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Walk into a conversation about greenhouse polycarbonate and the thickness question comes up almost immediately. Is 4mm enough? Is 6mm worth the extra cost? What does the difference actually mean in practice — in a real UK garden, across a real growing year?
This guide answers those questions directly and honestly. It covers what the two thicknesses deliver in measurable terms — insulation, light, UV performance, structural rigidity — and it gives you a clear recommendation based on how you plan to use your greenhouse.
One important framing point before we start: both 4mm and 6mm options available on KLASIKA and BALTIC LT greenhouses use Brett Martin twin-wall polycarbonate with co-extruded UV protection — a named European product carrying a 10-year manufacturer’s UV warranty. Understanding what that means, and why it matters, is as important as understanding the thickness difference itself.
Both panel thicknesses share the same fundamental structure: two parallel polycarbonate faces connected by internal ribs running the length of the panel, creating a series of enclosed air channels. It is this multi-wall construction — not the polycarbonate itself — that provides the thermal insulation the panels are known for.
The faces are the transparent outer walls. The ribs are the structural connectors. The air channels between them are the insulating layer.
In a 4mm panel, the two faces and their air channel together measure 4mm total. In a 6mm panel, the wider air channel between the two faces accounts for the additional 2mm. The structural faces themselves are approximately the same thickness in both cases — it is the width of the insulating air gap that changes.
This tells you something important: moving from 4mm to 6mm is primarily about insulation performance and panel rigidity, not about the transparency or quality of the faces themselves.
Insulation is measured in two complementary ways. The R-value measures resistance to heat flow — higher is better, more resistance means heat stays where you want it. The U-value measures the rate of heat transfer — lower is better, a lower U-value means less heat escapes through the panel per unit of time.
For greenhouse glazing in the UK climate, these numbers have direct practical consequences. A higher-insulating panel means warmer nights in spring and autumn — the shoulder seasons where the difference between a comfortable growing temperature and a damaging frost can be measured in one or two degrees. It means lower heating costs if you run a heater through winter. It means a longer effective growing season without any additional energy input.
| Panel | R-value | U-value (W/m²K) | Insulation vs single glass |
|---|---|---|---|
| Single-pane glass (3mm) | ~0.95 | ~5.8 | Baseline |
| 4mm twin-wall Brett Martin | ~1.43 | ~3.9 | ~50% better than glass |
| 6mm twin-wall Brett Martin | ~1.54 | ~3.5 | ~65% better than glass |
The headline figure: 6mm polycarbonate insulates approximately 65% better than single-pane glass, and around 8% better than 4mm polycarbonate. That 8% difference between the two panel options may sound modest, but in terms of overnight temperature retention — the metric that matters most for growing — it translates to a meaningful real-world gap.
Consider a representative April night — not unusual in the UK — where outdoor temperature drops to 4°C. In an unheated glass greenhouse, internal temperature might reach 6–7°C: marginal for many seedlings and genuinely risky for tender transplants. In a 4mm polycarbonate greenhouse, internal temperature holds at approximately 8–9°C — safe for most crops. In a 6mm polycarbonate greenhouse, that figure rises to approximately 9–11°C — comfortably safe, with meaningful headroom.
These are not theoretical figures — they reflect the real consequences of the insulation difference across typical UK spring and autumn conditions. For gardeners who want to start seeds in February, transplant in March, or keep tender crops growing into November, those extra degrees matter.
If you heat your greenhouse through winter — even minimally, with a frost-stat thermostat set to 3–4°C — the insulation difference between 4mm and 6mm has a direct impact on your energy bill. A higher-insulating panel envelope means the heater runs less frequently and for shorter durations to maintain the target temperature. Over a heating season, the energy saving from 6mm over 4mm is small but consistent — the kind of quiet, compounding efficiency that adds up across years of use.
Both panel thicknesses are visually transparent and allow the full visible spectrum of light to pass through to your plants.
| Panel | Light transmission |
|---|---|
| Single-pane glass (3mm) | ~90–92% |
| 4mm twin-wall Brett Martin | ~80–85% |
| 6mm twin-wall Brett Martin | ~80–82% |
Two things stand out here.
First, the difference in light transmission between 4mm and 6mm panels is minimal — approximately 1–3 percentage points. In practical growing terms, this difference is not measurable in crop performance. Both thicknesses deliver the same growing environment from a light perspective.
Second, both polycarbonate options transmit somewhat less light than glass. This is the trade-off that is most frequently cited by glass advocates, and it is worth addressing directly.
The 8–12% reduction in light transmission compared to glass is real — but it applies to new glass in ideal conditions. In practice, horticultural glass accumulates algae, mineral deposits, and surface crazing over time, and older glass greenhouses typically transmit considerably less than their theoretical maximum. The light diffusion effect of polycarbonate — scattering light evenly throughout the growing space rather than focusing it in direct beams — also means that more of the plant canopy receives usable light simultaneously, partially offsetting the transmission difference.
For all commonly grown greenhouse crops — tomatoes, cucumbers, peppers, aubergines, chillies, herbs, salad leaves, brassicas, and flowers — the light levels inside a polycarbonate greenhouse are more than sufficient for excellent growth and yield. This is not a qualified claim: growers around the world produce outstanding crops in polycarbonate structures year after year. The light transmission figure is a specification, not a practical limitation.
Thickness also affects the physical properties of the panel itself — its resistance to flex under load and its ability to absorb impact without cracking or puncturing.
A 6mm panel is measurably stiffer than a 4mm panel of the same width. Under snow load, under wind pressure, or under accidental impact — a football, a falling branch, a knock from a gardening implement — the additional depth provides more resistance. Both panels are many times more impact-resistant than glass; the question is the degree of that advantage.
For standard UK conditions, both 4mm and 6mm panels perform well. In exposed positions — hilltop gardens, coastal sites, locations known for heavy snowfall — the additional rigidity of 6mm provides a more comfortable margin of structural confidence.
The arch spacing of your greenhouse frame also interacts with panel thickness. Closely-spaced arches — 50cm or 67cm centres, as found on the KLASIKA ARCHED, KLASIKA HOUSE, and KLASIKA TUBE — provide more support points and reduce the unsupported span of each panel. In these models, 4mm panels are well-supported and perform excellently. In models with wider arch spacing — 99cm or 100cm — the additional stiffness of 6mm panels provides a more solid result.
This is the section that most greenhouse buyers miss — and it is arguably the most important thing to understand about polycarbonate quality.
Polycarbonate is vulnerable to ultraviolet radiation. Without protection, UV exposure causes progressive degradation of the polymer at the panel surface: a yellowing discolouration that develops over months and years, a gradual loss of optical clarity, and eventually a reduction in structural integrity as the material becomes brittle. This is not a design flaw — it is a known material property, and the quality difference between greenhouse polycarbonate products is almost entirely determined by how well the manufacturer has addressed it.
There are two approaches to UV protection in polycarbonate panels:
Surface-applied coating is the method used by lower-cost panel manufacturers. A UV-absorbing compound is applied to the outer face of the panel after extrusion — either as a spray coating or a laminated film. The result is UV protection that functions adequately when new but is inherently vulnerable to degradation. Surface coatings are exposed to the same weathering, cleaning, and physical abrasion as the panel surface itself. Over time — typically within three to five years of outdoor use — the coating thins, becomes uneven, and eventually fails in patches. Once the coating has failed, UV degradation of the underlying polycarbonate accelerates rapidly. The panel yellows noticeably, light transmission drops, and the greenhouse begins to look and perform like a structure that is overdue for replacement.
Co-extruded UV protection is the method used by Brett Martin and by a small number of other quality European manufacturers. Instead of applying UV protection to the surface of the panel after it is made, co-extrusion incorporates the UV-stabilising compound into the outer polycarbonate layer during the extrusion manufacturing process itself. The UV-absorbing material is not a coating on top of the polycarbonate — it is part of the polycarbonate, chemically bonded at the molecular level throughout the outer layer of the sheet.
The consequence of this manufacturing difference is profound. A co-extruded UV layer cannot be removed by cleaning. It cannot be worn away by rain or weathering. It cannot be damaged by the normal abrasion that panel surfaces experience outdoors. It degrades very slowly — at approximately the rate the UV-absorbing compound is consumed by UV exposure — from the outermost surface inward through the layer’s depth.
Brett Martin’s co-extruded UV layer is typically 50–80 microns thick. At the rate of degradation determined by UV exposure in Northern European climatic conditions, this depth of UV-stabilised material provides effective protection for a minimum of ten years — which is precisely what Brett Martin guarantees.
The Brett Martin 10-year UV warranty is a manufacturer’s guarantee that their polycarbonate panels will not experience unacceptable yellowing or loss of light transmission attributable to UV degradation within ten years of installation in normal outdoor conditions.
This is a commitment by the panel manufacturer — not the greenhouse manufacturer, not the retailer — that the panels are built to perform to specification for a decade of continuous outdoor use. It is backed by Brett Martin’s manufacturing quality systems, their material testing protocols, and their reputation as one of Europe’s most established polycarbonate producers.
In practical terms, it means that a buyer purchasing a KLASIKA or BALTIC LT greenhouse with Brett Martin panels can expect:
Years 1–3: Panels fully clear and performing at full light transmission specification. No yellowing. No haziness. Full insulation performance.
Years 3–7: Gradual, very slight reduction in light transmission as the UV layer does its intended work. Panels remain visually clear and functionally excellent. No structural degradation.
Years 7–10: Panels at or near the end of their warranted specification, with light transmission potentially reduced by a few percentage points from new. Still functional, still providing insulation, still structurally sound. The point at which replacement planning is sensible rather than urgent.
Beyond year 10: Panels can be replaced individually and inexpensively, restoring the greenhouse to full performance. The galvanised steel frame, built to a completely separate and longer service specification, remains intact and functional for many years beyond this point.
Compare this trajectory to what happens with unbranded, surface-coated polycarbonate: visible yellowing beginning within 18–24 months, significant light loss by year three, structural brittleness emerging by year four or five. The greenhouse does not fail dramatically — it simply stops performing as intended, well within the period a buyer had every right to expect it to last.
Both 4mm and 6mm Brett Martin polycarbonate are high-quality, co-extruded panels that deliver the same fundamental quality assurance and 10-year UV warranty. The decision between them is a practical one based on your growing priorities and the UK climate.
4mm Brett Martin polycarbonate is not a budget compromise. It is a capable, warranted, European-manufactured panel that provides meaningful insulation over glass and will perform reliably for a decade. Many thousands of gardeners grow excellent crops in 4mm polycarbonate greenhouses throughout the UK. It is the right choice for buyers who prioritise the entry price and whose growing ambitions sit comfortably within its thermal performance.
6mm Brett Martin polycarbonate is the recommended panel for most UK greenhouse buyers who are serious about getting the maximum growing return from their structure. The thermal performance improvement over 4mm — approximately 8% better insulation, translating to 1–2°C warmer overnight temperatures in the shoulder seasons — directly extends the effective growing season without requiring any additional energy expenditure. Over ten years of use, that improvement compounds significantly.
If there is one point to take from this guide, it is this: the UV protection method matters more than the thickness choice.
A 6mm panel with a surface-applied UV coating is a worse investment than a 4mm panel with co-extruded UV protection. The surface-coated 6mm panel will begin to yellow and degrade within three to five years regardless of its thickness; the co-extruded 4mm panel will still be performing to specification in year nine.
Both the 4mm and 6mm options on KLASIKA and BALTIC LT greenhouses are Brett Martin co-extruded panels. When you compare either of these to the panels used in low-cost imported greenhouse kits — typically unbranded sheets from manufacturers who do not publish their UV protection method — you are not comparing thicknesses. You are comparing fundamentally different products with fundamentally different performance trajectories.
This is why the panel specification matters when evaluating greenhouse cost. A greenhouse with Brett Martin polycarbonate and a galvanised steel frame is not expensive because of premium branding. It is priced to reflect materials that are engineered to work properly for ten years — not to look good in year one and degrade quietly through years two, three, and four.
| 4mm Brett Martin | 6mm Brett Martin | |
|---|---|---|
| UV protection | Co-extruded | Co-extruded |
| UV warranty | 10 years | 10 years |
| R-value | ~1.43 | ~1.54 |
| U-value | ~3.9 W/m²K | ~3.5 W/m²K |
| Light transmission | ~80–85% | ~80–82% |
| Best for | Entry price, seasonal growing, sheltered positions | Year-round growing, colder climates, best thermal performance |
Both options are co-extruded European polycarbonate — not the thin, unbranded sheet that gives polycarbonate a bad reputation. The choice between them is a performance upgrade decision, not a quality tier decision.
Greenhouse styles and structures explained: a guide to the KLASIKA and BALTIC LT range
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