Glass or polycarbonate — which greenhouse glazing is better?

This is genuinely a trade-off rather than a right answer. Glass is the better growing medium; polycarbonate is the better material to own. Which matters more depends on whether you are growing seedlings in February, whether children use the garden, and how exposed the site is.

Safety

Step by step

1. Weigh light transmission against insulation.Single glass transmits the most light and holds the least heat. Twin-wall polycarbonate transmits noticeably less but retains heat far better. If your main use is overwintering plants, insulation wins; if it is raising early seedlings, light does.
2. Consider diffused versus direct light.Polycarbonate scatters the light it transmits. That gives more even illumination through the canopy and less leaf scorch in high summer, which many growers genuinely prefer, at the cost of less total light in winter.
3. Be honest about breakage risk.Football, wind-blown branches, ladders and grandchildren. If any of those are part of your garden, horticultural glass will not survive and toughened glass or polycarbonate is the right call.
4. Think about lifespan.Glass is effectively permanent — sixty-year-old greenhouses still have their original panes. Good polycarbonate lasts ten to fifteen years before it yellows and loses transmission, then the whole lot needs replacing at once.
5. Check how each is held in the frame.Glass sits in glazing bars held by spring clips and W-clips, which can slip. Polycarbonate sheets need proper clips or bar caps and are the classic thing found in a neighbour's garden after a gale, because sheets act as sails.
6. Factor in cleaning.Glass cleans easily inside and out and comes back to full clarity. Twin-wall polycarbonate cannot be cleaned inside the flutes, so algae and dust accumulate there permanently and transmission drops over the years.

Tips

• You can mix: toughened glass to the roof and upper panes for light, polycarbonate to the lower panes where breakage risk is highest and light matters least.
• Polycarbonate has a UV-protected face that must go outward. Fitting a sheet the wrong way round halves its life — check the printed film before you strip it.
• If you inherit a greenhouse with horticultural glass and small children arrive, replacing just the lower panes with polycarbonate is a cheap and worthwhile safety upgrade.

Common mistakes

• Choosing single-skin polycarbonate to save money — It has the light penalty of polycarbonate with none of the insulation benefit, and it degrades fastest of all. Twin-wall or glass — single-skin is the worst of both.
• Not clipping polycarbonate sheets properly — A loose sheet lifts in wind and takes the neighbouring ones with it. Proper clips at the stated spacing, checked each autumn, is the whole of the maintenance regime.
• Assuming toughened glass is unbreakable — It is far stronger against impact but vulnerable at the edges, and when it does go it goes completely and instantly. It is safer, not indestructible.

Alternatives

• Horticultural glass, 3mm: The traditional and cheapest glazing. Highest light transmission, stays clear indefinitely, easy to clean and to replace pane by pane. Breaks readily and dangerously, and gives almost no insulation.
• Toughened safety glass, 4mm: Same optical performance as horticultural glass, four to five times the impact strength, and it shatters into blunt granules instead of shards. Costs noticeably more and cannot be cut — every pane must be ordered to size.
• Twin-wall polycarbonate, 4 to 10mm: Two skins with an air gap. Insulates far better than single glass, is effectively unbreakable, and diffuses light so plants get less scorch. Light transmission is lower, it gathers algae and dust in the flutes, and it degrades in UV over ten to fifteen years.
• Acrylic or single-skin plastic: Found on cheap mini greenhouses. Light, clear and inexpensive, but it goes brittle and cloudy quickly and gives no insulation. Fine for a season or two of hardening off, not for a permanent structure.

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