Glass Advice · 3 min read
Solar Control Glass Explained
Large glazed openings on sunny elevations can overheat rooms even in a cool climate. Solar control glass is the usual solution.
How Do I Stop a Large Glazed Extension Overheating?
Deal with the sun before it reaches the glass, specify solar control glass where you cannot, and give the room a real way to purge heat. Blinds alone are a poor substitute for all three.
3 min read
Written by Windows in the Lakes · A trading name of Home Glazing Ltd · Last reviewed 21 September 2026
Short answer
In short: overheating is solved in layers — orientation and shading first, glass specification second, ventilation third. Retrofitting any of them after the room is built is harder and less effective than designing them in.
The layers, in order of effectiveness
- 011. Shading outside the glassAn overhang, brise soleil, deep reveal, pergola or planting stops heat before it enters. External shading is far more effective than internal blinds.
- 022. Solar control glassCoated units reduce the proportion of solar energy transmitted while keeping light levels high. Specified correctly the tint is barely perceptible.
- 033. VentilationOpenings at high and low level, or on opposite sides of the room, let hot air leave. A single sliding door does not ventilate a deep room well.
- 044. Rooflight strategyHorizontal glass takes the most solar gain of any orientation. Fewer, better-specified rooflights beat a fully glazed roof.
- 055. Internal blindsUseful for glare and privacy, limited against heat because the energy is already inside.
Understanding the glass specification
Two numbers matter. The U-value describes heat loss; the g-value describes how much solar energy the glass lets through. A low g-value reduces overheating but also reduces useful winter warmth, so the right choice depends on the orientation of each elevation rather than one specification everywhere.
| Elevation | Typical approach |
|---|---|
| South | Solar control glass and external shading |
| West | Solar control glass — low evening sun is hard to shade |
| North | Clear, high-clarity glass; solar control rarely needed |
| Roof | Solar control as standard on any significant area |
Terms used in this guide
- Solar-control glass
- Glass with a coating that reduces the amount of solar heat transmitted while keeping the glazing visually clear, used to limit overheating in heavily glazed rooms.
- g-value
- The proportion of solar energy passing through the glass, between 0 and 1. A lower g-value admits less solar heat, which matters on south and west-facing glazing.
- U-value
- The rate at which heat passes through a building element, in W/m²K. A lower number means less heat loss. Window U-values may be quoted for the glass alone or for the whole window; the whole-window figure is the comparable one.
Sources and further reading
Regulatory and technical points in this guide should be checked against the current published guidance below, and against the specific circumstances of your property.
- GOV.UK — Approved Document O, overheating
- GOV.UK — Approved Document L, conservation of fuel and power
Guidance last reviewed 21 September 2026 by Windows in the Lakes
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