When someone calls asking about "the glass," they usually mean one of two things: is it going to hold up? and what happens if a pane breaks? Fair questions when you're spending real money on something that's mostly, well, glass. The short version is that a proper greenhouse doesn't use ordinary window glass at all — and the difference is the whole point.
Why greenhouse glass is tempered safety glass
The glass in a quality Janssens greenhouse is 4mm tempered safety glass. Tempering is a heat-treating process that makes the glass several times stronger than standard glass and changes how it fails. That second part is what matters most.
Ordinary glass breaks into long, dangerous shards. Tempered glass shatters into small, dull, pebble-like pieces instead — the same reason it's used in car windows and shower doors. In a structure you'll be walking through, working in, and standing inside during a storm, that's not a nice-to-have. It's the reason we won't sell you plain horticultural glass and call it a day.
Tempered glass also gives you the thing glass is loved for: clarity. Nothing beats the crisp, true light and the clean look of real glass at eye level. If the aesthetic of a classic greenhouse is what pulled you in, glass walls are a big part of that feeling.
The one thing you must know: tempered glass can't be cut
This trips up more people than anything else, so I'll be blunt: tempered glass cannot be cut, trimmed, or drilled after it's made. It shatters if you try.
Regular glass gets scored and snapped to size all the time. Tempered glass doesn't work that way — all its strength comes from the tension locked in during tempering, and any cut releases that tension and blows the whole pane apart. Anyone who tells you they'll "just trim a pane to fit" either means a different material or is about to make an expensive mess.
Two practical consequences worth planning around:
- Every glass pane is a fixed, factory size. Your foundation and frame have to be right, because you're not shaving glass to cover a gap.
- When you need a non-standard opening — say, mounting an exhaust fan in the gable — you don't cut a glass pane for it. You swap in a polycarbonate (lexan) panel, which can be cut cleanly to fit the fan. More on that in our ventilation guide.
If a pane ever does break, you order a replacement in that exact factory size — you don't fabricate one on site.
Glass or polycarbonate for the walls?
Both are legitimate, and plenty of buyers mix them. Here's the honest trade-off for the wall glazing:
| 4mm tempered glass | 10mm twin-wall polycarbonate | |
|---|---|---|
| Clarity | Crystal clear, true view | Translucent, diffused light |
| Insulation | Single pane — less | Air gap between walls — more |
| Impact / snow | Strong, but rigid | Flexes instead of shattering |
| Look | Classic greenhouse | Modern, utilitarian |
| Best for | Aesthetics, mild-to-moderate climates | Heat retention, hail and heavy snow |
Glass wins on looks and light. Polycarbonate wins on insulation and impact resistance — its twin-wall construction traps an insulating layer of air, and it flexes rather than cracks under a snow load or a falling branch.
That's why so many of our buyers land on the hybrid setup: tempered glass on the walls, polycarbonate on the roof. You keep the clear, classic look where you see it most — at eye level — and put the tougher, better-insulating material overhead where snow, hail, and heat loss actually hit. If you're weighing the roof specifically, our roof material guide goes deep on all three options, and the cold-climate guide covers what winter demands.
(One note if you're looking at our cedar line: the Alton Cheltenham glazes with 3mm tempered safety glass — same safety principle, sized for that frame.)
So which is right for you?
If you're in a mild-to-moderate climate and you love the classic, crystal-clear greenhouse look, glass walls are a beautiful, durable choice. If you're fighting hard winters, hail, or you want to hold heat overnight without running a heater constantly, lean toward polycarbonate or the hybrid. Most people are somewhere in between — which is exactly why we talk it through case by case rather than pushing one answer.
Every Janssens greenhouse is backed by a 15-year warranty and built in Lier, Belgium — engineered so the glazing you choose lasts as long as the frame around it.
Not sure which glazing suits your climate and your snow load? Call us or send your zip code and we'll match glass, polycarbonate, or a hybrid to where you actually live. It's a five-minute conversation that saves a lot of second-guessing.