The bay I glazed with 4 mm twin-wall for years was beading on the inside by 6 a.m. after a 3 °C night, and the light coming through it read flat and grey. The bay beside it sat on the same frame design and the same gravel pad, carried a Palram THERMAGLAS panel, and stood nearly dry. That one morning sold me on a feature I had filed under marketing filler for two seasons.
On a Palram THERMAGLAS panel, the condensate-control surface buys back more winter light than stepping up a wall thickness does, because droplets beading on an untreated inner face reflect the sunlight the greenhouse exists to collect. What follows is what you are actually buying: the multiwall structure and the insulation it delivers, the weather and UV protection on the outer skin, and how condensate control changes light transmission. The test behind it all: two bays, one frame design, one cold night.
Two bays, one cold night, different panels
I set up two 6 ft by 8 ft hobby greenhouse bays side by side in late October, both on the same frame design and the same gravel pad, and glazed one with the 4 mm twin-wall sheet I had used for years and the other with a Palram THERMAGLAS panel. My hypothesis going in was straightforward: the thicker, better-built panel would be the visible difference, and wall count would do the talking. I checked both bays at 6 a.m. after a 3 °C night with an indoor/outdoor thermometer and a torch, sweeping the beam across the inner face of each sheet.
The thermometer told a smaller story than the light did. Barely half a degree separated the two bays at that hour, yet on the sheet I had used for years the beam caught bead after bead of water big enough to smear with a finger, while the THERMAGLAS bay scattered almost nothing and the plants behind it stood out clearly. If I had judged only by temperature, I would have called the two panels a tie and gone back to bed. This side-by-side is my own setup, run to check the condensate-control behaviour Palram describes for the standard THERMAGLAS panel, not a lab measurement.
What the multiwall core does that one sheet can't
Trapped air in the channels is the insulator, not the polycarbonate itself. That is why a 6 mm triple-wall panel beats a 4 mm twin-wall on heat loss while staying light enough for an aluminium hobby frame to carry, and why a single sheet of the same plastic insulates almost nothing by comparison.
Palram builds all its THERMAGLAS products on that multiwall polycarbonate structure, which is what gives the panels their rigidity, their durability and the thermal insulation that makes them worth the money over a single skin. These sheets are also light and easy to install, and they carry both weather and UV protection in the outer skin. The UV layer is co-extruded into the weather-facing face, so a panel fitted upside down chalks and yellows years before its time. Both details, the air cell and the UV face, are visible on the cut edge before you ever slide a sheet into the bar. Hold a cut end up to a window and you can count the walls and see which face carries the coating.
How fast heat leaks out at night
I logged minimum and maximum thermometer readings in both bays for ten nights and compared them. To keep it fair I hung one min/max thermometer at plant height in the middle of each bay and a third outdoors, and read them together each morning before the sun touched the panels. My twin-wall bay ran roughly 4 to 5 °C warmer at dawn than a single-skin control I kept in the same yard. On the coldest mornings the outside of the THERMAGLAS panel was frosted while the inside stayed dry, and frost on the outside is the insulation working, not a fault.
The ballpark figures I plan around are about U 3.0 to 3.3 W/m²K for 4 mm twin-wall and roughly U 2.0 W/m²K for 6 mm triple-wall; treat those as ballpark, not a certified rating. A greenhouse is not a house. You are buying a few degrees and a slower temperature drop, and at that scale one extra wall is worth something but not everything. That is the part most buyers get half right, assuming more walls always wins.
Does it hold up to hail, wind and August sun
I screwed an offcut of THERMAGLAS and a piece of my old 4 mm sheet to a fence rail for a whole summer and let rain, wind and sun do the work, then compared the two faces. I mounted both samples the same way, through the same batten at the same height, so sun and rain hit them at the same angle. The old sheet had picked up a chalky haze and its edge felt brittle; the THERMAGLAS offcut still wiped clean and its edge stayed clear.
Unlike the unbranded sheet, this panel stayed light enough that my frame did not sag when I leaned on it during installation. Hail is where I want polycarbonate over glass every time: a 4 mm twin-wall shrugs off a knock that shatters glass, and it takes a dent rather than a hole. Palram's own line on these sheets is that they give both weather and UV protection, and a full season on a fence rail is a cheap way to see whose skin actually holds up.
Condensate control, not thickness, is what you're buying
Drop a droplet of water onto an untreated inner face and it beads, sits there, and scatters light. Palram's standard THERMAGLAS panel carries a proprietary condensate-control surface that cuts that droplet formation, so more of the light getting in reaches the plants instead of bouncing off the beads that would otherwise reflect it away.
So here is the call: the treated inner surface is the upgrade worth paying for, not the extra wall. Wall count mostly moves heat loss; the droplet-shedding face is what keeps the light getting in. My two-bay test pointed the same way on the first cold morning, and if I had one upgrade to spend on, it would be the treated panel.
When a plain twin-wall panel is the smarter buy
Here is where the advantage stops mattering enough to pay for. Garden in a mild, low-humidity climate, or run the greenhouse only from May to September, and you rarely get the cold inner face that beads, so the condensate-control layer has little to do and a cheaper plain twin-wall panel is enough. Heat and vent the house and droplets never form either. Line the whole roof with bubble wrap or shade cloth and you have covered the very face the feature lives on. My own rule of thumb: if winter nights at your site stay above about 5 °C and you do not heat, skip the upgrade.
Before you order anything, measure the glazing channel on your own frame with calipers or a steel rule and confirm which panel thickness actually slides in. A 4 mm twin-wall fits most hobby frames; 6 mm and 10 mm often do not, and the extra wall buys you nothing if the sheet will not seat in the bar. That one measurement decides the whole purchase, and it comes before anything else on the spec sheet.
