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Key takeaways
- Triplewall insulates better than twinwall because it has an extra layer and more trapped air. It also transmits somewhat less light.
- One retailer's published specs show clear 10mm twinwall at about 82% light transmission and R-1.9, versus clear 16mm triplewall at about 74% and R-2.5.
- Choose twinwall for mild climates, unheated season extension and light-hungry crops. Choose triplewall for cold climates and heated winter growing.
- Snow and wind performance depend mostly on the frame, panel thickness and installation. Check the greenhouse maker's ratings.
- Panel thickness must match the frame's glazing channels. You usually can't swap to a thicker panel without a different frame.
Quick verdict
Choose twinwall if you grow in a mild climate, run an unheated or lightly heated greenhouse for season extension, or want maximum light for summer crops. It's lighter, cheaper and brighter.
Choose triplewall if you have long, cold winters and plan to heat the greenhouse. The extra insulation cuts heat loss, which matters more than the modest light loss when you're paying for heat.
Side-by-side comparison
| Factor | Twinwall | Triplewall |
|---|---|---|
| Structure | Two skins with one layer of air channels | Three skins with two layers of air channels |
| Insulation | Good | Better |
| Light transmission (clear) | Higher | Somewhat lower |
| Light diffusion | Good | Good to very good |
| Common thicknesses | Thinner sheets typical of hobby kits | Thicker sheets typical of premium and cold-climate kits |
| Weight | Lighter | Heavier |
| Price tier | $–$$ | $$–$$$ |
| Best climate | Mild to moderate | Cold, with heated winter growing |
What the numbers say
Manufacturers and retailers publish insulation (R-value) and light transmission figures for their sheets. The figures below come from the Polycarbonate Store's published product specifications for clear sheets. Other brands will differ, so treat them as a sense of scale, not universal values.
| Clear sheet (Polycarbonate Store specs) | Light transmission | R-value |
|---|---|---|
| 6mm twinwall | 80% | R-1.6 |
| 8mm twinwall | 81% | R-1.7 |
| 10mm twinwall | 82% | R-1.9 |
| 10mm triplewall | 74% | R-2.1 |
| 16mm triplewall | 74% | R-2.5 |
Two things stand out. First, at the same 10mm thickness, the triplewall sheet insulates a little better and lets through noticeably less light. Second, thickness matters too: the 16mm triplewall sheet insulates better than any of the twinwall sheets in the list.
For context, the same retailer lists opal and bronze tints with much lower light transmission. Tinted sheets are for shading and patio covers, not most growing greenhouses.
Insulation and heating
The extra wall in triplewall adds another pocket of still air, which slows heat loss. If you heat your greenhouse through a cold winter, that difference shows up in your heating bills and in how steady night temperatures stay.
The formula shows why R-value matters most when the temperature difference is large. On a greenhouse kept frost-free through a mild winter, the gap between twinwall and triplewall is small. On a greenhouse kept warm through a hard northern winter, it's much larger.
If you already have twinwall and want less heat loss, you don't need to reglaze. Bubble insulation, thermal screen curtains and draft-proofing can close some of the gap. See how to insulate a greenhouse for winter.
Light transmission and diffusion
Twinwall's advantage is light. More light matters most in winter, when days are short, and in summer for fruiting crops like tomatoes and peppers. In a mild-climate greenhouse where you're growing through winter without much heat, light can be the bigger limit than cold. See the 10-hour light rule.
Both types diffuse light, which spreads it more evenly and reduces scorching compared with clear glass. Our glass vs polycarbonate greenhouse comparison covers diffusion in more depth, and greenhouse lighting science explains how light levels affect growth.
Snow load and wind
Snow and wind performance depend mostly on the greenhouse frame: rafter spacing, profile strength, bracing and anchoring. Thicker panels are stiffer and resist flexing better, so a cold-climate kit with thicker triplewall will often be rated for heavier loads. But the rating belongs to the whole greenhouse, not the panel.
- Check the manufacturer's snow and wind ratings for the complete greenhouse. See greenhouse snow load ratings.
- Install panels with channels vertical (running from ridge to eave) so condensation drains, and follow the maker's instructions for sealing the ends.
- Anchor properly. A strong panel won't help if the frame lifts. See how to anchor a greenhouse against wind.
Cost and compatibility
Triplewall sheets cost more and usually come in thicker gauges, so greenhouses glazed with them also cost more. The more important point is fit: the panel thickness must match the glazing channels in the frame. You usually can't upgrade a twinwall hobby kit to thick triplewall without a different frame. If you're replacing panels, match the existing thickness. Our replacement panels roundup covers options.
Durability and UV protection
Both types are only as durable as their UV coating. Quality sheets have a UV-protected outer face that must face outward. Install one upside down and it will yellow and turn brittle much faster. The label or protective film usually marks the UV side.
Choose twinwall if…
- You're in a mild or moderate climate.
- You run an unheated or lightly heated greenhouse for season extension.
- You grow light-hungry crops and want the brightest possible house.
- You're on a tighter budget or buying a hobby kit.
- You're replacing panels in an existing twinwall frame.
Choose triplewall if…
- You have long, cold winters.
- You'll heat the greenhouse through winter and want lower heating costs.
- You're buying a cold-climate kit rated for heavy snow. See best greenhouses for cold climates.
- You value steady night temperatures over maximum light.
The bottom line
The difference in light is modest and the difference in insulation is real but not dramatic. Climate and heating plans should decide it. If you heat through a cold winter, triplewall usually earns its keep. If you mostly extend the season or grow in a mild climate, twinwall's extra light and lower cost make more sense.
Frequently asked questions
Is triplewall polycarbonate worth it?
It's worth it if you heat a greenhouse through cold winters, because the extra insulation reduces heat loss. In mild climates or unheated greenhouses, twinwall's extra light and lower cost usually make more sense.
Does triplewall polycarbonate let in less light?
Yes, somewhat less. One retailer's published specs show clear 10mm triplewall at about 74% light transmission versus about 82% for clear 10mm twinwall. Figures vary by brand.
What is the R-value of twinwall polycarbonate?
It depends on thickness and brand. The Polycarbonate Store lists its clear twinwall at about R-1.6 for 6mm up to about R-1.9 for 10mm. Check the manufacturer's specs for your sheet.
Can I replace twinwall panels with triplewall?
Usually only if the triplewall panel is the same thickness as your frame's glazing channels. Most hobby kit frames are made for a specific panel thickness.
Which polycarbonate is better for snow?
Snow performance depends mostly on the greenhouse frame. Thicker panels flex less, but check the whole greenhouse's snow load rating from the manufacturer.
Which way up does polycarbonate go?
The UV-protected side faces out. It's usually marked on the protective film or label. Install channels running vertically so condensation can drain.