Toughened Glass vs Horticultural Glass vs Polycarbonate: An Honest Comparison
By The Rhino Growing Team— Greenhouse specialists, Norfolk ·
Glazing is where greenhouse budgets are quietly cut, and where cheap greenhouses fail first. Here is the comparison we wish every buyer saw before choosing.
The three options at a glance
| 4mm toughened glass | 3mm horticultural glass | Twin-wall polycarbonate | |
|---|---|---|---|
| Relative strength | ~5x horticultural | Baseline | High impact, low rigidity |
| Breakage behaviour | Crumbles into blunt granules | Long, sharp shards | Doesn’t shatter; panels blow out |
| Light transmission | ~90%, stays clear | ~90%, stays clear | ~80%, yellows over years |
| Wind performance | Excellent (rigid, heavy) | Poor in panes over 2ft | Poor — lightweight panels lift |
| Typical lifespan | Decades | Until the first accident | 10–15 years before clouding |
Why we only fit toughened glass
Every Rhino ships with 4mm toughened safety glass — the same specification as a patio door. Three reasons:
- Safety. In a family garden, glazing that breaks into rounded granules rather than blades is not a luxury.
- Wind. Glazing failure in storms is rarely the glass breaking — it’s light panels flexing and popping out of their channels. Heavy, rigid toughened panes held by capped bars stay put.
- Light, for decades. Polycarbonate’s UV coating degrades; glass doesn’t. The light your plants get in year 20 is the light they got in year one.
When polycarbonate makes sense
We’ll be fair: twin-wall polycarbonate insulates better (roughly 40% lower heat loss than single glazing) and makes sense for heated tropical collections. For the way most European gardeners actually use a greenhouse — unheated, season-extending — toughened glass wins on every measure that matters.
Questions to ask any greenhouse seller
- Is the toughened glass 4mm, or thinner?
- Are the glazing bars capped (mechanically clamped) or just clipped?
- Is glass included in the headline price, or an upgrade?
If you’d like to feel the difference rather than read about it, our Norfolk show site has every range built and glazed — bring a windy day if you can.