Polycarbonate: performance characteristics and limitations
Polycarbonate conservatory roofing is a multiwall thermoplastic sheet material typically available in 16mm, 25mm, or 35mm wall thickness, used as a lower-cost alternative to glass in conservatory roof glazing systems. It has been the dominant material for entry-level conservatory roofs since the 1980s and remains widely used in PVCu conservatory packages from volume suppliers. The thermal performance of polycarbonate is poor relative to glass. A 16mm twin-wall polycarbonate sheet achieves a centre-pane U-value of approximately 2.8–3.0 W/m²K. A 25mm multiwall sheet achieves 1.5–2.0 W/m²K. Even the best 35mm polycarbonate sheet — typically a premium product from Suntuf or Palram — achieves only 1.2–1.5 W/m²K, compared with 1.0–1.2 W/m²K for a standard double-glazed solar-control glass unit and 0.6–0.8 W/m²K for triple glass. The difference in heat loss through a 20m² polycarbonate roof versus a glass roof is equivalent to the heat loss of a well-insulated wall approximately 50–100m² in area. The solar heat gain characteristic of polycarbonate is its most significant practical limitation. Standard clear or opal polycarbonate has a Solar Heat Gain Coefficient (SHGC) of 0.7–0.85 — meaning 70–85% of incident solar energy is transmitted into the space as heat. By comparison, a solar-control low-E glass unit achieves SHGC of 0.3–0.4. The result is that a polycarbonate-roofed conservatory in a south or west-facing orientation in London becomes effectively unusable in summer — internal temperatures regularly exceed 40°C in direct sun. Polycarbonate also discolours over time: UV degradation causes yellowing and hazing of clear sheets within 10–15 years, and crazing and cracking can occur as the material becomes brittle at 20+ years.
Glass conservatory roofs: types, performance and cost premium
Toughened glass conservatory roof systems are the standard specification for mid-range and above conservatory projects in London. Glass conservatory roofs are available in several configurations: standard float glass (no coating), hard-coat low-E glass (coating applied during manufacture, durable), soft-coat low-E glass (coating applied post-manufacture, higher performance, requires sealed unit), self-cleaning glass (pyrolytic coating that breaks down organic dirt in UV light), and solar-control low-E glass (the premium option that balances thermal insulation with solar gain reduction). For London conservatories, the recommended specification is solar-control low-E glass in a sealed double-glazed unit: outer pane of 6mm toughened solar-control glass, 16mm argon-filled cavity, inner pane of 4mm toughened glass with soft-coat low-E inner surface. This configuration achieves a centre-pane U-value of 1.0–1.2 W/m²K, SHGC of 0.3–0.5, and visible light transmission (VLT) of 60–75% — meaning it allows plenty of natural light while blocking most solar heat. Self-cleaning coating adds approximately 15% to the glass unit cost but significantly reduces maintenance for roof panels where manual cleaning is difficult. Supply cost for standard solar-control double-glazed roof units: £120–£180/m². Supply cost for premium triple-glazed units: £200–£280/m². These costs cover glazing only — the supporting bar and rafter structure, ridge, hips, flashings, and installation are additional. For a 20m² conservatory roof (a typical 4m × 5m footprint): polycarbonate roof supply only: £900–£1,700. Standard solar-control glass supply only: £2,400–£3,600. Premium triple glass: £4,000–£5,600. The cost differential of £1,500–£3,300 for solar-control glass over polycarbonate is recovered within 3–5 years in reduced cooling and heating costs, and the glass roof adds meaningfully more to the property value on sale.
Part O overheating assessment and rain noise
Part O of the Building Regulations, effective from June 2022, requires that all new residential extensions in England demonstrate they will not create overheating conditions. For London (classified as a high solar exposure area under the Simplified Method assessment), Part O sets specific maximum glazing limits relative to floor area and requires that high-SHGC glazing be mitigated by solar shading or cross-ventilation if limits are exceeded. A polycarbonate conservatory roof with SHGC 0.8+ significantly exceeds Part O compliance thresholds for most south and west-facing orientations. A building regulations application that specifies polycarbonate roofing must include either a Simplified Method compliance calculation demonstrating the glazing ratio is within permitted limits, or a Dynamic Thermal Modelling (CIBSE TM59 methodology) demonstrating acceptable overheating hours despite the high SHGC. In practice, most polycarbonate conservatory specifications that trigger Part O assessment (because they require building regulations approval — for example, because they are over 30m² or heated) will fail the Simplified Method and require either a specification change to solar-control glass or the provision of external shading devices. Conservatories that are building-regulations exempt (under 30m², unheated) do not require Part O assessment — but may still overheat uncomfortably in practice. Rain noise is a consistently reported issue with polycarbonate conservatory roofs. The lightweight thermoplastic sheet transmits rain impact noise significantly more than glass — a heavy summer shower on a polycarbonate roof generates noise levels of 65–75 dB inside the conservatory (comparable to a busy restaurant), making conversation and any audio-visual use impossible during rain. Glass roofs — particularly those with a PVB acoustic interlayer (acoustic laminated glass) — reduce rain noise transmission to 45–55 dB, with laminated acoustic units achieving 40–50 dB. In a city like London where rain is a frequent occurrence, the difference in rain noise performance is a significant quality-of-life factor in conservatory use frequency.
Decision framework for London homeowners: which roof is right for your project
The choice between polycarbonate and glass for a London conservatory roof is, in reality, a question of budget, duration of ownership, and intended use of the space. Polycarbonate is the right choice only in a very narrow set of circumstances: very tight budget constraints, short-term ownership (less than 5 years), a conservatory used as an occasional garden room or storage space rather than a year-round habitable room, or a north-facing aspect with low solar gain and limited overheating risk. For all other London conservatory projects — and certainly for any orangery — glass is the correct specification. The reasons are compelling: glass provides meaningfully better thermal performance (reducing heating bills), dramatically lower solar gain (making the space usable year-round in all weather), better sound attenuation in rain, resistance to discolouration and brittleness, and a higher contribution to property value on sale. Estate agents and valuers in London consistently assign a lower value to conservatories with polycarbonate roofs than equivalent conservatories with glass — the aesthetic difference is visible to buyers, and the practical limitations are well understood. The addition of a self-cleaning coating (approximately £200–£400 premium for a 20m² roof) is almost always worthwhile on conservatory roofs where ladder access for cleaning is awkward — the pyrolytic coating reduces the frequency of cleaning required from twice yearly to once every 12–18 months. For the south or west-facing orientations that are common in London rear gardens (terraced houses oriented on east-west streets), solar-control low-E glass is not a luxury — it is a functional necessity for comfortable year-round use.
