Greenhouses
Greenhouse glazing for short seasons
In a short-season climate the glazing is the part of a greenhouse that does the most work. It decides how much light reaches the plants on a grey day, how long the structure holds the day's warmth after sunset, and whether the roof survives a wet spring snowfall. The three materials most gardeners actually choose between are horticultural glass, twin-wall polycarbonate, and polyethylene film.
What the glazing has to balance
Every glazing material trades off three properties that matter in cold regions: light transmission, insulation, and durability under load. A single sheet of glass transmits a great deal of light but loses heat quickly. A twin-wall panel insulates better because of the air gap between its layers, but the same structure diffuses and slightly dims the light. Understanding the trade-off is more useful than chasing one ideal material.
The recurring tension: the materials that hold heat best tend to scatter light, and the materials that transmit the most light tend to leak heat. In a short season you are usually willing to accept softer light in exchange for warmer nights.
The three common choices
Horticultural glass
Glass offers high light transmission and does not yellow or degrade with sun exposure, so a glass house can stay clear for decades. Its weaknesses are weight, fragility under hail or falling branches, and poor insulation when used as a single layer. In snow country a glass roof needs a steep enough pitch to shed snow before the weight becomes a problem.
Twin-wall and triple-wall polycarbonate
Polycarbonate sheets have internal channels that trap air, giving them noticeably better insulation than single glazing. They are light, hard to break, and forgiving to cut and fasten. The light they pass is diffused rather than direct, which actually helps even out shading inside the structure. Over many years exposed polycarbonate can cloud, though sheets sold with a UV-protective coating resist this.
Polyethylene film
Film stretched over hoops is the least expensive way to enclose growing space, which is why polytunnels are common on small farms. Greenhouse-grade film is treated to resist UV breakdown and is typically replaced on a multi-year cycle. A double layer of film with air gently blown between the sheets adds an insulating gap and reduces flapping in wind.
A quick comparison
| Property | Glass | Twin-wall polycarbonate | Polyethylene film |
|---|---|---|---|
| Light transmission | High, direct | Good, diffused | Good when new, diffused |
| Insulation | Low (single layer) | Higher (air gap) | Higher with double layer |
| Impact resistance | Low | High | Moderate, tear-prone |
| Typical lifespan | Very long | Long | Shorter, replaceable |
| Relative cost | Higher | Moderate | Lowest |
Matching glazing to a short season
For a small permanent greenhouse meant to run from late winter onward, twin-wall polycarbonate is a common compromise: it holds heat through cold nights and shrugs off impacts. Where a gardener wants maximum light for a sunroom-style house and snow is managed by a steep roof, glass remains attractive. Film suits anyone who needs the most enclosed area per dollar and is comfortable replacing the cover periodically.
Practical detail: whatever the glazing, plan ventilation first. A sealed structure overheats on a sunny winter afternoon long before the outside air feels warm, and trapped humidity encourages disease. Roof vents, louvres, or roll-up sides matter as much as the glazing choice.
Further reading
For background on greenhouse structures and growing under cover, see the overview articles maintained on Wikipedia's greenhouse entry and the Agriculture and Agri-Food Canada public resources. Continue with the companion guides on cold frames and the season-extension calendar.