Glass Wiki
U-values, Part L compliance, condensation risk and energy efficiency for roof glazing.
Heat escapes through roof glazing via conduction, radiation and air leakage. Lower U-values, thermally broken frames, quality IGUs and careful installation all reduce losses over heated rooms.
BS EN 1279 governs insulating glass units (IGUs) — sealed multi-pane assemblies with gas fill and spacer systems. It defines durability, moisture control and conformity assessment for glazing used in roof lights.
Light transmission describes how much visible daylight passes through glazing. For roof lights, it affects room brightness, colour rendering and whether solar-control or privacy coatings are needed.
Solar gain is the heat admitted when sunlight passes through glazing. The g-value (or solar factor) quantifies total solar energy transmittance — critical for balancing winter warmth, summer overheating and Part O assessments.
A thermal break is an insulating barrier within a metal frame that separates inner and outer aluminium sections, reducing heat loss and cold bridging. Thermally broken frames are standard for roof lights over heated rooms.
Condensation on roof lights forms when warm, moist indoor air meets a surface below the dew point — often at cold frame sections or poorly ventilated cavities. Thermally efficient units and correct detailing reduce risk.
Approved Document L sets energy efficiency requirements for buildings in England. Roof lights must meet applicable U-value limits and are modelled as part of the whole building envelope in SAP and SBEM.
A U-value measures how readily heat passes through a building element. For roof lights, the declared value usually covers the whole unit — glazing, frame and spacer — and lower numbers mean better insulation.
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