Twinwall polycarbonate is two thin sheets held apart by ribs, with air in the channels between them. That trapped air works like double glazing: it slows heat loss at night. The thicker the panel, the more air it holds.
The common thicknesses
- 4 mm: the usual choice on budget greenhouses. Fine for summer crops and spring seedlings, thin for keeping plants frost free over winter.
- 6 mm: noticeably warmer. A good middle ground if you want to start sowing early.
- 10 mm: the warmest panels on garden greenhouses. The Palram Canopia Glory uses 10 mm twinwall.
Our score gives 2 points from 8 mm, 1.5 points from 6 mm and 1 point below that. If the maker does not publish the thickness, the glazing part drops out, and many makers do not. Palram, for example, gives no thickness for the Mythos and Hybrid. Ask the shop before you buy.
Light and clarity
Twinwall panels let in a little less light than clear glass, and the light comes in diffused. You cannot see clearly through them. For growing that is rarely a problem, and diffused light reduces scorch on hot days.
What to check before you buy
- UV protection: good panels have a UV-protected outer side. Fit that side facing out, the instructions show which one.
- Sealed ends: the channels should be taped or capped at the ends, or dirt, algae and insects get in.
- Panel fixing: panels that clip into the frame stay put better in wind than ones held by a few wire clips.
- Guarantee: check whether the guarantee covers the panels. Palram's covers the frame only.
Twinwall or glass plus insulation?
A glass greenhouse lined with bubble insulation in winter holds heat well too, and you take the bubble wrap down again in spring for full light. Twinwall gives you the insulation all year round without the extra job. The full trade-off is in polycarbonate or glass.
See all polycarbonate greenhouses.
