To prevent solar glare in a south-facing roof lantern, specify a solar control glass unit with a total solar energy transmittance (g-value) of 0.35 or below and a light transmittance (LT) of 60% or above. This combination blocks the majority of solar heat gain while still admitting high-quality diffused daylight - keeping the space below bright, comfortable, and free from overheating throughout the year.
This guide explains exactly how solar control glass roof lantern works, what specifications to demand from your supplier, and how to combine glazing performance with Building Regulation compliance for the ultimate south-facing extension. Browse our full roof lanterns collection to see available options as you read.
1. How Solar Control Glass Roof Lantern Works?
Solar control glass uses a microscopically thin metallic or metal oxide coating applied to the inner surface of the outer pane (Position 2 in a double-glazed unit). This coating performs three simultaneous functions:
The Three-Layer Performance Model
- Reflects a portion of incoming solar radiation before it enters the unit
- Absorbs a further portion within the glass itself, dissipating it as heat to the outside air
- Transmits only the filtered remainder - primarily visible light into the interior
The result is a glass that lets in high-quality daylight while blocking the bulk of solar heat gain. The two critical performance values are:
|
Performance Metric |
What It Measures |
Target for South-Facing |
|
g-value (SHGC) |
Total solar energy transmittance |
0.35 or below |
|
LT (Light Transmittance) |
Visible light passing through |
60% or above |
|
U-value |
Thermal insulation (heat loss) |
1.2 W/m²K or below |
|
UV transmittance |
Ultraviolet radiation passing through |
Below 1% (best spec) |
2. Understanding the South-Facing Problem
In the UK, a south-facing roof sits at an almost perpendicular angle to the summer sun between May and September. At solar noon on a clear June day in southern England, direct solar irradiance on a horizontal surface reaches approximately 900 W/m². A standard clear double-glazed unit transmits roughly 76% of this energy - meaning a 2m x 1.5m lantern pane is admitting the equivalent of a 1,000-watt electric heater into the room below, every hour the sun is out.
This is the physics behind the "greenhouse effect" in south-facing extensions. The glass admits short-wave solar radiation freely, which then converts to long-wave heat radiation inside the room and cannot escape back through the glass.
3. Solar Control Glass Coatings: The 2026 Specification Guide
Not all solar control coatings are equal. In 2026, there are three main coating types specified in UK residential roof lanterns:
Hard Coat (Pyrolytic) Solar Control
Applied during the float glass manufacturing process, hard coat coatings are baked into the glass surface. They are durable and easy to clean but deliver a more limited g-value reduction - typically 0.50 to 0.60 - making them suitable for east or west-facing orientations but insufficient for true south-facing performance.
Soft Coat (Sputter) Solar Control
Applied in a vacuum chamber after manufacture, soft coat coatings achieve significantly better solar control - g-values of 0.25 to 0.40 — with high light transmittance values of 60–70%. This is the gold standard for south-facing solar control glass roof lanterns. Soft coat coatings must be sealed inside the double-glazed unit (on Position 2 or 3) and cannot be exposed to the atmosphere.
4. Building Regulations and Solar Control Glass in 2026
Solar control glass does not exempt you from Building Regulation compliance - it must work alongside your thermal specification, not instead of it.
The Overheating Risk Assessment (TM59)
Since 2022, CIBSE TM59 overheating risk assessment has been increasingly referenced by Building Control officers on residential projects - particularly those with large south-facing glazing. If your extension has a total glazed area exceeding 25% of the floor area, your architect or SAP assessor may need to demonstrate through dynamic thermal modelling that the space will not overheat in a 2026 climate scenario. Solar control glass with a g-value of 0.35 or below is one of the primary design tools for passing this assessment.
Read more about the 25% glazing rule in our flat glass rooflights guide.
5. Combining Solar Control with Self-Cleaning Glass
One common misconception is that solar control coatings and self-cleaning coatings are mutually exclusive. They are not. In 2026, the best-performing south-facing roof lantern units combine:
- Outer pane: Self-cleaning coating on Position 1 (exposed to atmosphere), which uses UV light and rainwater to break down and wash away organic dirt
- Inner surface of outer pane (Position 2): Soft coat solar control coating, sealed inside the unit cavity
- Cavity: Argon gas fill (typically 90% concentration) for improved U-value
- Inner pane: Low-emissivity coating on Position 3 to reduce winter heat loss
This four-layer performance stack delivers a unit that is self-maintaining on the outside, solar-controlling in summer, and thermally insulating in winter - the complete solution for a south-facing solar control glass roof lantern. Our Brett Martin Roof Lantern can be specified with self-cleaning glass; speak to our team for solar control options on the same unit.
Conclusion:
A south-facing extension should be the best room in the house not the hottest. At Skylights Roof Lanterns, all our roof lanterns can be discussed with our technical team to ensure the correct glass specification for your orientation, extension size, and budget. Call us on 0204 538 3079 or email sales@skylights-rooflanterns.co.uk to speak to a glazing specialist before you order.
