Polycarbonate vs Glass Greenhouse: Which Is Actually Better

Most comparisons of polycarbonate and glass greenhouses are written for a market that does not exist in North America. They read as a straight glazing choice, two materials at broadly similar prices, pick the one that suits your garden. That is a fair description of the British market, where glass has been the default hobby greenhouse material for a century and a toughened glass 6×4 starts around the same price as a decent polycarbonate kit.

On this side of the Atlantic the picture is different enough to change the question. Polycarbonate kits run from roughly three hundred dollars to about thirty five hundred. Glass hobby greenhouses effectively start near eight thousand and climb steeply from there. For most buyers, that is not a choice between two glazing materials. It is a choice between two entirely different budgets, and the glazing follows from the budget rather than the other way round.

That said, the material differences are real, and they matter for anyone at the top of the market, anyone building custom, and anyone deciding whether the premium is worth reaching for. This page covers what each material actually does for light, heat, impact and service life, and where the received wisdom about both is wrong.

The Price Reality in North America

Glass at hobby scale here means importing a European structure. The Exaco Janssens range dominates the segment, with the smallest Junior Victorian around eight thousand dollars and the Royal Victorian VI 23 near ten thousand as of 2026. Larger and insulated models run into the tens of thousands. Custom builders like BC Greenhouse Builders occupy the same tier with engineered structures priced per project.

Against that, a Palram Mythos sits around seven hundred dollars, a Riverstone MONT around three thousand, and a Grandio Ascent somewhere between. The entire polycarbonate kit market fits inside the gap between zero and the cheapest glass greenhouse you can buy assembled.

The practical consequence is that most people asking whether glass is better are really asking whether the premium tier is worth it, and the honest answer is that the glazing is only part of what the premium buys. A Janssens is a heavier frame, thicker aluminium, stainless fittings, rubber-sealed panels and a structure meant to outlive the person who assembled it. Comparing the glass to a kit’s polycarbonate in isolation misses most of the difference.

Light: More Is Not Automatically Better

Glass transmits around 90% of available light. Twin-wall polycarbonate transmits roughly 80 to 85% depending on thickness and wall count. On paper glass wins comfortably, and this is the argument that sells glass greenhouses.

It is also the argument that deserves the most scrutiny, because the two materials transmit light differently. Glass is directional. Sunlight passes through in parallel rays, which produces bright patches where the beams land and shadow everywhere else. Twin-wall polycarbonate diffuses the light as it crosses the internal channels, spreading it more evenly across the growing space and reducing the scorch risk on whatever sits directly under the sun’s path.

For an ornamental glasshouse holding specimen plants, the clarity of glass is part of the point. For raising seedlings and growing vegetables, even distribution generally matters more than raw transmission, which is why commercial growers use diffusing materials deliberately rather than reluctantly.

Insulation: The Number That Surprises People

This is where the intuition fails hardest. Glass feels solid and permanent and polycarbonate feels like plastic sheeting, so most people assume glass holds heat better. It does not, and it is not close.

Glazing	R-value	Light transmission
Single-pane glass	0.9	90%
4 mm twin-wall polycarbonate	1.47	85%
8 mm twin-wall polycarbonate	1.72	81%
10 mm twin-wall polycarbonate	1.92	82%

A single pane of glass is one solid layer with no trapped air. A twin-wall polycarbonate panel is two skins with an insulating cavity between them, which is the same principle as a double-glazed window applied to a much cheaper material. The cheapest polycarbonate kit on the market insulates better than the most expensive single-glazed glasshouse.

Glass only wins this comparison when it is double-paned, and double-glazed glass greenhouses exist at the very top of the market at prices well into five figures. If year-round growing in a cold climate is the goal and the budget is anything short of that, polycarbonate is the better material, not the compromise.

The Three Kinds of Glass

Glass greenhouse listings use three terms that are not interchangeable, and the difference between them is a safety difference rather than a performance one.

Horticultural glass is standard annealed glass, usually 3 mm, supplied in small panes of roughly two feet square that overlap and are held with S-clips. It is the cheapest option and it transmits light well. It also breaks easily, and when it breaks it produces large sharp shards. The overlapping joints collect dirt and algae over the years and are difficult to clean properly.

Toughened or tempered glass is the same material heat-treated and rapidly cooled, which leaves the surface in compression and makes it roughly four to five times stronger. When it does break it granulates into small blunt fragments rather than shards, in the way a car side window does. It comes in 3 mm and 4 mm, and the 4 mm has meaningfully better impact resistance than the 3 mm.

Premium glass greenhouses use 4 mm toughened glass in full-length panels rather than overlapping small panes, sealed with rubber gaskets instead of clips. That eliminates the algae traps and the draughts, and it is a large part of why the premium tier stays looking good for decades while a cheap glass greenhouse looks tired in five years.

If there are children around, the horticultural glass tier is worth avoiding entirely. A football through a 3 mm annealed pane produces exactly the outcome you would expect.

Impact, Hail and Snow

Polycarbonate is the clear winner on impact by a wide margin. It is the material used for bullet-resistant glazing and aircraft canopies, and a twin-wall panel will take hail, a stray branch or a thrown ball without damage in conditions that would break any glass.

Snow is a more nuanced question, because the failure mode differs. A glass pane held in clips is rigid and strong in compression but it is only as secure as its fixings, and glass greenhouses in high snow country need engineering rather than assumption. Polycarbonate flexes, which is a virtue up to a point and a problem past it, since a thin panel deflecting under load can escape its channel entirely. Neither material makes a structure snow-proof. The frame does that, and the glazing thickness contributes to it.

On wind, the weight of toughened glass is an advantage. It is heavy enough that the wind does not lift or shift it, whereas the most common way a light polycarbonate kit fails in a storm is a roof panel flexing out of its retaining channel.

Service Life and the Replacement Cycle

This is the strongest argument for glass and it is usually made badly.

Glass does not degrade. A pane installed in 1970 transmits the same light today as it did then, assuming nobody has broken it. Polycarbonate is a consumable. The co-extruded UV layer that protects it thins with sun exposure, and a good panel yellows and hazes gradually over ten to twenty years, at which point light transmission has dropped enough to justify replacement. Many sheet manufacturers warrant against yellowing for ten years, which is a fair indication of the expected working life.

So the honest cost comparison is not sticker price against sticker price. It is a polycarbonate kit plus a re-glazing somewhere in the second decade, against a glass structure that does not need one. That closes some of the gap, though on North American prices it does not close eight thousand dollars of it.

The caveat is that this only holds for panels installed correctly. Polycarbonate fitted with the UV-coated side facing inward yellows within one or two seasons, which turns a fifteen year material into a three year one. That single installation detail moves the service life comparison more than the choice of material does.

Replacing a Broken Panel

The expectation here runs backwards from what most people assume.

A broken glass pane is a local problem with a local solution. Any glazier can cut a replacement to size, usually within a few days and for very little money. A damaged polycarbonate panel generally has to come from the manufacturer, in the specific size and profile that kit uses, and availability depends on whether the model is still in production. Owners of discontinued kits regularly find that the structure is intact and unrepairable at the same time.

That is a real ownership cost that does not appear on any spec sheet, and it argues for buying polycarbonate kits from manufacturers with long production runs and an accessible parts channel rather than from whoever is cheapest this month.

Weight, Frame and Foundation

Glass is heavy, and everything else follows from that. A glass greenhouse needs a frame stiff enough to carry the load without racking, and it needs a level, permanent foundation, because a frame that moves cracks panes. That usually means a concrete base or a properly built perimeter footing, which is real cost and real work on top of the purchase.

Polycarbonate kits are light enough to sit on a prepared gravel bed or a timber perimeter, provided they are anchored properly. Lighter is not automatically better here, since the same low weight is what makes anchoring non-negotiable, but it does mean the site preparation is a weekend rather than a contractor.

What the Premium Glass Makers Do

The most telling detail in this whole comparison is buried in the options list of the premium glass brands.

Janssens offers a 10 mm polycarbonate upgrade on its Royal Victorian glass greenhouses, marketed specifically for cold climates. A manufacturer selling ten thousand dollar glasshouses will sell you polycarbonate instead of glass when insulation is the priority, because at that point the material genuinely performs better and they know it.

That is worth holding onto when reading marketing from either side. Glass is chosen for clarity, permanence and appearance. Polycarbonate is chosen for insulation, impact resistance and price. Anyone claiming glass is the better growing material in a cold climate is selling something, and so is anyone claiming polycarbonate is just as good to look at.

Which to Choose

Choose polycarbonate if the budget is under about five thousand dollars, if the climate is cold enough that heat retention decides whether the structure is useful in winter, if hail is a regular feature of your weather, or if children play anywhere near where it will stand. That covers the large majority of buyers, and it is not a compromise choice in any of those cases.

Choose glass if the greenhouse is a permanent piece of garden architecture as much as a growing space, if you want a structure that will still be transmitting full light in forty years, if you are keeping specimen plants where clarity matters, and if the budget clears eight thousand dollars with room for a proper foundation on top. Specify 4 mm toughened rather than 3 mm horticultural, and treat full-length rubber-sealed panels as a requirement rather than an upgrade.

What does not work is buying at the bottom of the glass market to get glass. A cheap horticultural glass greenhouse gives you worse insulation than a four hundred dollar polycarbonate kit, a genuine safety hazard, and panes that will be cracking one at a time for as long as you own it.