Understanding glazing performance: U-values, SHGC and g-values
Glazing performance for a conservatory or orangery is described by three key metrics. The U-value (measured in W/m²K) quantifies heat loss — a lower number is better. Standard sealed double-glazed units with no special coating achieve approximately 2.8 W/m²K centre pane. Argon-filled units (the standard in modern conservatories) achieve 1.6–1.8 W/m²K. Low-emissivity (low-E) coating on the inner face of the outer pane, combined with argon fill, achieves 1.0–1.4 W/m²K. Triple glazed units with two low-E coatings and two argon-fill cavities achieve 0.5–0.8 W/m²K. Building regulations Part L requires all glazed elements in regulated extensions to achieve ≤1.4 W/m²K overall window U-value (which means the sealed unit must achieve ≤1.2 W/m²K to allow for frame heat loss). The Solar Heat Gain Coefficient (SHGC), also called the g-value in European standards, describes how much solar heat passes through the glass into the room — a lower number means less solar heat is admitted. Standard clear glass has an SHGC of approximately 0.75–0.85. Solar-control coatings reduce this to 0.20–0.40 depending on the coating specification. A low SHGC reduces summer overheating risk but also reduces the passive solar gain that warms the space in winter. The optimum balance for London's climate: for south and west-facing aspects, specify SHGC 0.25–0.35 (good solar control while still allowing some passive heating); for north and east-facing aspects, SHGC 0.4–0.55 (better passive solar gain). Light transmission (LT) is the third metric — solar-control coatings inevitably reduce visible light transmission. High-performance solar-control low-E units achieve LT of 0.55–0.70, meaning 55–70% of visible light is transmitted. Lower-SHGC units have lower LT (0.35–0.50) and give the glass a slightly tinted or reflective appearance.
Glazing options for conservatory roofs — glass types and configurations
The roof is the most thermally and optically critical element of a conservatory. Options from worst to best performing: Polycarbonate (16mm twin-wall or 25mm multi-wall): cheap, lightweight, achieves U-values of 1.8–3.2 W/m²K depending on wall count. Scratches easily, yellows over time, sounds loud in rain, and provides poor thermal performance. Not compliant with current Part L if building regulations apply. Avoid for any permanent living space. Standard float glass in a patent glazing bar system: achieves approximately 2.0–2.8 W/m²K without special coating. No solar control. Cheap but performs poorly. Only appropriate for budget lean-to structures used seasonally. Argon-filled soft-coat low-E sealed units (standard specification): achieve 1.0–1.4 W/m²K centre pane. Compliant with Part L when building regulations apply. This is the minimum specification Builderr installs on any conservatory roof. Solar-control low-E argon-filled sealed units: add a hard or soft solar-control coating that reduces SHGC to 0.25–0.40 while maintaining U-value at 1.0–1.4 W/m²K. Add £2,500–£5,000 for a 20m² roof but are essential on south and west-facing roofs to prevent overheating. Self-cleaning glass (hydrophilic coating, Pilkington Activ or Saint-Gobain Bioclean are the leading products): a photocatalytic coating that breaks down organic material in sunlight and a hydrophilic surface that causes rain to sheet rather than bead, carrying dirt away. Add approximately £500–£1,200 for a 15–25m² conservatory roof. Worth specifying on any inaccessible roof section where cleaning requires scaffold. Triple glazing on roof sections: achieves 0.5–0.8 W/m²K at significantly higher cost and weight. Structurally, the glazing bars and frame must be designed to carry the additional weight — typically 10–15 kg/m² additional dead load. Appropriate for north-facing conservatories, orangery lanterns and any application where maximum winter thermal performance is the priority.
Roof lanterns, rooflights and full glass roofs compared
The overhead glazing configuration profoundly affects the character, light quality and thermal performance of an orangery or conservatory. A full glass roof (as in a traditional conservatory) provides maximum daylight but also maximum solar gain in summer and maximum heat loss in winter. The visual experience is dramatic — direct sky views — but thermal management requires high-performance solar-control glass. A roof lantern sits on a solid flat or low-pitched roof, typically as a rectangular raised structure with angled glazing bars and a ridge bar. Sizes range from 1.0m × 1.5m up to 3.0m × 5.0m or larger for wide-span applications. Lanterns provide a column of diffuse overhead light — similar to a traditional atrium light — without the full solar exposure of a glazed roof. They are the defining feature of a classic orangery. Modern aluminium lanterns with thermally broken frames and argon-filled units achieve roof U-values of 1.4–1.8 W/m²K, which is adequate but not exceptional. Upgrading to a structural glass lantern (frameless or minimal-frame) with advanced solar-control glass improves performance to 1.0–1.2 W/m²K but costs 2–3× more. Flat rooflights (flush or low-profile) are the most thermally efficient option for an orangery roof — triple-glazed frameless rooflights can achieve 0.7 W/m²K and are virtually invisible from outside, which suits conservation area applications where a raised lantern might not be approved. The trade-off is less dramatic interior character. For large south-facing orangeries in London's climate, the optimum configuration is: a combination of MVHR (mechanical ventilation with heat recovery) or passive stack ventilation, automatic roof-light ventilators, solar-control SHGC 0.25–0.30 glass on the lantern, and a heat pump system capable of cooling as well as heating.
Overheating risk and ventilation solutions for London conservatories
Overheating is the single most common failure mode in London conservatories and orangeries. A south-facing glazed room in London can reach 40–50°C on a July afternoon without solar control glass and adequate ventilation. The 2021 Part L/Part O changes require any regulated extension to pass an overheating risk assessment — but even PD-exempt conservatories frequently overheat and become unusable, which is a practical problem regardless of regulatory compliance. Overheating solutions, in order of effectiveness: solar-control glazing with SHGC ≤0.30 on all south, southwest and west-facing elements (walls and roof); automatic roof ventilators (ACO, Velux or aluminium frame-integrated) on ridge or lantern — minimum 5% of floor area in openable roof section; mechanical ventilation (MVHR or simple through-ventilation): for a 20m² orangery, a 150mm MVHR unit with summer bypass can maintain 22–25°C in summer without air conditioning; high-level openable windows (awning or tilt-and-turn) in addition to low-level bifolds, to create stack ventilation; retractable or fixed external shading (pergola, louvred pergola or full automated solar blind system in the roof). External shading is 4–8× more effective than internal blinds because it stops solar heat entering the glass before it reaches the interior. Part O of the Building Regulations 2010 (effective June 2022) requires a simplified overheating check for all new regulated extensions. The check assesses the glazing area, orientation, shading, and ventilation provision. If the simplified check fails, a dynamic thermal simulation is required. Builderr's specifications are designed to pass the Part O simplified check as standard — we do not rely on fabric improvements alone but design in appropriate ventilation and shading from the outset.
