sheets with an air gap between them. That gap insulates, stiffens the panel, and scatters the light so it reaches lower leaves and the shaded side of a plant rather than throwing the hard shadows you get under glass. Two other things follow from it:
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Impact resistance. Polycarbonate is around 200 times stronger than glass on impact. A hailstone that cracks glass bounces off. So does a stray baseball.
- Weight. Panels are light enough for one person to carry, which is why a polycarbonate greenhouse goes up over a weekend. Our manual recommends three people and a full day at minimum.
Are polycarbonate greenhouses as good as glass?
Not identical - better at some things, worse at others. Polycarbonate passes over 80% of available light and spreads it evenly. Glass passes around 95%, undiffused.
Put plainly, this is where each one wins:
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Choose polycarbonate if you want maximum growing space for the money, if hail or heavy snow are a fact of life where you are, if you're assembling it yourself, or if you want to grow through winter without heating a large volume of air.
- Choose glass if you're growing light-hungry plants - citrus, orchids, Mediterranean herbs - where the difference between 80% and 95% actually changes the outcome. Or if the building has to look like part of the garden rather than equipment in it.
There's one difference no spec sheet lists. Twin-wall polycarbonate is translucent, not transparent - light comes through, but you can't see out of it. For growing, that doesn't matter at all. For a building you'd sit in on an autumn evening, it decides everything.
How thick should polycarbonate panels be?
For a backyard greenhouse, 6mm twin-wall is the sensible standard, and it's what we use - our panels are 1/4 inch double-wall. 4mm is thinner than it sounds and struggles under snow and wind. 8mm and 10mm insulate better but cost more and add weight most home structures don't really need.
Panel thickness is the one spec that genuinely separates a cheap greenhouse from a serious one. What the numbers mean in practice:
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4mm - the default on budget kits. Adequate in mild climates, but it flexes, it carries less snow, and the inner walls can collapse over time.
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6mm - roughly 50% thicker, and the jump where a panel stops being a compromise. Better insulation and impact resistance, and rigid enough that the panel contributes to the structure rather than just filling it.
- 8mm and 10mm - used in commercial growing and in the harshest winter zones. Genuinely better insulation, at a price and a weight that most backyard frames aren't designed to carry.
For context on what "thin" actually looks like: budget kits are typically glazed in 5/32″ single-wall panels, which have no air gap at all. A double-wall panel of our thickness is around 70% stronger than single-wall, and it insulates, which single-wall effectively doesn't.
How long do polycarbonate panels last?
Ten to twenty years, depending on the UV coating and your climate. Ours block 99% of UV, which protects both the plants and the panel itself. Polycarbonate panels don't usually crack or fail suddenly - they yellow slowly and lose light transmission. A UV-protected panel fitted the right way up will still be working in fifteen years. Ours carry a 5-year panel warranty and a 10-year warranty on the steel frame.
What shortens that life, in order of how often we see it:
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The panel fitted upside down. The UV layer is on one face only. Reversed, the panel is unprotected and will yellow in a few years instead of fifteen.
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Algae in the channels. Water that gets into the flutes and can't drain grows green and blocks light. Sealed ends and vertical installation prevent it.
- The wrong cleaner. Mild soapy water and a soft cloth. Solvents, acetone and paint thinners destroy polycarbonate on contact, and a pressure washer forces water into the channels.
Will a polycarbonate greenhouse blow away?
Not if it's anchored properly. Our polycarbonate greenhouses are rated to 65 mph winds and 3 feet of snow (32 psf), on a 3.07-inch galvanized steel frame. Nearly every failure we hear about is an anchoring failure, not a panel or frame failure.
The arched profile does a lot of the work - wind passes over it rather than pushing against a flat wall - and the anchoring does the rest. Our manual is blunt about it: a greenhouse left unanchored is not a warranty claim.
One caveat catches people out. Wind ratings are measured with the doors and vents shut. An open door turns the structure into a sail and the rating stops applying. If a storm is forecast, close everything.
Snow works the same way: the rating assumes you're clearing the roof after heavy falls, not leaving three feet up there all winter. And in real snow country the model matters as much as the anchor. The Classic Arch, for instance, is rated to 49 psf.
Does a polycarbonate greenhouse get too hot in summer?
It gets hot, and it handles heat better than glass. Twin-wall panels diffuse light rather than focusing it, so there are no burning hot spots and no scorched leaves under a lens effect. But any sealed greenhouse overheats without airflow. You need the vents open on warm days and, in a hot climate, shade cloth through midsummer.
The panels hold up from -4°F to 104°F, so heat isn't a material problem. Air is: a closed greenhouse in full sun can run 30°F above the outside temperature within an hour. The fix is simple:
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Roof and side vents open together. Hot air leaves at the top, cooler air comes in low. One vent alone does very little.
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Automatic openers. A heat-sensitive strip opens the vent as the temperature climbs and closes it as it drops - ours work between 62°F and 77°F, with no power and no attention from you. In a hot climate that's not an accessory.
- Shade cloth in midsummer, in the hottest zones, on the roof rather than the walls.
Will a polycarbonate greenhouse stay warm in winter?
Warmer than outside, and warmer than glass of the same thickness - the air gap between the two walls is insulation that single glazing doesn't have. Expect it to hold several degrees overnight and carry hardy crops through without any heating. Keeping it reliably frost-free through a hard winter needs a heat source.
That trapped layer of air works like an insulated mug: on our own figures it cuts heating costs by up to 40% against an uninsulated structure. What it won't do is create heat - it slows the loss of what the day gave you. In practice that's the difference between a greenhouse that grows spinach in January and one that just keeps the frost off pots.
Two things matter more than the glazing here: sowing early enough that the crops reach size before the light goes, and keeping the structure sealed.
So what makes a good one?
Five things. Everything else on a spec sheet is detail.
- 6mm twin-wall panels - not 4mm, and never single-wall.
- UV protection on the outward face, and fitted that way round.
- A wind and snow rating that matches your zone, stated as a number rather than a claim.
- Anchoring designed into the structure, not left to you to improvise.
- Vents at the roof and the sides, opening on their own.
Get those right and a polycarbonate greenhouse will outlast the weather that breaks cheaper ones, for a fraction of what the same growing space costs in glass.
Buy glass instead if you want to see out of it, or if you're growing light-hungry plants through the darkest months. Those are the two places polycarbonate genuinely loses, and we'd rather say so now than after delivery.
If you already know polycarbonate is right and just need to pick a model, our three-model comparison goes width by width.
Explore all the Polycarbonate Greenhouses
