Double Poly, Polycarbonate, or Glass? The Honest Glazing Verdict for a Canadian Winter
Every greenhouse supplier will tell you their glazing is the right choice. None of them will tell you it’s the wrong one for your budget, your climate, or how many years you actually plan to run the structure. Here’s the honest version.
The short answer
If you’re heating through a real Canadian winter, single poly film is the wrong glazing at any price — it loses heat fast enough to make your fuel bill the whole story. Twin-wall polycarbonate is the honest default for almost everyone reading this: it roughly halves your heat loss against single poly, survives hail and heavy snow load, and costs a fraction of glass. Glass wins only if you want the clearest light transmission for a very long time and you’re prepared to pay for that clarity twice — once at installation, and once every time hail finds it.
The rest of this page is the numbers behind that answer, so you can weigh them against your own site instead of taking our word for it.
The numbers, side by side
Our heating cost article uses the formula heat loss (watts) = U × A × ΔT — glazing efficiency (U), times surface area, times the temperature gap you’re heating against. U is the number that actually changes your bill, and it’s the one glazing brochures bury behind marketing names like “energy saver” or “climate glass.” Here it is in plain terms, converted to the same U-value units our heating calculator uses (W/m²K — lower is better):
| Glazing | U-value (W/m²K) | Light transmission | Typical lifespan | Snow & hail |
|---|---|---|---|---|
| Single poly film | ~6.3 | 85–90% | 3–5 years (UV-treated) | Flexes with load; punctures, doesn’t shatter |
| Single-pane glass | ~5.9 | 90–95% | 20–30+ years, if it survives | Shatters under hail or impact |
| 6mm twin-wall polycarbonate | ~3.5 | ~82% | 10–15 years | High resistance to hail and snow load |
| 16mm double-pane glass | ~2.7 | ~90% | 20–30+ years, if it survives | Better than single glass; still glass |
| 16mm triple-wall polycarbonate | ~1.9 | 60–65% | 10–15 years | High resistance to hail and snow load |
| High-performance Low-E double glass | ~1.7 | ~70% | 20–30+ years, if it survives | Better than single glass; still glass |
These are typical published figures, not a guarantee for any specific product — a supplier’s real spec sheet for the exact panel you’re buying overrides this table every time. R-values and light transmission both vary by manufacturer, and we’ve rounded to the nearest figure that’s consistent across the sources we checked.
What that actually costs you, in a Canadian winter
Run the two glazings everyone actually compares — single poly against 6mm twin-wall polycarbonate — through the heat-loss formula on a 30 m² greenhouse holding 12°C against a −25°C prairie night (ΔT of 37°C):
- Single poly (U 6.3): roughly 7,000 watts of heat loss to replace, continuously, all night.
- 6mm twin-wall polycarbonate (U 3.5): roughly 3,900 watts — about 45% less.
That gap doesn’t shrink in a mild December — it’s a fixed percentage of your heat loss, every night you run a heater, for as long as you own the structure. Poly film’s low sticker price is the first bill. The glazing’s U-value is every bill after that.
Where glass actually wins
Glass isn’t obsolete — it’s just solving a different problem than most home greenhouses have. If you’re growing something that needs every available photon in December (citrus is the case we’ve written about directly), the 10–15 percentage points of light transmission glass holds over multiwall polycarbonate is a real difference, not a rounding error. And unbroken glass simply outlasts polycarbonate — 20 to 30 years against 10 to 15 — if it survives your climate’s hail and snow load, which is the whole bet.
Glass makes sense when: you’re not building near a hail corridor, you can afford the higher upfront material and structural cost (glass is heavier and needs a sturdier frame), and light matters more to what you’re growing than insulation does. For most cold-climate season-extension use — getting greens through November, starting seedlings in March, holding tomatoes past first frost — that combination of conditions is rare.
Where triple-wall earns its extra cost
Stepping up from 6mm twin-wall to a thicker triple- or five-wall polycarbonate roughly doubles your R-value again — but it also drops light transmission into the 60–65% range, noticeably dimmer than either glass or 6mm twin-wall. That trade only makes sense if you’re heating to a genuinely warm setpoint through a long, dark winter, where the fuel savings are large enough to be worth growing under duller light. For frost protection or light-frost tolerance — most of what a home season-extension greenhouse is actually doing — 6mm twin-wall is very hard to beat on cost per degree.
What it costs to buy, honestly
Material pricing moves with the exchange rate and whoever’s warehouse is closest to you, so treat this as a snapshot, not a quote. One Ontario supplier’s current pricing for full 4×8 ft polycarbonate sheets: roughly $5.80–$6.40 CAD per square foot for 6mm twin-wall, and closer to $11 CAD per square foot for 9mm. Single poly film runs a small fraction of that per square foot but needs replacing every 3–5 years, which is the trade you’re actually making — low capital cost against a recurring one. Glass, installed, is typically several times the per-square-foot cost of twin-wall polycarbonate once you account for the heavier framing it requires; get a real quote for your structure rather than trusting a per-square-foot rule of thumb, because framing cost swings more than the glass itself does.
So: which one, for you?
- Frost protection or light season extension, on a budget: 6mm twin-wall polycarbonate. It’s the honest default for a reason.
- Heating to a warm setpoint through a long winter: step up to 10mm or a triple-wall panel and accept the dimmer light — run the fuel-cost math first using the heating cost calculator.
- Growing something genuinely light-hungry in winter (citrus, some flowering crops): glass, if your site isn’t a hail risk and your budget covers the frame.
- Testing the idea before committing money: single poly film, on the understanding you’re buying three to five seasons, not twenty.
Whichever glazing you land on, the U-value only tells you how much heat you’ll lose — not what that costs in dollars for your specific structure, location and setpoint. The Canadian Greenhouse Planner runs that full calculation on your own postal code: your design temperature, your degree-days, and an honest payback figure before you buy anything.
Sources: figures in this comparison are cross-checked against published specifications from Growing Spaces, the University of Arkansas greenhouse extension program, BC Greenhouses, and current Ontario polycarbonate retail pricing. Where sources disagreed, we used the figure consistent with the U-values already published in our own heating-cost article.