Polycarbonate vs glass greenhouses: which is better for a UK garden?

Glass and polycarbonate compared

The rest of this guide explains each of those rows, and where the differences are big enough to change your decision.

The three glazing types

Horticultural glass is the traditional 3mm greenhouse glass. It comes in small panes, usually about 2ft (610mm) square, that overlap like roof tiles and are held in with W or S shaped clips. It is the cheapest glass, easy to replace and very clear. Its drawback is how it breaks: into large, sharp pieces.

Toughened glass is safety glass, typically 4mm thick on greenhouses. It is heat-treated so that when it does break it shatters into thousands of small, blunt granules instead of shards. It comes in larger panes with no overlaps, which looks cleaner and seals better. It costs more and is heavier, and toughened panes cannot be cut to size afterwards, so replacements have to be ordered to the exact dimensions.

Twin-wall polycarbonate is a rigid plastic sheet made of two skins joined by thin ribs, with air channels between them. Budget greenhouses use 4mm sheets; better ones use 6mm, 10mm or even 16mm. The trapped air is what makes it insulate. Some very cheap greenhouses use thin single-skin styrene or acrylic instead: these are brittle, go cloudy quickly and are not the same thing as twin-wall polycarbonate.

Light: glass wins, but not by as much as you might think

Clear glass lets through about 90% of daylight. Twin-wall polycarbonate lets through less: around 81% for good 6mm and 10mm sheets, and about 74% for 16mm, going by one UK manufacturer's figures. Older or cheaper sheets let in less again as they yellow.

In a British winter, light is the thing plants are shortest of, so that 10% matters most from November to February. If you start seeds early, overwinter salads or grow under cover all year, glass gives you a real advantage.

In summer the picture flips. Polycarbonate scatters the light it lets through, so it arrives from every direction rather than as a direct beam. That means fewer scorched leaves, more even growth and less need for shading paint or blinds in June and July.

Warmth: polycarbonate wins clearly

Single glass is a poor insulator. Its heat loss (U-value) is around 5.8 W/m²K. Twin-wall polycarbonate does much better: about 3.5 for 6mm, 3.0 for 10mm and 2.3 for 16mm.

In plain terms, a 6mm polycarbonate greenhouse loses about 40% less heat than the same greenhouse glazed in single glass, and 10mm loses nearly half as much. If you heat your greenhouse to keep frost out over winter, that is a direct saving on every unit of electricity or litre of paraffin you burn.

If you don't heat at all, the difference is smaller than the numbers suggest: an unheated greenhouse of either type will still drop close to the outside temperature on a clear, frosty night.

Safety

This is often the deciding factor for families.

• Horticultural glass breaks into large, sharp pieces. A football, a fall against a pane or a branch in a storm can leave dangerous shards in the beds and on the path.
• Toughened glass breaks into small, blunt granules. It is the safer choice if you want glass and have children, pets, or a greenhouse next to a path or play area.
• Polycarbonate almost never shatters. A ball bounces off it.

Wind and weather

Polycarbonate's lightness cuts both ways. It puts less strain on the frame, but in a strong gale a sheet that isn't well secured can bow, pop out of its glazing bars and take off. Glass panes are heavy enough to stay put but can crack if the frame twists or something hits them.

Whichever you choose, the things that matter most in an exposed garden are the same:

• A strong frame with proper bracing, and glazing held by clips or rubber beading along its full length.
• A level base the greenhouse is anchored to. See our guide to greenhouse bases.
• Keeping doors and vents shut in a storm, so wind can't get inside and push panes out.

How long each lasts

Glass, looked after, lasts as long as the frame: decades. It doesn't yellow or go brittle.

Polycarbonate ages. Good sheets have a UV-protective layer on the outside face and are commonly sold with ten-year warranties; the cheapest have little protection and can go cloudy and brittle within a few seasons. Twin-wall sheets also need their open ends sealed with tape, or algae and dirt get into the channels and can't be cleaned out. Expect to reglaze a polycarbonate greenhouse after 10 to 15 years.

Cleaning and maintenance

Glass is easy to clean and can be scrubbed. Polycarbonate scratches, so use a soft cloth or sponge and plenty of water, and never abrasive cleaners or a pressure washer close up. Scratches collect dirt and cut the light.

Replacing a broken horticultural pane is cheap and simple. Toughened panes have to be ordered to size. Polycarbonate sheets can be cut at home with a fine-toothed saw.

Cost

As a rough guide for the same size of greenhouse:

1. Polycarbonate greenhouses are usually the cheapest, and lighter frames keep the price down further.
2. Horticultural glass comes next.
3. Toughened glass costs the most, and the extra weight needs a sturdier frame.

Running costs can reverse that order. If you heat through the winter, polycarbonate's lower heat loss can make it the cheaper greenhouse to own within a few winters.

Which should you choose?

• You'll start seeds early and grow all year: glass, for the light. Toughened if children are about.
• You'll heat it to overwinter tender plants: polycarbonate, 6mm or thicker.
• Children, pets or ball games near the greenhouse: toughened glass or polycarbonate. Never horticultural glass at low level.
• Exposed, windy site: either, but put your money into a strong frame and a proper anchored base before the glazing.
• Mainly summer tomatoes and cucumbers: either works. Polycarbonate's diffused light means less shading and fewer scorched leaves.
• Tight budget: polycarbonate, but buy twin-wall with UV protection, not thin single-skin plastic.
• Looks matter, or it's in full view of the house: glass, ideally toughened.

Questions people ask

Is polycarbonate or glass better for a greenhouse?

Neither is better for everyone. Glass lets in about 10% more light and lasts for decades, which suits seed-raising and year-round growing. Twin-wall polycarbonate keeps far more heat in, is safer and costs less, which suits heated greenhouses and gardens with children.

Do polycarbonate greenhouses get hotter than glass?

In winter a polycarbonate greenhouse holds its heat better because twin-wall sheets insulate. In summer both get very hot, but polycarbonate diffuses the light, so plants are less likely to scorch. Either way you need good ventilation.

Is toughened glass worth it for a greenhouse?

Yes if children, pets or people pass close to the greenhouse. Toughened glass breaks into small, blunt granules instead of sharp shards, and comes in larger panes that seal better. It costs more and panes have to be ordered to size.

How long does polycarbonate last on a greenhouse?

Good UV-protected twin-wall polycarbonate typically lasts 10 to 15 years before it clouds or turns brittle. Cheap sheets with little UV protection can go cloudy in a few seasons. Fit the UV-protected face outwards.

What thickness of polycarbonate is best for a greenhouse?

6mm twin-wall is a good all-round choice. 4mm is fine for an unheated summer greenhouse; 10mm or 16mm insulates better for heated greenhouses but lets in a little less light.

Sources

1. Greenhouse glazing: what are my options?, Rhino Greenhouses
2. Guide to polycarbonate sheet thickness for greenhouses, eRoofing