Architectural and industrial polycarbonate (PC) sheet is manufactured from bisphenol A polycarbonate with a density of 1.20 g/cm³ determined per ISO 1183-1:2019, a glass transition temperature near 145 °C by ISO 306, and a solid-state thermal conductivity of approximately 0.20 W/(m·K) measured in accordance with ISO 8302:2018 or ASTM C518-21. Clear uncoated monolithic sheet transmits between 88 % and 91 % of visible light when tested to ISO 9050, whereas multiwall configurations exhibit lower total transmittance because of the refractive effects of vertical rib geometry and multiple air interfaces. The thermal distinction between solid and multiwall polycarbonate arises because a solid panel conducts heat through a continuous polymer cross-section, while a multiwall panel inserts a series of air cavities separated by thin PC webs that reduce conductive area and add surface-film resistances. Consequently, the structural choice determines the center-of-glazing U-value under EN 673 or ISO 10292, the load-deflection behavior, and the permissible panel span for a given design pressure.

Solid or Multiwall Polycarbonate? The Structural Choice That Decides Your U-Value

The U-value, or thermal transmittance, is the steady-state heat flux through a building element divided by the temperature difference across that element, expressed in W/(m²·K). For monolithic PC, the center-of-glazing U-value is determined from the inverse of the sum of the internal and external surface resistances and the material thickness divided by its thermal conductivity: U = 1 / (R_si + t/λ + R_se). With R_si = 0.13 m²·K/W and R_se = 0.04 m²·K/W for vertical glazing, a 4 mm solid sheet has a nominal U-value near 5.3 W/(m²·K), while a 10 mm twin-wall multiwall sheet is commonly reported at 3.0–3.3 W/(m²·K) on manufacturer datasheets using EN 673. The difference is not linear with thickness: multiwall panels alter the thermal path length, increase internal surface resistances, and suppress convection within narrow cavities. Selection therefore begins with the building envelope U-value target, the structural side load, and the optical transmission required.

Where concentrated static loads, point fixings, or flat glazing require high bending stiffness, monolithic solid polycarbonate is specified. Tensile yield strength is commonly 60–65 MPa and flexural modulus is 2.3–2.4 GPa when tested per ASTM D638-14 and ASTM D790. Notched Izod impact resistance often exceeds 600 J/m at 3.0 mm thickness under ASTM D256. Because the solid sheet has no internal cavities, edge fixings can be drilled and sealed without generating internal condensation paths. Thermal expansion, however, is significant: the coefficient of linear thermal expansion is approximately 6.5 × 10⁻⁵ m/(m·°C) per ISO 11359-2, requiring slotted connections and clearance holes to avoid thermal buckling. In forced-entry and blast-resistant glazing, monolithic PC thicknesses of 6 mm to 12 mm are used with anti-fog coatings and edge retention; published data for specific blast-rated assemblies depends on the frame and interlayer, and certification is evaluated under ASTM F1233 or EN 356 rather than by sheet polymer properties alone.

When Rib Web Spacing and Cavity Depth Modify the Thermal Path

Multiwall PC transforms the solid polymer cross-section into a load-bearing shell of external skins and vertical webs. Heat transfer through the panel is separated into conduction through the PC webs and skins, convection within the air cavities, and long-wave radiation between cavity surfaces. Narrow cavity heights suppress convective loops; the thermal resistance of a 10 mm twin-wall cavity is therefore greater than a simple air gap of equivalent dimension. Manufacturer-tested values under EN 673 show that a 10 mm two-wall sheet with a mass of approximately 1.5 kg/m² has a U-value of 3.0–3.3 W/(m²·K), while increasing the panel thickness to 16 mm and adding a third wall reduces the U-value to 2.4–2.6 W/(m²·K). X-structure and five-wall panels in the 20–25 mm range reach 1.7–1.9 W/(m²·K) but with a mass penalty and reduced visible transmittance, often 40–55 % depending on the number of internal layers. The rib spacing also changes panel stiffness anisotropy: flexural stiffness is higher along the rib direction than transverse to it, so span tables from extrusion suppliers govern the allowable spacing. Condensation control for multiwall sheet requires ventilated edge closure and anti-dust tape to prevent cavity contamination and maintain the designed cavity thermal resistance.

On a production-scale sheet extrusion line, polycarbonate pellets are dried in desiccant hoppers at 120 °C for 3–4 h to a residual moisture content below 0.02 %. Residual moisture above 0.02 % hydrolyzes bisphenol A polycarbonate in the melt, reducing molecular weight and lowering viscosity, which changes die flow and produces silver-streak defects and bubble-containing webs. A co-rotating twin-screw extruder with an L/D ratio between 32:1 and 36:1 and closed-loop barrel temperatures from 240 °C to 290 °C is used; the downstream die is generally maintained at 280–300 °C. Melt temperature must not exceed 350 °C because thermal degradation initiates at the isopropylidene linkage, producing yellowing and molecular weight reduction. Vacuum venting at approximately -0.08 MPa removes volatiles from regrind streams. A gear pump after the screw is used to reduce pressure pulsation to less than ±1 %, because wall-thickness variation in the final sheet is directly related to die-lip pressure stability. Multiwall sheet is formed through a specialized multi-channel die and vacuum calibration; the ribs must be pulled at controlled haul-off speed to maintain vertical orientation. If the calibration vacuum is too low, the internal webs can be wavy or collapsed; if the melt temperature is too high, the rib walls can thin unevenly and the panel U-value and bending stiffness diverge from the design values.

Flame, Weathering, and Chemical Compatibility Matrix

Regulatory compliance for polycarbonate glazing depends on the building code pathway. Under UL 94, unfilled PC grades typically achieve V-2 at 3.0 mm, while mineral-filled and phosphorus-based flame-retardant formulations may achieve V-0; European classifications are assigned under EN 13501-1 with the final rating linked to thickness, substrate, and mounting. Long-term outdoor exposure requires coextruded UV-stabilized cap layers on one or both surfaces because PC undergoes photo-Fries rearrangement and yellowing when exposed to UV below 350 nm. Accelerated weathering per ISO 4892-2:2013 uses xenon-arc exposure with a 0.35 W/(m²·nm) irradiance at 340 nm and 65 °C black-panel temperature; uncoated PC can lose tensile strength and develop yellowing after extended cycles, while protected grades maintain transmission longer. Chemical compatibility limitations are significant: aromatic hydrocarbons, ketones, esters, and chlorinated solvents cause environmental stress cracking or partial dissolution, and ammonia or amine-based compounds degrade the carbonate linkage. Cleaning is limited to milder agents such as aliphatic hydrocarbons or dilute soap solutions. Contact with plasticized PVC films or neoprene gaskets should be avoided because plasticizer migration can embrittle the polycarbonate surface.

Overhead glazing in industrial skylights, covered walkways, and greenhouse roofs imposes a coupled thermal and structural specification. The selection between solid and multiwall PC is often determined by the design snow load and the maximum allowable deflection, usually expressed as a fraction of the span such as L/30 for multiwall roof panels under uniformly distributed load. Multiwall sheet reduces U-value but requires extruded polycarbonate or aluminum framing, thermal expansion joints, and sealing against water ingress into the flutes. Solid sheet is used where the glazing must carry point loads or where transparency above 80 % is required; it has a higher U-value and therefore may not satisfy local energy codes without a secondary glazing layer or low-emissivity coating. Hail impact resistance is verified by ASTM E822 or FM 4473 depending on the assembly; the sheet type alone does not define the final rating, because frame support spacing and attachment method are part of the tested system.

Condensation risk evaluation under ISO 13788 shows that multiwall PC cavity surfaces remain closer to the exterior temperature than the inner surface of monolithic sheet at equivalent external conditions. The top edges of multiwall panels must be sealed with vapor-impervious tape and the bottom edges with breathable filter tape to prevent moisture accumulation and fungal growth inside the flutes. Sealant compatibility with polycarbonate must be confirmed, since incompatible gun-grade sealants containing acetoxy-cure silicone acetic acid or high-aromatic solvents can stress-crack sheet edges. These boundary conditions are not secondary installation details; they directly preserve the U-value and load capacity of the selected solid or multiwall polycarbonate panel.