When choosing a greenhouse, many buyers first focus on the thickness of the polycarbonate. Is 4 mm enough, or is it worth paying more for 6 mm? The difference may look small on paper, but it affects heat retention, panel rigidity and the length of the growing season.
The short answer is straightforward: 4 mm polycarbonate is a sensible choice for normal seasonal use from spring to autumn. We recommend 6 mm if you want to start earlier, keep growing later, or place the greenhouse in a colder or more exposed position.
Panel thickness is not the only factor, however. The quality of the polycarbonate, UV protection, frame construction, spacing between the arches, anchoring and ventilation are just as important.
Quick comparison: 4 mm or 6 mm?
| Feature | 4 mm polycarbonate | 6 mm polycarbonate |
|---|---|---|
| Best suited to | spring-to-autumn growing | an earlier start and later end to the season |
| Thermal insulation | good for seasonal gardening | better; the greenhouse cools more slowly |
| Panel rigidity | lower | higher |
| Suitable position | sheltered and mild | colder, windier or more exposed |
| Cost | lower | higher initial investment |
| Typical user | hobby gardeners and summer crops | gardeners wanting a longer season or tougher specification |
When is 4 mm polycarbonate a good choice?
A 4 mm covering makes sense if you mainly use your greenhouse for tomatoes, peppers, cucumbers, lettuce and raising seedlings during the usual growing season. On a sheltered plot, where the greenhouse will not be heated in winter, thicker panels are often unnecessary.
For a small or medium-sized greenhouse, 4 mm can offer the most practical balance between price and performance. It is particularly suitable when:
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you use the greenhouse from spring to autumn;
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the site is mild and sheltered;
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you do not plan to heat it in winter;
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you want an affordable greenhouse for a home garden;
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you would rather spend part of the budget on a stronger frame, extra vents or irrigation.
One example is the 3 m wide VRTNI garden greenhouse, along with the more compact 2 m and 2.4 m wide models. VRTNI has one-piece arches made from 20 × 20 mm profile, spaced 0.67 m apart, and is available with either 4 mm or 6 mm polycarbonate.
When is 6 mm worth choosing?
We generally recommend 6 mm polycarbonate to buyers who want to use their greenhouse for more of the year. A thicker twin-wall panel retains heat better and cools more slowly, with the difference most noticeable at night, in early spring and during autumn.
It is worth considering 6 mm when:
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you want to sow and plant earlier in spring;
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you want to continue harvesting later in autumn;
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your garden is colder, higher or exposed to more wind;
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you are choosing a larger greenhouse;
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you raise temperature-sensitive young plants;
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you want a more rigid covering.
With larger greenhouses such as the 3 m wide TITAN and the 4 m wide COMFORT PRO, buyers often select 6 mm because they want a longer season and expect to use the greenhouse for many years. Both models are also available with 4 mm panels, so the choice can be matched to the position of the garden and the available budget.
Important: 6 mm polycarbonate does not replace a strong frame
This is one of the details most easily missed when comparing greenhouses. A thicker panel is more rigid and provides better insulation, but it does not determine how much snow the complete greenhouse can carry.
Load capacity depends mainly on:
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the shape and dimensions of the steel profile;
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the thickness of the steel;
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the distance between the arches;
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cross-bracing and reinforcements;
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the quality of the joints;
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how the greenhouse is anchored to the ground or base.
In an area with heavier snowfall, choosing 6 mm polycarbonate alone is not enough. The frame must also be designed for those conditions. Options in the TIM-DAM range include EURO KLASIKA TUBE, with a stated snow load of up to 220 kg/m², and STANDART, with a stated load of up to 240 kg/m².
Whichever model you choose, check snow accumulation during winter, follow the manufacturer's instructions and make sure the greenhouse is assembled correctly and securely anchored.
Which greenhouse suits the way you plan to use it?
| What you need | Suitable choice | Suggested covering |
|---|---|---|
| A small garden and normal seasonal growing | 2 m or 2.4 m wide model | 4 mm |
| An affordable 3 m greenhouse for a home garden | VRTNI | 4 mm or 6 mm |
| A stronger 3 m model with one-piece arches | TITAN | 4 mm for seasonal use, 6 mm for a longer season |
| A spacious 4 m wide greenhouse | COMFORT PRO | 6 mm is sensible for longer use |
| A site with more snow | EURO KLASIKA TUBE or STANDART | 6 mm together with a suitable frame |
| Straight side walls and more usable height | VICTORIA LINE | 4 mm or 6 mm, depending on site and season |
The table is a guide rather than a fixed rule. Two gardens in the same town can face very different conditions. One greenhouse may be protected by a house, while another stands at the open edge of a plot and receives much stronger wind. The exact position therefore matters.
Thickness is not the same as quality
Two panels of the same thickness are not necessarily equal in quality. When comparing polycarbonate, look at the following points as well:
UV protection
The UV-protected side of the panel must face the sun. Installing it the wrong way round can shorten its service life, so check the markings on the protective film before fitting the panels.
Panel weight and structure
A very lightweight 6 mm sheet may be less rigid than a better-quality sheet of the same thickness. Do not compare offers using the 4 mm or 6 mm figure alone.
Warranty
Check what the warranty actually covers and how long it applies. Most of the main TIM-DAM greenhouse models state a ten-year warranty on the polycarbonate.
Protection of the open channels
The open ends of twin-wall polycarbonate should be protected correctly. Suitable sealing tapes and finishing profiles help reduce the amount of dust, insects, algae and moisture entering the channels.
What about light?
Plants need more than a certain amount of light; even distribution also matters. Clear twin-wall polycarbonate diffuses sunlight, creating fewer harsh shadows inside the greenhouse than ordinary glass.
For most home gardeners, the difference in light transmission between 4 mm and 6 mm is not the deciding factor. Good panel quality, a clean surface, correct orientation and installation without damage to the UV-protective layer matter more.
Even the best covering needs good ventilation
Thicker polycarbonate retains heat more effectively, but on a sunny day the greenhouse can also overheat quickly. Doors and vents are not merely convenience features; they are essential to the way a greenhouse works.
On a shorter 4 m greenhouse, doors with windows at both ends are often sufficient. For a 6 m length, we generally recommend two additional ventilation windows. Automatic openers are particularly useful when nobody is at home during the day, as they respond to temperature without requiring electricity.
Our practical recommendation
If you want a straightforward greenhouse for tomatoes and other summer vegetables, good-quality 4 mm polycarbonate on a properly anchored frame will usually be sufficient.
If you want to begin earlier in spring, extend the autumn harvest, or place the greenhouse in a cold and exposed position, choose 6 mm.
If snow is your main concern, first check the stated capacity of the frame and the spacing between the arches. A 6 mm label alone does not guarantee a stronger greenhouse as a whole.
If you are still unsure, send us the width and length you need, the location of the greenhouse and how you intend to use it. With those details, we can recommend a suitable model and polycarbonate thickness. You can also browse the full range of TIM-DAM polycarbonate greenhouses.
