A cold snap can expose every weakness in a greenhouse. Cracked panes, water pooling in channels, frost damage to seedlings, and structures that simply don’t hold heat. What you glaze your greenhouse with makes a genuine difference to how well your plants cope through the darker months.

With several materials on the market, each with a different set of trade-offs, it’s worth knowing what you’re actually comparing before you spend money on a replacement.

Polycarbonate Twin-Wall: Popular, But Not Perfect

Twin-wall polycarbonate is one of the most common glazing upgrades from glass. The two layers create an insulating air gap, which does help retain warmth overnight. It’s also impact-resistant, lightweight, and much safer to handle than glass.

Where it falls down is light diffusion. The hollow channels scatter light rather than transmitting it directly, which can reduce intensity on already short winter days. The channels also collect dust, algae and moisture over time, and cleaning them out properly is frustratingly difficult. Long-term, twin-wall polycarbonate can yellow and lose clarity, particularly on south-facing structures that see a lot of UV.

Solid Polycarbonate: Stronger, But With Its Own Limits

Solid polycarbonate sheet sits between glass and twin-wall in terms of both performance and price. It’s significantly tougher than glass, handles impacts well, and doesn’t have the channel problem that twin-wall faces. Light transmission is better than twin-wall too.

That said, solid polycarbonate scratches more easily than glass or acrylic and can be prone to surface hazing over time. It also tends to flex in larger panels, which can pull away from frame seals if it isn’t fitted carefully.

Acrylic: Where Light Transmission and Durability Meet

Acrylic sheet is increasingly the preferred choice for greenhouse glazing, and it’s easy to see why once you look at the numbers. Clear acrylic transmits around 92% of light, which is actually higher than standard horticultural glass. On a grey January day in the UK, that extra light reaching your plants genuinely matters.

It’s also many times stronger than glass, which means it can take the kind of knocks a British winter delivers without cracking or shattering. For anyone replacing broken panes or upgrading an existing structure, replacement acrylic greenhouse glazing can be cut to your exact dimensions, which makes fitting straightforward even on older or non-standard frames. Acrylic also doesn’t yellow the way cheaper plastics do, and quality sheet comes with a 10-year guarantee against discolouration.

Thermal Captabilities of Acrylic

On the thermal side, acrylic won’t perform quite as well as twin-wall polycarbonate in terms of pure insulation value, since it’s a single-skin material. If your greenhouse runs a heater overnight, that’s less of a concern. If you’re relying entirely on passive heat retention, pairing acrylic with a thermal fleece or bubble wrap lining for the coldest months is a practical solution.

Acrylic handles frost well. Its service temperature range runs from -40°C to 80°C, so there’s no risk of the material becoming brittle in cold weather the way some cheaper plastics can. It doesn’t absorb water either, so you won’t get the kind of freeze-thaw damage that can degrade other materials over time.

Glass: The Traditional Choice With Real Drawbacks

Horticultural glass has been the standard for generations. It offers excellent light transmission and holds heat reasonably well when combined with a good frame seal. The clarity is hard to beat, and it doesn’t yellow with age the way some plastics can.

The problem is fragility. Standard horticultural glass shatters easily, and in a UK winter that brings wind, falling debris and the occasional hard frost, that’s a real risk. It’s also heavy, awkward to handle, and can create cold spots if a pane breaks and isn’t replaced quickly. For anyone with children or pets nearby, the safety concerns are hard to ignore.

Final Take

For most UK gardeners, the choice really comes down to what matters more: insulation or light. If you want passive heat retention above everything else, twin-wall polycarbonate is worth considering. If you want maximum light transmission, durability and clarity that lasts, acrylic is the stronger all-round option for winter growing.

Glass remains a solid performer in mild conditions but its fragility makes it harder to justify as a long-term material in exposed or busy gardens. Solid polycarbonate sits in the middle but lacks the optical clarity of acrylic at a comparable price point.

For a simple replacement that won’t let you down when temperatures drop, acrylic sheet is the most practical and reliable choice for the majority of UK greenhouses.