Greenhouse Glazing Thickness: What 4mm, 6mm, 8mm and 10mm Actually Buy

Every greenhouse kit listing puts a millimetre figure in the specification block, and almost none of them explain what that figure buys. Four millimetre, six, eight, ten. The implication is always the same, that thicker means warmer, and that a thicker panel is what turns a spring and summer structure into a year-round one.

That implication is mostly wrong, and it is wrong in an expensive direction. The insulation difference across the entire hobby greenhouse range is smaller than the marketing suggests, and it is not where the money goes. What panel thickness actually buys is structural capacity, panel retention in wind, and a frame built to carry the extra weight.

This page covers what the millimetre number measures, what the insulation figures really are once you line them up, why light transmission does not behave the way people expect, and the installation details that decide whether a panel lasts fifteen years or two.

What the Millimetre Figure Measures

The number is the overall depth of the sheet, not the thickness of the plastic. A 6 mm twin-wall panel is two thin skins of polycarbonate separated by ribs, with the whole assembly measuring 6 mm front to back. The skins themselves are a fraction of a millimetre. Most of what you are buying is the air trapped in the channels between them, which is where the insulating performance comes from and also where the rigidity comes from.

Wall count is a separate specification from thickness and it matters as much. Twin-wall has two skins and one layer of channels. Triple-wall, four-wall and five-wall subdivide the same depth into more layers, which traps more still air and stiffens the sheet. A 10 mm four-wall panel and a 10 mm twin-wall panel are not the same product, and listings frequently give the thickness without the wall count. When a kit advertises 10 mm and does not say four-wall, assume twin-wall.

Single-wall polycarbonate is a different material again. It is a solid sheet with no channels, it insulates barely better than glass, and it appears on the clear side panels of some hybrid kits where light transmission is the priority and heat retention is not.

The Insulation Numbers, Lined Up

Almost no greenhouse kit brand publishes an R-value on its own product page. The figures below come from polycarbonate sheet manufacturer specification data rather than from kit makers, which is the closest thing to a like-for-like comparison available.

Panel	R-value	Light transmission
Single-pane glass	0.9	90%
4 mm twin-wall	1.47	85%
6 mm twin-wall	1.64	80%
8 mm twin-wall	1.72	81%
10 mm twin-wall	1.92	82%
16 mm five-wall	2.78	65%

Figures vary between sheet manufacturers by more than a rounding error, and Palram’s architectural sheet data reads higher than the commodity sheet suppliers at 8 mm and 10 mm, most likely because of different internal rib geometry. Treat the table as a set of ranges rather than exact values.

The important thing in that table is the size of the gap. Going from 4 mm to 10 mm, which is essentially the full width of the hobby kit market, moves you from about R-1.5 to about R-1.9. For context, a typical insulated house wall runs somewhere from R-13 to R-20. The entire greenhouse glazing range sits in a narrow band down at the bottom, and no thickness available in a kit changes the fundamental fact that a greenhouse is a thin-skinned building that loses heat fast.

What that means practically is that panel thickness is not the variable that decides whether you can grow through a Canadian January. Supplemental heat decides that. Thickness decides how much heat you need, and the difference between 4 mm and 10 mm is real but incremental, on the order of a quarter more thermal resistance rather than a doubling.

Light Transmission Does Not Fall the Way People Think

The received wisdom is that thicker panels are darker, and the table above shows why that is only half true. Transmission drops from 85% at 4 mm to 80% at 6 mm, then goes back up slightly at 8 mm and 10 mm before falling sharply at 16 mm five-wall.

The reason is that the number of internal walls the light has to cross matters more than the total depth. Each skin and each rib scatters and absorbs a little light, so a five-wall sheet loses considerably more than a twin-wall sheet of similar depth. Within the twin-wall range, the differences are small enough to be swamped by how clean the panels are.

There is also a distinction between how much light gets through and how useful it is. Glass transmits more total light and transmits it directionally, which produces bright patches and shadows inside the structure. Twin-wall polycarbonate diffuses light as it passes through the channels, spreading it more evenly across the growing space. For most hobby growing, the diffusion is worth more than the few percent of transmission it costs, and it is the reason many growers choose polycarbonate roof panels even where glass is affordable.

What Thickness Actually Buys

The honest answer to why thickness matters is structural, not thermal.

A thicker panel is stiffer, and a stiffer panel deflects less under load. In a twin-wall sheet the internal ribs act like small beams, so the panel is contributing structure rather than just filling a hole in the frame. That is why snow load ratings track glazing thickness closely across the market, and why the step from the 4 mm tier to the 8 mm tier is roughly where published ratings jump from around 15 pounds per square foot to the mid twenties.

Panel retention in wind follows the same logic. The most common way a budget greenhouse fails is not the frame buckling but a thin roof panel flexing enough under wind uplift to escape the channel that holds it. Once one roof panel leaves, the frame loses bracing, the opening becomes a scoop, and the rest goes quickly. A thicker panel is harder to flex out of its channel, which is a durability difference that shows up as a survival difference in the first bad storm.

The third thing thickness buys, indirectly, is a better frame. Thicker panels are heavier, so kits built around 8 mm and 10 mm glazing carry more aluminium and use tighter framing spacing to support them. When you buy up the thickness ladder you are usually buying up the frame ladder at the same time, and it is difficult to separate how much of the durability difference comes from which. That is not a reason to discount it. It is a reason to read thickness as a proxy for overall build quality rather than as an isolated specification.

Framing Spacing and the Span Chart

Panel thickness only performs to specification if the framing behind it is spaced to suit. Sheet manufacturers publish span charts giving the maximum unsupported distance for each thickness, and a 4 mm panel spanning a gap sized for 8 mm will deflect, pond water and eventually fail regardless of what the sheet is rated for.

This matters mostly to anyone building from parts or re-glazing an existing structure with thicker panels. Fitting 10 mm sheets to a frame designed for 4 mm does not deliver a 10 mm structure. It delivers a heavier roof on framing that was never sized for it, and the weight of the upgrade eats into the capacity it was supposed to add.

UV Coating and Which Way the Panel Faces

Polycarbonate on its own degrades in sunlight. It yellows, hazes and turns brittle. What prevents that is a co-extruded UV layer applied during manufacture, and on most greenhouse panels it is applied to one side only.

That single-sided coating creates the most consequential installation mistake in the category. Fit the panel with the coated side facing inward and the raw side takes the sun. Panels installed the wrong way round begin to yellow and haze within one or two seasons and become brittle enough to crack under hail or a dropped tool. Correctly installed and reasonably maintained, the same panel should give ten to twenty years, and many sheet manufacturers back that with a ten-year warranty against yellowing and light transmission loss.

The panels arrive with protective film on both faces and the UV side is printed or labelled on the film. Once the film is off there is no reliable way to tell the sides apart by eye, so the rule is to leave the film in place until the panel is positioned and fixed, and to mark the coated face before peeling anything. Double-sided UV coating exists and is worth asking about, though it is uncommon in the hobby kit tier.

Flute Orientation, Sealing and Expansion

The channels inside a twin-wall panel collect condensation, and they need somewhere for it to go. Flutes must run vertically on walls and along the slope on the roof, so water drains to the bottom edge rather than sitting in a horizontal channel. A panel fitted with the channels running sideways traps moisture and dust inside the sheet, and the result is a permanently clouded panel that cannot be cleaned from the outside or the inside.

The edges want different treatment at top and bottom. Solid aluminium tape seals the top edge against water and insects. Vented or breather tape goes on the bottom edge so trapped condensation can drain while dust stays out. U-profiles cap the taped edges and H-profiles join panels. Skipping the tape leaves the channels open to algae, which is the other route to a permanently green panel.

Polycarbonate also moves a great deal with temperature. Fixing holes should be drilled oversize so the panel can expand and contract, and screws want rubber washers and a gentle hand. A panel screwed down rigidly will buckle in summer and crack around the fixings, which is a slow failure that looks like a manufacturing defect and is not.

Choosing a Thickness

For a small structure in a mild climate used from spring to autumn, 4 mm twin-wall is adequate and the extra light is a genuine benefit. This is what most kits under about six hundred dollars ship with, and within its limits it is not a bad specification.

For anything unheated that has to stand through a winter with real snow, 6 mm is a practical floor and 8 mm is the point where the structural argument becomes convincing. This is also where kits start publishing snow load ratings worth reading, because the manufacturers building at this level have something to publish.

For year-round growing with supplemental heat in a cold zone, 10 mm twin-wall or a four-wall panel of similar depth is the sensible target, and the heating cost saved over a decade closes most of the price gap. Sixteen millimetre multiwall belongs on heated commercial structures where the transmission loss is acceptable because the energy saving is large and measurable.

Above all, read the thickness alongside the snow load rating and the frame specification rather than on its own. A thick panel on a light frame is a mismatch, and so is a strong frame with thin glazing. The kits that survive are the ones where the two are specified together.