6mm vs 4mm Polycarbonate Greenhouse Panels: When the Extra 2mm Actually Matters

Technical guide · 2,010 words · ~9 min read · Published [DATE-TBC] by Waldenhaus

TL;DR for the impatient: 4mm twin-wall cellular polycarbonate is the UK retail standard on timber-framed greenhouses and most aluminium frames — it fits the glazing channel, it performs well in sheltered to moderate sites, and the math works for the typical British back garden. 6mm twin-wall sits in a different category: it costs more, weighs more, requires a steel frame to mount reliably, and earns its keep in exposed sites, snowy regions, and serious year-round growing programmes. The choice isn't "which is better" — it's "which fits your frame, your site, and your growing."

You'll find both thicknesses sold across the UK polycarbonate greenhouse market. The default on most entry-tier polycarbonate aluminium greenhouses is 4mm. The default on most heavy-duty market-garden steel-arch greenhouses is 6mm. This guide is the honest comparison — what each actually delivers, where the trade-offs land, and which fits your specific use case.

We make greenhouses with both thicknesses — 4mm CrystalLight™ on the NORDIC timber range, 6mm CrystalLight™ on the SteelRoot steel-arch range. So you should know our bias, and also that we have direct experience with both choices and the engineering reasons we landed on different specifications for different ranges.

What twin-wall cellular polycarbonate actually is

Both 4mm and 6mm panels share the same structure: two thin polycarbonate sheets joined by perpendicular ribs running the length of the panel, creating sealed air pockets between the layers. This is the "twin-wall" or "cellular" construction — the air gap is what gives cellular PC its thermal insulation advantage over single-skin glass.

The numerical difference is overall panel thickness. The chemistry, the UV-stabilised top layer, the light transmission characteristics, and the impact resistance behaviour are broadly similar within the 4mm and 6mm bands. What changes is the air-gap volume, the panel weight, the panel stiffness, and the frame channel required to hold it.

These are representative figures from the UK retail market in 2026. Individual products vary slightly.

What the extra 2mm actually buys you

Three measurable improvements, in order of practical impact:

1. Better thermal retention

The U-value drop from ~3.9 to ~3.5 W/m²K is roughly a 10% improvement in heat retention per panel. In practical terms, a 6mm-glazed greenhouse holds 1–2°C above ambient for slightly longer after sunset and is roughly 8–12% cheaper to heat through a UK winter if you're running any supplementary heating.

This matters most for: - Year-round growing programmes where overnight temperature drives crop survival - Heated greenhouses where energy cost is a recurring operational concern - Northern UK sites with sub-zero winter overnight temperatures

It matters less for: - May-to-September seasonal growers - Sheltered southern UK gardens - Greenhouses used primarily for season extension rather than year-round production

2. Better impact resistance

Twin-wall cellular polycarbonate of any thickness is significantly more impact-resistant than glass — that's the category-wide advantage. Within the cellular PC category, 6mm provides better resistance to medium-sized hailstones, falling branches, and the routine garden knocks that thin polycarbonate doesn't handle well.

The Northern European and Scandinavian markets — which experience more frequent hail damage to glazing — have moved increasingly toward 6mm as the default for serious greenhouses, while the UK retail market has largely stayed at 4mm because UK hail events are typically smaller-stone, lower-frequency. This is a market expectation more than an engineering optimum.

3. Better wind-load performance with longer panel spans

This is the engineering reason 6mm exists at all. Polycarbonate panels deflect under wind load, and the deflection is roughly inversely proportional to thickness cubed. A 6mm panel is roughly 3.4× stiffer than a 4mm panel of the same span.

What this means practically: 4mm panels at 0.67m arch spacing perform well across the UK design wind envelope. 4mm panels at 1.0m arch spacing approach deflection limits in moderate wind. 6mm panels at 1.0m spacing recover the structural margin that the wider spacing loses.

For our SteelRoot range, we chose both 0.67m UK-spec arch spacing and 6mm cellular PC — a deliberate belt-and-braces approach for British wind exposure. It's not necessary for sheltered Continental sites; we chose it because the UK weather we actually have warrants the additional structural margin.

Why NORDIC stays at 4mm and SteelRoot at 6mm

We get this question regularly. The honest answer is a physical constraint, not a commercial choice.

The NORDIC timber range uses a 45 × 45mm FSC Swedish pine frame with glazing channels routed into the timber. The channel geometry that holds 4mm panels cleanly — with proper EPDM washer compression and a clean weather seal — is incompatible with 6mm panels. The thicker panel doesn't seat properly in the channel; over time the seal compromises and you lose the weatherproof integrity.

You could redesign the frame to accept 6mm, but it would require a larger timber section (likely 60 × 60mm or 70 × 70mm), which changes the structural language, the cost, and the visual proportion of the greenhouse. We chose to keep NORDIC at 45 × 45mm pine + 4mm CrystalLight™ — proven, dimensionally proportionate, suited to the architectural integration NORDIC is designed for.

SteelRoot uses 40 × 20mm closed RHS galvanised steel arch with mechanical screw-fixing to the frame (not channel retention). The thickness flexibility is structural, not channelled — we can specify 6mm without changing the frame section. We chose 6mm for SteelRoot because the steel-arch market includes serious-growing customers in more exposed sites, and the engineering case for 6mm is genuine.

[FOUNDER QUOTE FQ-1: In one sentence — what's the most common 'wait, you only do 6mm on the steel range?' question, and your honest answer?]

The two ranges genuinely serve different growing contexts. NORDIC is the architectural timber greenhouse for the formal-garden and small-plot UK buyer. SteelRoot is the engineered steel-arch greenhouse for the serious-growing UK buyer in exposed or year-round contexts.

When 4mm is genuinely the right answer

Most UK domestic greenhouse buyers should be perfectly happy with 4mm twin-wall cellular polycarbonate. Specifically:

• Sheltered or partially-sheltered sites — walled gardens, suburban back plots with mature hedging, valley positions away from prevailing wind corridors
• Standard-rectangle greenhouses with close rafter spacing — most timber-framed greenhouses run rafters at 600–700mm centres, well within 4mm panel deflection limits
• Season-extension growing programmes — March-to-October growing where overnight temperature retention is less critical than peak-summer light transmission
• Aesthetic-priority installations — the slightly higher light transmission of 4mm matters for greenhouses that double as visible garden buildings, where bright interior light through the glazing is part of the look
• Budget-conscious buyers — 4mm panels cost roughly half what 6mm panels cost; for buyers optimising total project cost, 4mm is the sensible choice if site and frame allow

When 6mm earns its keep

Specifically:

• Exposed sites — coastal positions within 1 km of Atlantic shoreline, elevated sites above 300m, rural plots in known wind corridors
• Steel-arch greenhouses with wider arch spacing — if the structure uses 1m arch spacing, 6mm is essentially mandatory to maintain reasonable wind-load margin
• Snowy regions — Pennines, Scottish Highlands, Outer Hebrides, North-East coast in sustained snow conditions; 6mm carries snow load better
• Year-round growing programmes — overwintering hardy crops, January seed starts, October-to-December harvesting where overnight thermal retention drives crop survival
• Heated greenhouses — the ~8–12% heating cost saving over a UK winter pays back the panel upgrade over 5–7 years for active heated growers
• Hailstone-prone regions — eastern UK coastal zones that occasionally see medium-stone hail events
• Commercial-style growing — market-garden microbusinesses where reliability through any UK weather event matters operationally

Frequently asked questions

Is 6mm polycarbonate twice as good as 4mm? No — it's roughly 10–15% better on thermal retention, roughly 3.4× stiffer in wind-load deflection, and slightly better on impact resistance and sound. "Twice as good" oversells the difference. The choice depends on whether your site and growing programme need the extra structural margin or whether 4mm is genuinely sufficient.

Will 4mm polycarbonate panels bow in UK wind? At close rafter or arch spacing (600–700mm) and proper screw-fixing to the frame, no — 4mm panels perform well across the UK design wind envelope. At wider spacing (1m+), 4mm panels begin to flutter at sustained 25 m/s (~56 mph) winds, which is where reports of "the panels are drumming in storms" come from. The fix is closer spacing or thicker panels.

Why don't more UK brands sell 6mm? Three reasons: cost (roughly 1.5–2× the panel cost), weight (limits to what timber frames can carry without redesign), and historical market expectation (the UK got used to 4mm as the default and consumer perception hasn't shifted). The Scandinavian and Northern European markets sell more 6mm because their weather warrants it.

Can I upgrade my existing 4mm greenhouse to 6mm panels? Usually not without significant work. The glazing channels in most UK greenhouses are sized for 4mm; retrofitting 6mm panels requires either a frame replacement or aftermarket adapter strips that compromise weatherproofing. If you're committed to upgrading thermal performance, secondary glazing (a second 4mm panel layer inside the existing one) is sometimes a more practical retrofit than swapping to 6mm.

Does 6mm polycarbonate transmit less light? Slightly — typically 85–88% vs ~90% for 4mm. The 2–5% difference is rarely material for typical UK crops; plants don't respond proportionally to small light-transmission changes. For seedling propagation specifically (where light intensity matters more), some growers prefer the brighter 4mm.

What's the difference between cellular and solid polycarbonate? Cellular (twin-wall or multi-wall) has internal air pockets that provide thermal insulation; solid polycarbonate is a single-thickness sheet without internal cavities. Solid PC is used for impact-rated applications (riot shields, security glazing) but is significantly more expensive and lacks the thermal advantage. For greenhouses, cellular is the relevant category.

How long does cellular polycarbonate last? With a UV-stabilised co-extruded top layer (standard on quality cellular PC including CrystalLight™), 10–20+ years is typical. Yellowing is the failure mode — typically begins as a slight haze at year 8–10, becomes visible at year 12–15. SteelRoot panels carry a two-year manufacturing defect warranty; the frame carries a ten-year anti-corrosion warranty.

Are there other thicknesses besides 4mm and 6mm? Yes — 8mm, 10mm, 16mm, and 25mm multi-wall cellular PC exist, with progressively better thermal performance. These are typically used for commercial horticulture, conservatory roofs, and architectural glazing rather than domestic greenhouses. For greenhouses, 4mm and 6mm cover essentially the entire market.

What to do with this information

If you're buying a freestanding timber-framed greenhouse for a typical UK domestic garden, 4mm cellular PC is the right answer — and what we ship with the NORDIC range. The frame is engineered around it, the channels are sized for it, and it performs well for the typical UK growing context.

If you're buying a steel-arch greenhouse for a more exposed site, larger growing area, or year-round growing programme, 6mm cellular PC at 0.67m arch spacing is the engineering match — and what we ship with every SteelRoot. The thicker panel and the closer spacing together create the structural margin the British weather warrants.

For the NORDIC wooden range — 4mm CrystalLight™ on FSC Swedish pine with Galvanised Ground Anchors included: → explore NORDIC. For the SteelRoot engineered steel-arch range — 6mm CrystalLight™ at 0.67m UK-spec spacing, Ground Screw Anchors INCLUDED: → explore SteelRoot.

See also: Polycarbonate vs Glass Greenhouse UK · Storm-Resistant Greenhouse UK · Polytunnel vs Polycarbonate Greenhouse UK