6mm vs 4mm Polycarbonate Greenhouse Panels: When the Extra 2mm Actually Matters
Technical guide · 2,010 words · ~9 min read · Published 21 May 2026 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 maths 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 real choice is not "which is better", it is "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: NORDIC and SteelRoot both start with 4 mm CrystalLight twin-wall polycarbonate, with 6 mm available as an option when the extra stiffness and heat retention justify the cost. So you should know our bias, and also that we have direct experience with both choices and the engineering reasons behind each specification.
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, and 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.
| Spec | 4mm twin-wall cellular PC | 6mm twin-wall cellular PC |
|---|---|---|
| Air gap | ~3.4mm | ~5.4mm |
| Weight per m² | ~0.8 kg | ~1.3 kg |
| Light transmission (new) | ~90% | ~85–88% |
| U-value (panel only) | ~3.9 W/m²K | ~3.5 W/m²K |
| Sound reduction | Lower | Slightly higher |
| Hailstone resistance | Good (small hail) | Better (medium hail) |
| Frame channel required | 4–5mm slot | 6–7mm slot |
| Typical retail price per m² | £8–£14 | £14–£22 |
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, and that is 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 towards 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: 6 mm panels are stiffer at the same span, while closer arch spacing reduces the unsupported panel span in the first place. For SteelRoot, 4 mm twin-wall polycarbonate is the standard/base glazing; 6 mm glazing and 0.67 m close-arch spacing are available when the site or growing programme warrants the extra margin.
For our SteelRoot range, 4 mm twin-wall polycarbonate and 1.0 m arch spacing form the standard/base configuration. Buyers can add 6 mm glazing and the 0.67 m close-arch option when the plot is more exposed or when they want the extra seasonal reassurance.
Why both ranges start at 4 mm, and when 6 mm earns its keep
We get this question regularly. The honest answer is no longer a fixed range split. NORDIC and SteelRoot both use CrystalLight twin-wall polycarbonate, with 4 mm as the standard/base specification and 6 mm available where the buyer wants a thicker glazing option.
The NORDIC timber range uses a 45 × 45mm FSC Swedish pine frame with screw-fixed glazing. The 4 mm panel is the base fit for the timber system; the 6 mm option is now available when thermal retention and extra panel stiffness matter more than keeping the entry specification lighter.
SteelRoot uses a 40 × 20mm closed RHS galvanised steel arch with mechanical screw-fixing to the frame. Its base configuration is 4 mm twin-wall polycarbonate and 1.0 m arch spacing. Buyers can add 6 mm glazing and the 0.67 m close-arch option for more exposed plots or more serious winter use.
The two ranges serve different growing contexts. NORDIC is the architectural timber greenhouse for the formal-garden and small-plot UK buyer. SteelRoot is the galvanised-steel arch greenhouse for buyers who want a configurable, utility-first growing structure.
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 lower heating cost over a UK winter helps offset the panel upgrade over time 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 is 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 in typical UK wind conditions. At wider spacing (1m+), 4mm panels begin to flutter in sustained strong 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% versus ~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: it typically begins as a slight haze at year 8–10 and becomes visible at year 12–15. SteelRoot carries a 2-year manufacturing defect warranty on cellular polycarbonate panels plus a 10-year anti-corrosion warranty on the galvanised steel frame.
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 it is 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, choose the 6 mm glazing option and 0.67 m close-arch spacing when the plot warrants the extra margin. SteelRoot starts with 4 mm standard/base glazing and 1.0 m standard/base arch spacing, then lets the buyer specify the exposed-site upgrades explicitly.
For the NORDIC wooden range: 4 mm standard / 6 mm option CrystalLight on FSC Swedish pine with Galvanised Ground Anchors included. → explore NORDIC. For the SteelRoot galvanised steel-arch range: 4 mm standard/base CrystalLight, 6 mm option, 1.0 m standard/base spacing and 0.67 m close-arch option, with Ground Screw Anchors included. → explore SteelRoot.
See also: Polycarbonate vs Glass Greenhouse UK · Storm-Resistant Greenhouse UK · Polytunnel vs Polycarbonate Greenhouse UK · Best polytunnel alternatives UK
