Polycarbonate for Greenhouses: U-Value, Light Transmission and Ten-Year Decay is evaluated as three linked engineering parameters rather than a single material property. The thermal performance of a greenhouse roof or sidewall is expressed as a U-value in watts per square metre per kelvin. The optical performance is defined by total luminous transmittance and photosynthetically active radiation transmittance between 400 nm and 700 nm. Ten-year decay is assessed through retained notched impact energy, change in yellowness index, and loss of light transmission after natural or xenon-arc exposure. Multiwall polycarbonate sheet for horticultural glazing is manufactured by multi-slit die extrusion followed by vacuum calibration. The weather-facing side receives a coextruded UV-stabilised cap layer of approximately 35–50 µm thickness. The base resin is an extrusion-grade polycarbonate with a melt volume-flow rate in the range of 3–6 cm³/10 min measured at 300 °C and 1.2 kg load per ISO 1133-1:2022.

On a production-scale line, polycarbonate resin is dried to residual moisture below 0.02% by weight before entering the extruder. A desiccant dryer delivering inlet air at 120 °C and -40 °C dew point is typical for multiwall sheet. Melt temperature is maintained between 280 °C and 310 °C; sustained operation above 320 °C accelerates molecular weight reduction and produces gel specks. Regrind addition above 30% can reduce luminous transmittance by 1–2% and increase haze in the finished sheet, a batch-to-batch variance observed on twin-screw extruders with L/D ratio ≥ 32:1. These process boundaries are relevant because the optical and thermal values specified for greenhouse service are determined at the die and calibration table, not after installation.

Does Multiwall Cell Geometry Depress U-Value Independently of the Polymer Thermal Conductivity?

The U-value of multiwall polycarbonate is an assembly property, not a resin property. Solid polycarbonate exhibits a thermal conductivity near 0.20 W/(m·K) at 23 °C under ISO 8302, yet a 4 mm solid sheet has a U-value close to 5.7 W/(m²·K) because the indoor and outdoor surface film resistances control the total heat flux. In twin-wall and triple-wall sheet, structural ribs create still-air cavities with low thermal conductivity. The polymer webs occupy only a small fraction of the cross-section, and narrow cavities below 10 mm in width suppress free convection. Additional horizontal ribs and X-shaped internal webs divide the air space into smaller cells and reduce radiative exchange between the exterior and interior faces. Table 1 lists representative U-values and luminous transmittance data for clear, unpigmented, UV-protected sheet configurations tested under ISO 12567-1 hot-box conditions. Values shift with colour, opacity, low-emissivity coatings, slope, and frame design.

Greenhouse U-value is not a static dry-sheet number. When internal relative humidity exceeds 90% during night heating, condensation forms on the inner surface, altering the surface film coefficient and reducing the effective cavity temperature difference. Under clear-night longwave radiation, the exterior surface can fall below ambient air temperature by several kelvin, increasing the heat flux above the hot-box value. Aluminium frame profiles and steel purlins have thermal conductivities in the range of 160–230 W/(m·K) and 40–50 W/(m·K) respectively. When glazing bars are placed at 1.0–1.5 m centres, the linear thermal transmittance at the frame-to-sheet junction increases total roof heat loss. Total greenhouse U-value therefore combines centre-of-sheet U-value with linear and point thermal bridges, calculated under EN ISO 10211 or national energy codes. Published data for the combined condensation-radiation-framing configuration in operational greenhouses is limited; conservative design practice selects the next lower U-value class when night humidity remains above 85% for more than four consecutive hours.

Light transmission through clear multiwall polycarbonate is measured under ASTM D1003 or ISO 13468-1 using CIE D65 and a 2° observer. The total hemispherical transmittance of a 6 mm twin-wall sheet is typically 78–82%, while the haze value often exceeds 85%. Haze and total transmittance are independent; internal ribs act as diffuse light guides, producing a high diffuse fraction without eliminating total transmittance. Clear solid polycarbonate has a refractive index of approximately 1.586, so normal-incidence reflectance at each air-polymer interface is about 5.1%. Each additional wall set and rib plane increases reflection loss and edge scattering. For horticultural use, PAR transmittance between 400 nm and 700 nm should be measured with a spectroradiometer or quantum sensor, not inferred from luminous transmittance alone. At incidence angles above 55°, Fresnel reflection rises rapidly and total transmittance declines. Latitude, roof angle, and greenhouse orientation therefore change the effective daily light integral independent of material quality.

Optical Losses from Rib Interfaces, Reflection, and the Coextruded UV-Blocking Cap Layer

The coextruded cap layer on UV-stabilised multiwall polycarbonate contains UV-absorbing additives in a 30–50 µm skin. The cap blocks most radiation below 385 nm, with transmittance at 350 nm typically below 1%, while preserving most PAR transmission. The spectral transition region between 385 nm and 410 nm is the critical boundary for crops in which UV-A exposure modulates secondary metabolite production or pathogen responses. Opal, pearl, and coloured grades shift total transmittance and diffuse fraction in ways that cannot be predicted from clear-sheet datasheets. Selection requires spectral transmission data collected under ISO 9845-1 or a calibrated spectroradiometric method. For clear sheet, internal reflection at rib interfaces reduces direct beam transmission and increases path length, contributing to high haze values that are sometimes specified for diffused-light greenhouse covers to reduce leaf shadowing. Published comparative crop-response data for specific polycarbonate cell geometries is limited. Condensation on the interior face further modifies reflection: a water film changes the refractive-index step and can increase back-reflection of low-angle morning light. Published data for the angular-condensation configuration is limited.

Polycarbonate also transmits near-infrared radiation up to approximately 1500 nm. This contributes to solar heat gain but is commonly separated from PAR. In cooling-limited greenhouses, near-infrared transmission is undesirable; in cold-season greenhouses it reduces heating demand. The solar direct transmittance per ISO 9050 or EN 410 is the correct metric for total energy transmittance. Luminous transmittance alone understates heat load. Clear multiwall polycarbonate can have a solar factor in the range of 0.75–0.85, depending on sheet thickness, colour, and cell structure. Published data for specific greenhouse configurations using newer low-emissivity or near-infrared-blocking additives is limited.

When Ten-Year Weathering Shifts the Failure Mode from Impact Ductility to Surface Microcracking

Polycarbonate is susceptible to photo-Fries rearrangement under ultraviolet radiation. The carbonate linkage rearranges to produce salicylate-type and dihydroxybenzophenone-type chromophores that absorb in the blue-visible region and create yellowing. Simultaneous chain scission and hydrolysis in condensing humidity reduce surface molecular weight. The coextruded UV cap layer delays this mechanism by absorbing UV before it reaches the bulk polymer, but decade-scale service exposes the cap layer to erosion, microcracking, and loss of stabiliser by diffusion. Once the cap layer is breached, surface microcracks form at notch sites such as rib intersections, cut edges, and clamping points. The failure mode shifts from ductile shear yielding to brittle crack propagation. Notched Izod impact of virgin extruded polycarbonate sheet falls in the range 600–850 J/m under ASTM D256. Weathered sheet can retain much of its tensile strength while losing a disproportionate fraction of impact energy because surface defects dominate fracture initiation. Tensile yield strength is measured under ISO 527-3, and impact retention is measured under ISO 180 or ASTM D256.

Accelerated weathering for greenhouse-grade polycarbonate commonly follows ISO 4892-2 method A: daylight filter, irradiance 0.51 W/(m²·nm) at 340 nm, black-standard temperature 65 °C, chamber temperature 38 °C, and a cycle of 102 min dry light followed by 18 min light with water spray. Outdoor exposure under ISO 877 or ASTM G7 supplements accelerated data. After 4000 h xenon-arc exposure, UV-stabilised clear multiwall polycarbonate grades may show yellowness index changes in the range 2–5 as measured under ASTM E313. Some technical datasheets report total transmittance retention above 95% of the initial value after the same interval. Direct equivalence to a ten-year greenhouse service interval is not standardised because accumulated UV dose depends on geography, orientation, glazing slope, and condensation cycles. Published data for this specific configuration is limited.

Polycarbonate multiwall sheet also interacts with mechanical load during decade-scale service. Flexural modulus is approximately 2300–2400 MPa under ISO 178, and tensile yield stress is in the range of 60–65 MPa under ISO 527-3. Short-term allowable spans for greenhouse roofs are based on deflection limits of L/20 or L/30. Long-term creep under continuous snow load reduces the apparent modulus; manufacturer load tables include creep factors for continuous loads. Field failures on insufficiently supported roofs are usually not brittle fracture of the polymer but buckling of internal ribs, loss of air-cavity integrity, and local stress whitening at fasteners. Once internal walls buckle, the U-value increases because the cavity dimension and radiative barrier are disrupted.

Installation determines whether the ten-year decay of a polycarbonate greenhouse sheet remains within specification. The UV-protected side must face the exterior; reverse installation exposes the unstabilised bulk to direct UV and accelerates yellowing and microcracking. Neutral-cure silicone and EPDM gaskets are used at clamped edges because amine-cured silicones, methyl methacrylate sealants, and plasticised PVC tapes can induce environmental stress cracking in loaded polycarbonate. Cell ends should be sealed with breathable tape and polycarbonate or aluminium profiles to exclude dust while allowing condensation drainage. Blocked cells in high-humidity service retain water and support biofilm growth that reduces PAR transmittance by an amount that varies with sanitation and is not captured by accelerated weathering standards. Polycarbonate sheets must also accommodate thermal expansion. Fixed glazing without expansion clearance can buckle and alter cell geometry, changing the effective U-value and inducing stress concentration at fasteners.