Selecting the correct glazing material determines the long-term economics, growing conditions, and practical performance of a greenhouse. Each material—glass, polycarbonate, and polythene—offers distinct advantages and trade-offs regarding light transmission, heat retention, durability, cost, and structural resilience. For UK growers, understanding these differences is essential for matching material choice to crop type and regional climate conditions.
Glass glazing: maximum light and durability
Glass transmits 90% of available light, the highest of all common greenhouse materials, making it the preferred choice for high-light-demand crops such as seedlings, cuttings, and plants requiring intensive light levels during establishment. Glass does not degrade in sunlight, maintaining light transmission and structural clarity year after year.
Thermally, glass reflects heat radiated from within the glasshouse back into the structure, improving heat retention compared to plastic materials. A single layer of glass provides moderate insulation; double-glazed glass offers enhanced thermal performance, though at significantly higher initial cost. The reflective property means that in summer, some growers apply shading compounds or internal screens to prevent overheating.
Structurally, glass offers rigidity and does not sag or warp. It resists wind loads and snow loads more effectively than thin plastic films. However, safety regulations now require toughened glass in certain greenhouse areas to minimise injury risk from breakage, increasing material costs. Regular maintenance includes cleaning to remove algae and mineral deposits, particularly important as even light dust reduces light transmission.
Initial capital costs for glass greenhouses exceed those of plastic alternatives by 30–50%, and installation requires skilled fitting. Repairs involve replacing individual panes, which is moderately expensive but straightforward for most damage patterns. A well-maintained glass greenhouse operates efficiently for 25–30 years, amortising the initial investment over a long production period.
Polycarbonate glazing: durability with compromise
Twin-walled polycarbonate sheets transmit 83% of available light—approximately 7% less than glass. This reduction may limit use for high-light seedlings and cuttings but is acceptable for general propagation, vegetable and herb growing, and ornamental production in the UK climate.
Polycarbonate excels in structural performance. Twin-wall construction provides stiffness that single-thickness films cannot achieve, allowing panels to withstand significant wind and snow loads without support. The cellular structure creates insulating air pockets, offering better thermal retention than single-layer glass—approaching the performance of double-glazed glass whilst remaining lighter and easier to install.
Polycarbonate does yellow slightly over 10–15 years under continuous UV exposure, progressively reducing light transmission. Some discolouration is normal and does not necessarily impair crop growth unless accumulated algae also blocks light. The material remains flexible enough to resist high-impact damage: hail and falling branches rarely shatter polycarbonate as they would glass.
Installation is simpler than glass: panels slot into aluminium profiles or attach via clips, reducing labour costs. Repairs often involve replacing entire panels rather than individual panes, which is cost-effective given the panel size. Initial costs run 20–30% lower than glass, and the reduced weight lowers structural foundation requirements.
Polycarbonate typically survives 15–20 years before significant yellowing or degradation necessitates replacement. The combination of lower initial cost, durability, and ease of maintenance makes it a pragmatic choice for mixed-use growing where glass is unnecessary but polythene is inadequate.
Polythene glazing: budget option for seasonal use
Polyethylene films cost £0.50–£1.50 per square metre, making them the most economical glazing material. Light transmission varies with film thickness and UV-inhibitor package but typically exceeds 80% initially. Polythene readily admits diffuse light and is suitable for summer-grown crops like tomatoes, peppers, and cucumbers in polytunnels.
Thermally, polythene provides minimal insulation. A single layer offers almost no barrier to heat loss, and even double-layer films with an air gap between them provide only moderate winter retention. As a result, polythene greenhouses are rarely economical to heat and maintain frost-free over winter. The RHS notes that polythene tunnels are “not usually economical to heat and keep frost-free over winter,” limiting their use to seasonal production schedules.
Structurally, thin polythene lacks rigidity. Unidirectional snow loads can deform or tear thin films, and sustained wind can create flutter and vibration. Snow melt and pooled water stress fixings and accelerate UV degradation. Annual or biennial replacement is normal; many commercial operators replace polythene film every 12–18 months.
Polythene films are quick to install and repair—often a one-person job with basic tools. For growers with capital constraints or those testing new crop types in single-season trials, polythene offers low financial risk. However, the recurring replacement cost, frequent maintenance requirements, and poor winter performance limit utility for year-round production or high-value perennial crops.
Heat retention and energy costs
In cold UK winters, heating demand dominates operating costs. Glass and double-layer polycarbonate provide roughly equivalent insulation. Single-layer polycarbonate falls between glass and polythene. Pairing any glazing material with internal thermal screens—deployed overnight—recovers much of the heat loss differential.
A greenhouse with thermal screens can reduce winter heating demand by 50–60% regardless of base glazing material. This means that polycarbonate with screens often outperforms unscreened glass in terms of energy efficiency, shifting the balance toward polycarbonate for cost-conscious operators.
Structural resilience: snow and wind loading
UK winter weather includes significant snow events in northern regions and coastal wind in the south. Glass, due to its rigidity, supports snow loads well but can fracture if overloaded. Polycarbonate’s flexibility and cellular structure distribute loads, making it more forgiving of heavy snow without structural damage.
Wind loads preferentially damage thin polythene by causing vibration and material fatigue. Polycarbonate and glass, with low surface deflection, resist wind damage more effectively. Rounded roof designs—common in modern polycarbonate greenhouses—further reduce wind loading by minimising pressure pockets that accumulate water and wind force.
Cost of ownership
Total cost of ownership encompasses initial capital, installation labour, energy use, maintenance, repairs, and eventual replacement. Glass carries high initial costs but low annual operating costs (beyond heating) and long service life. Polycarbonate offers moderate initial costs and moderate ongoing expenses with mid-range durability. Polythene minimises upfront capital but demands annual or biennial replacement and provides poor winter performance.
For a grower planning 10 years of production, polycarbonate often delivers the lowest total cost. For permanent installations targeting 25+ years, glass justifies its higher capital cost through longevity and minimal maintenance burden.
Key points
- Glass transmits 90% of light and lasts 25–30 years but costs 30–50% more than polycarbonate.
- Polycarbonate transmits 83% of light, resists impact better than glass, and offers a 15–20 year lifespan at moderate cost.
- Polythene transmits 80%+ light initially but is impractical for winter heating and requires annual or biennial replacement.
- Thermal screens reduce heating demand by 50–60% with any glazing material, significantly improving winter energy economics.
- Polycarbonate withstands UK snow and wind loads better than polythene; glass provides equivalent performance at higher cost.
Related service: Greenhouse project management services