The depth of the foam core sets how warm a panel roof is. Using one foam throughout, 25 mm gives a U-value near 1.14, 35 mm near 0.86, 50 mm near 0.62, 75 mm near 0.43 and 100 mm near 0.32. Eco Tech fits 75 mm on every roof: it removes about 80% of the heat loss of 16 mm polycarbonate without the weight and height of deeper panels.
The foam does the work
Inside every insulated roof panel sits a foam core with a thin aluminium skin bonded to each face. The skins make it rigid and weatherproof. They do almost nothing for warmth. All the insulating happens in the foam, and how much you get comes down, nearly entirely, to how deep that foam is.
So the core depth is the number to ask about first. From the garden two panels can look the same, while one lets out over twice the heat of the other simply because its foam is half as thick.
Checking a U-value yourself
One short sum will check any panel. Convert the core depth to metres and divide by the thermal conductivity of the foam. In the extruded polystyrene (XPS) core of our UK made panels that figure is 0.034 W/mK. Then add 0.14, the standard allowance for the thin still air layers on the two faces of a roof, and divide 1 by the result.
Written out: U = 1 / (0.14 + d / 0.034), where d is the depth of foam in metres. Run it for the 75 mm core we use: 0.075 / 0.034 is about 2.21, adding 0.14 gives 2.35, and 1 divided by 2.35 is roughly 0.43 W/m²K.
Other foams conduct heat a little better or worse, so their results shift slightly. The pattern, with each extra slice of foam doing less than the one before, holds for all of them.
Five core depths, side by side
Below, the sum is run for each depth and applied to a 12 m² roof. Watts are at 20°C indoors and 0°C outside. The yearly columns take about 2,150 heating degree days (England's average, base 15.5°C) and 26.32p per kWh, the electricity cap for October to December 2026.
| Core depth | U-value (W/m²K) | Saving against 16 mm polycarbonate | Yearly electricity cost | kWh lost a year | Watts lost by 12 m² |
|---|---|---|---|---|---|
| Polycarbonate sheet, 16 mm (baseline) | 2.3 | None | £375 | 1,424 | 552 W |
| Panel, 25 mm | 1.14 | 50% | £186 | 706 | 274 W |
| Panel, 35 mm | 0.86 | 63% | £140 | 533 | 206 W |
| Panel, 50 mm | 0.62 | 73% | £101 | 384 | 149 W |
| Panel, 75 mm (what we fit) | 0.43 | 81% | £70 | 266 | 103 W |
| Panel, 100 mm | 0.32 | 86% | £52 | 198 | 77 W |
Those running costs suppose the room is heated right through the winter. Use less heat and all the figures fall together, keeping the same gaps in proportion.
Reading the table
A thin panel is not far ahead of thick polycarbonate
At 1.14, a 25 mm panel beats 16 mm sheet comfortably, yet it is only a modest gain on 35 mm polycarbonate at around 1.4. If you are going to the trouble of stripping off old polycarbonate, fitting 25 mm throws away much of what you are paying for.
Returns shrink with every step
On a 12 m² roof, doubling 25 mm to 50 mm saves roughly 322 kWh a year. The next 25 mm, up to 75 mm, saves roughly 118 kWh, about £31 a year on electricity at the cap. Adding another 25 mm to reach 100 mm saves roughly 68 kWh, about £18. The early millimetres earn the most.
Ask how any figure was reached
Approved Document L (2021 edition) gives 0.15 as the target for a new roof element, and our 75 mm panel at about 0.43 does not claim to meet it. Whoever quotes you, ask them to explain where their U-value comes from and what the core is.
The reasons Eco Tech settled on 75 mm
We picked 75 mm because the benefit there is still big while the drawbacks remain small. It removes about 80% of the roof heat loss of 16 mm polycarbonate and about 73% of 25 mm polycarbonate. Stepping up to 75 mm from 50 mm is worth paying for; going on to 100 mm gives much less back.
Deeper panels bring practical problems too. They weigh more, they need bigger trims where they meet the gutter and wall, and they sit higher above the bars, which can foul the brickwork over a lean-to or an upstairs window sill. At 75 mm the panel is still light enough for the majority of existing uPVC and aluminium frames, and it finishes tidily on most conservatories.
On a cold night the underside of our 75 mm panel stays near room temperature, which largely ends condensation, and the foam muffles rain as well. Thin panels do both less well because their inner face runs colder.
Lining up quotes? Write each one's core depth and U-value in two columns next to each other. Where a quote shows only one of the two, ask for the missing figure before you choose.
Depth is not everything
A deep core decides the insulation but not the quality of the roof. Each panel has to be cut to measure so it meets the bars without gaps, the seals must be right, and the junctions with the wall and the gutters must keep weather out. A thick panel fitted carelessly will still let in draughts and water, which is why our fitters, with more than 15 years on conservatory roofs, spend most of the day on those details.
Prices for a 75 mm roof
Our prices cover removing and recycling the old roof, made to measure 75 mm panels in your chosen colour and the fitting itself, plus a 10 year insurance backed warranty through HomePro. Roofs to 12 m² start at £3,000, 12 to 25 m² at £4,000 and large or P-shaped roofs at £5,000. Most homes spend £3,000 to £7,500, or roughly £250 to £450 per m².
See how that stacks up against other roofs on our roof panel prices page. What goes into each panel is set out on our insulated conservatory roof panels page, and the shades on offer are on conservatory roof colours. For the wider picture on heat loss, see our guide to conservatory roof U-values.
Getting it in writing
Put your measurements into the roof price calculator for a range. Then ask for a free quote: send photos first if you like, and after a short survey you get a fixed price with the core depth and U-value printed on it.
Questions people ask
How thick are typical conservatory roof panels?
A lot of panels sold have cores between 25 and 50 mm. At Eco Tech every roof gets a 75 mm core, for a U-value of roughly 0.43.
Should I pay more for 100 mm panels?
On a 12 m² roof, choosing 100 mm over 75 mm cuts roughly 68 kWh a year, close to £18 on electricity at the cap. The deeper panel is heavier and stands taller at the edges, so 75 mm suits most conservatories better.
Will a thicker panel make my conservatory darker?
No. Every solid panel blocks light over its own area, however deep it is. Glazed roof lights fitted into the panel roof let daylight back in, and most customers have one or two.
