Selecting a covering material for a vegetable greenhouse is a strategic decision that directly determines how much light reaches the crop, how much heat is retained or lost, how long the cover will last, and ultimately how profitable the winter or off-season harvest will be. The main options used today are compared below on the five criteria that growers most often cite.

1. Light transmission  

   - Single-layer glass gives the highest PAR (90–93 %) and is preferred for high-wire tomato or cucumber crops that need maximum solar energy in winter.  

   - Single polyethylene (PE) film transmits 88–92 % when new, but drops 5–7 % per year as dust and UV haze build up.  

   - Multi-wall polycarbonate (PC) 8 mm sheets transmit 82 % initially; the diffusion they create can actually improve canopy penetration by 10–15 % compared with clear glass.  

   - Acrylic and fibreglass-reinforced polyester (FRP) panels are in the same 80–85 % range, but FRP yellows faster unless it has a UV-resistant gel coat.

2. Thermal insulation  

   - A single glass pane has a U-value of 6.2 W m⁻² K⁻¹—good for sunny days but poor at night.  

   - Double-layer inflated PE (two 150–200 µm films with a 30–50 mm air cavity) cuts heat loss by 35–40 %, giving U ≈ 3.5 W m⁻² K⁻¹, which is usually enough to keep night temperatures 2–3 °C higher than outside in a Chinese solar-type vegetable greenhouse without extra heating.  

   - Twin-wall PC 16 mm drops U to 2.2, close to that of double glass, while weighing only 1/16 as much.  

   - Corrugated PVC and FRP are single-skin, so they perform like glass unless a second layer or thermal screen is added.

3. Service life and maintenance  

   - Glass lasts ≥ 25 y but breaks under hail and requires aluminium bar glazing that increases shade.  

   - PE film with 200 µm thickness, UV stabiliser and anti-drip additive is guaranteed 3–4 y; commercial growers often re-skin every 5 y to maintain light. Replacement is cheap (≈ 0.5 USD m⁻²) and can be done by two workers in a day.  

   - PC carries 10–15 y warranties against yellowing and loss of impact strength; hail damage is rare and cleaning is easy.  

   - FRP sheets last 8–12 y but surface erosion gradually reduces light by 10–15 %.  

   - Acrylic is the longest-lived plastic (15–20 y) but is brittle in sub-zero installs.

4. Structural load and design freedom  

   - Because PE weighs < 0.2 kg m⁻², it can be used on low-cost bamboo or galvanized-steel hoops with 0.8–1.0 m purlin spacing; this is why 80 % of new vegetable greenhouses worldwide are still single- or double-layer film houses.  

   - Glass (12 kg m⁻²) and PC (1.7 kg m⁻² for 8 mm) need stronger trusses, increasing steel cost by 15–25 %. PC, however, is available in 2.1 m × 6.0 m sheets that can bridge 2.0 m purlin centres, cutting shadowing by 5–7 %.  

   - In heavy-snow regions, the smooth surface of PE allows snow to slide off quickly, whereas PC twin-wall can trap cold air and slow melting; designers must increase roof pitch to ≥ 28° or provide supplementary heat.

5. Functional additives and special films  

   - IR-blocking PE reduces nighttime radiative loss by 20–25 %, raising leaf temperature 1–2 °C.  

   - Diffusion films scatter 30–50 % of incoming light, lowering the risk of leaf burn in summer crops such as sweet pepper and producing 3–5 % higher marketable yield.  

   - Panda film (white/black) or red mulch can be used on side-walls or as an inner thermal curtain to modify the spectrum—red light enhances tomato fruit colour, while the white outer face reflects heat in hot climates.  

   - Anti-drip surfactants prevent condensation droplets that cause leaf mould; they are standard on PE and PC grades sold for vegetable greenhouse use.

 Practical selection guide  

- Low-cost seasonal production (spring cucumber, autumn lettuce): 150–200 µm single PE, replaced every 3–4 y.  

- Year-round harvest in temperate zones (winter tomato, coloured sweet pepper): double-layer inflated PE + movable thermal screen; if heating is used, upgrade to 8 mm PC on the north roof for extra insulation.  

- High-insulation, snow-load region (northern China, Canada): twin-wall PC or 16 mm triple-wall PC on gothic-arch truss, gutter-connected, with natural ridge vents.  

- Premium retail display garden centre: glass or 16 mm acrylic for maximum clarity and public appeal, combined with PC side walls for safety.

No single covering is “best”; the optimal choice matches crop biology, local climate, energy price and the grower’s pay-back horizon. A good rule for a vegetable greenhouse is to spend just enough on the cover to keep night temperature 2 °C above the crop’s lower threshold while still achieving > 80 % of outside light at noon in mid-winter.