A polycarbonate cold frame is a miniature greenhouse judged on two results: do the seedlings come out stocky, and does the lid still open in March after a winter of frost.
An allotment society in Yorkshire glazed 40 frames with 3 mm twin-wall and fixed the lids with hinges bolted straight through the sheet. A third of the panels cracked at the hinge line over one winter.
Polycarbonate cold frame glazing and light
- Twin-wall: 4 mm, best night insulation
- Solid: 5 to 6 mm for exposed sites
- Light transmission: around 80 percent clear
- Slope: 1 in 20 to drain condensate
Light is the growth variable. Clear stock at 80 percent transmission keeps seedlings compact; a tinted panel drops below 70 percent and plants stretch toward the light, which is the failure a grower notices first.
Hinges, frost and ventilation
Hinges need clearance. A panel bolted hard to a hinge cracks on the first cold morning because the sheet contracts and the fixing does not move; a slotted hinge with 2 mm of travel prevents it.
Ventilation is part of the glazing design. A lid that opens to 100 mm at mid-day releases the humid air that would otherwise condense on the panel and drip cold water onto the seedlings overnight.
For the larger greenhouse and roofing case, UV resistant polycarbonate for roofing covers the UV cap layer.
Send your frame dimensions, crop use and climate to oversales@rpplexiglass.com for glazing pricing.
Frequently Asked Questions
What thickness polycarbonate for a cold frame?
4 mm twin-wall for insulation and diffuse light, 5 to 6 mm solid where wind load or snow is a factor. Twin-wall keeps night temperatures higher; solid transmits more light.
Does polycarbonate block light that seedlings need?
Clear polycarbonate transmits around 80 percent of visible light, which is enough for most seedlings. A bronze or opal tint drops it further and causes stretching, so clear is the right choice for a cold frame.
Why does condensation form inside a cold frame?
Warm moist air meeting a cold panel. Ventilation at mid-day and a slight slope on the lid move the condensate to one edge instead of dripping on the plants.