Thermal performance in a winter greenhouse is rarely about a single product. The total insulation value of a structure depends on how the glazing material, any added liner films, and framing details interact. In regions where overnight temperatures regularly drop below −15°C, a single-layer polyethylene film alone is insufficient to maintain above-zero growing conditions without substantial supplemental heat.
This article examines the materials most commonly used in small and mid-scale Canadian operations, focusing on thermal resistance, light transmission trade-offs, moisture behaviour, and practical installation considerations. Cost figures are not included, as regional pricing varies considerably and changes year to year.
Polycarbonate Twin-Wall Panels
Twin-wall polycarbonate — also called double-wall or multiwall polycarbonate — consists of two parallel sheets connected by internal channels. The air trapped in those channels provides measurable thermal resistance. The R-value of a standard 6mm twin-wall panel falls in the range of R-1.4 to R-1.6 (US units). Triple-wall and five-wall variants increase this further, with five-wall 25mm panels reaching roughly R-3 to R-3.5.
For a Zone 4 or Zone 5 operation in Ontario or Alberta, twin-wall polycarbonate is generally the minimum acceptable glazing for a structure intended to maintain crops above freezing without continuous high-output heating. Three-wall or thicker panels reduce heat loss proportionally but also reduce light transmission slightly — typically from around 82% for standard twin-wall down to 72–76% for triple and five-wall versions.
Light Transmission Note
Light transmission values from manufacturers assume clean, new panels. UV degradation, dust accumulation, and algae growth on the inner channels of horizontal-run panels all reduce actual transmission over time. OMAFRA technical notes recommend orienting twin-wall panels with channels running vertically to prevent moisture and debris buildup in the flutes.
Polycarbonate panels are durable and can be reshaped to follow curved roof profiles, which reduces the number of framing members and associated thermal bridges. The main limitations are cost relative to film options and the gradual yellowing that occurs with lower-grade UV coatings over a period of years.
Double-Layer Polyethylene Film with Air Inflation
Commercial greenhouse operations in Canada frequently use double-layer polyethylene (PE) film systems, where two layers of 4 or 6 mil film are inflated with a small squirrel-cage blower to maintain an air gap of 25–50mm between them. The inflated air gap raises the effective R-value of the assembly to approximately R-1.7 to R-2, which is competitive with twin-wall polycarbonate at significantly lower material cost.
Light transmission through quality IR-blocking or thermal PE films can exceed 85–90%, which is a meaningful advantage during the short days of December and January. Some film formulations include IR-blocking additives that reduce longwave radiation loss at night, further improving nighttime heat retention.
The trade-off with film systems is longevity. Standard greenhouse PE films are rated for one to four seasons depending on UV inhibitor grade, while polycarbonate panels are typically warranted for ten years or more. For a small-scale backyard operation, the annual labour of replacing film and the cost of replacement rolls must be factored into a realistic comparison.
Rigid Foam Board on Opaque Sections
North walls and lower wall sections that do not need to transmit light are candidates for rigid foam insulation. Extruded polystyrene (XPS) board at 50mm thickness provides approximately R-10, while 100mm gives roughly R-20. Expanded polystyrene (EPS) is slightly less per unit of thickness but is less moisture-resistant.
In practice, many small Canadian operations insulate the lower 90–120cm of north and east walls with rigid foam, leaving glazing only on south-facing and roof surfaces. This significantly reduces total heat loss without affecting the primary light-entry surfaces.
Bubble Wrap Film as a Secondary Liner
Horticultural bubble wrap (larger bubble diameter than standard packaging bubble wrap) is sometimes used as an interior liner inside existing single-layer structures. Attached to the inside of a frame just inside the glazing, it creates an additional air layer and can meaningfully reduce heat loss from an older single-skin structure. Published R-values for horticultural bubble wrap liners are modest — roughly R-0.8 to R-1.0 — but the improvement over no liner at all is relevant in marginal climate zones or for spring/fall extension rather than deep winter production.
The primary drawback is light reduction. Even relatively clear bubble wrap films reduce transmitted light by a measurable amount, and this cost is high during the already low-DLI months of November through January.
Thermal Mass and Passive Night Insulation
Thermal mass — water-filled barrels, stone, or concrete placed along the north wall — absorbs solar heat during the day and releases it at night, reducing temperature swings. This approach is well-documented in passive solar literature and suits structures in climates with predominantly clear winter days (parts of Alberta and BC interior). In cloudy climates like southern Ontario through December–January, thermal mass contributes less because solar gain on overcast days is limited.
Row covers or interior floating films placed directly over crops at night add a small additional temperature buffer at the plant level, allowing a slightly cooler overall greenhouse temperature while maintaining crop-level conditions within acceptable limits.
Practical Comparison for Small Canadian Operations
| Material | Approx. R-Value | Light Transmission | Lifespan |
|---|---|---|---|
| 6mm Twin-wall polycarbonate | R-1.4–1.6 | ~82% | 10+ years |
| Triple-wall polycarbonate (10mm) | R-2.0–2.2 | ~76% | 10+ years |
| Double-layer PE film (inflated) | R-1.7–2.0 | ~85–90% | 1–4 years |
| Horticultural bubble wrap liner | R-0.8–1.0 | ~60–75% | 3–5 years |
| 50mm XPS rigid foam (opaque sections) | ~R-10 | 0% (opaque) | 20+ years |
For most backyard and small commercial operations in Canada, a practical baseline is twin-wall or triple-wall polycarbonate on roof and south-facing walls, rigid foam on the north wall and lower sections, and a supplemental heat source rated to maintain at least 2–4°C on the coldest nights of the intended operating season. The specifics depend heavily on the climate zone, crop targets, and how much temperature fluctuation the chosen crop tolerates.
Further Reading
OMAFRA publishes a greenhouse heating and energy management guide relevant to Ontario operations. Agriculture and Agri-Food Canada's Greenhouse Production of Vegetables is available through their publications catalogue and covers thermal management in the context of Canadian growing conditions.