Reference · intermediate · 3 min read
Solar gain and g-value explained
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.
Roof lights admit daylight — and with it, solar energy that warms the room below. In winter that free heat is welcome; in summer it can contribute to overheating. The g-value (solar factor) is the standard metric for quantifying how much solar energy passes through glazing into the building.
Specifiers balancing Part L heat loss against Part O overheating need both U-value and g-value on the datasheet — neither tells the full story alone.
Definition
g-value (solar factor) — The ratio of total solar energy transmittance through glazing to the incident solar energy on the surface, expressed between 0 and 1. It includes directly transmitted radiation and the portion of absorbed energy re-radiated and convected to the interior.
How solar gain works
Sunlight contains visible light and shorter-wavelength infrared energy. When it strikes a roof light:
- Some energy reflects away from the surface.
- Some transmits directly through the glass into the room.
- Some is absorbed by the glass and frame, then partly re-radiated inward.
The g-value captures the combined heating effect of transmission and inward re-radiation. A g-value of 0.50 means half the incident solar energy becomes heat gain indoors (per unit area, under standard test conditions).
g-value vs U-value
Metric
What it measures
Design tension
U-value
Heat loss when inside is warmer than outside
Lower is better for winter insulation
g-value
Solar heat admitted when sun strikes the glass
Lower reduces summer overheating; higher aids winter passive heat
High-performance low-E coatings improve U-value by reflecting long-wave radiation from heated objects inside. They do not automatically block short-wave solar radiation from the sun. A unit can have an excellent U-value and still a relatively high g-value if specified with clear glass.
Solar-control coatings or tints target g-value — sometimes at the cost of lower visible light transmission. The right balance depends on orientation, room use and shading.
Part L, Part O and orientation
Part L models annual energy using solar gains through roof lights in SAP or SBEM. South- and west-facing glazing on a shallow-pitched or flat roof can deliver large mid-summer gains.
Part O (overheating, England) requires new dwellings to limit excessive summer temperatures. Large roof light areas with high g-values on vulnerable elevations may need mitigations: external shading, openable vents, reduced glazing area, or lower g-value glass.
Early coordination between architect, energy assessor and roof light supplier avoids redesign after structural openings are formed.
Reducing solar gain without losing all daylight
Options for roof glazing include:
- Solar-control pyrolytic or magnetron coatings on the outer or cavity-facing pane.
- Body-tinted glass absorbing solar energy before it enters (grey, bronze or blue tints).
- Laminated interlayers with solar-control properties.
- External shading — brise-soleil, louvres or neighbouring buildings — which reduces gain without changing the glass.
Declare the chosen g-value and light transmission together so interior designers understand daylight levels.
Reading manufacturer data
Request the declared g-value for the complete roof light unit at your size, determined per BS EN 673 / BS EN 410 methods as referenced in CE thermal-solar performance marking. Do not assume clear double glazing values from a textbook — low-E, gas fill and laminate interlayers all shift the result.
Pair this reference with light transmission through roof glass and U-value basics when preparing elevation schedules.
Every Vant Glass roof light is made to order in Britain, backed by a 20-year guarantee and free UK mainland delivery. Configure frameless or framed sizes in the online calculators or call 03330 902 592.
Frequently asked questions
What is a typical g-value for clear roof glass?
Clear double glazing without solar-control coating often has a g-value around 0.6–0.7, though the exact figure depends on pane coatings, gas fill and the proportion of frame in the unit. Always use the manufacturer's declared g-value for calculations.
Should I choose the lowest g-value possible?
Not always. Very low g-value glass reduces summer overheating but also blocks useful winter passive heat and daylight. South-facing living spaces may benefit from moderate solar gain; large north-facing roof lights may prioritise light over gain.
How is g-value different from light transmission?
Light transmission (LT or τv) describes visible daylight admitted. g-value includes near-infrared energy that heats the room but is not visible. Solar-control coatings can lower g-value while maintaining reasonable daylight — see light transmission guidance.
Does g-value appear in SAP calculations?
Yes. SAP uses solar gain through transparent elements when calculating dwelling CO₂ emissions and energy use. Roof lights on sunny elevations with high g-values increase summer solar gains.
Can solar-control glass be used on roof lights?
Yes. Body-tinted, coated or laminated interlayer options are available to reduce g-value. Balance against desired daylight and appearance — tinted or coated panes change internal colour rendering.
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