Are Triple Glazed Roof Lights Worth It?
Triple glazed roof lights cost more but perform better in winter. Here's when the upgrade is worth paying for, and when double glazing is sufficient.
Triple glazed roof lights sit at the premium end of the specification range. They cost more than equivalent double glazed units, typically 20–35% more, and they perform better thermally. The question is whether that performance improvement is worth the additional cost for your specific project. Here's the honest assessment.
The Thermal Performance Difference
A standard double glazed roof light with a low-e coating achieves a centre-pane U-value of approximately 1.0–1.2 W/m²K. A triple glazed equivalent achieves 0.5–0.7 W/m²K. The whole-unit U-value (including frame) is typically 1.2–1.5 for quality double glazed and 0.7–1.0 for triple glazed.
In practical terms: through a 2m² roof light, the additional heat loss through a double glazed unit compared to triple glazed on a cold winter day amounts to a modest but real increase in heating energy. Over a full UK heating season, across a typical family home extension, the energy saving from triple glazing a roof light is real but not transformative.
When Triple Glazing Makes Strong Sense
North-facing extensions
A north-facing extension receives no direct solar gain through the roof light to offset heat loss. The thermal performance of the glazing has maximum impact on comfort and running costs in this orientation. Triple glazing is a well-justified upgrade here.
Year-round garden rooms and home offices
A space used year-round, particularly through winter, where comfort and low heating cost are priorities. The improved U-value reduces the sensation of 'cold ceiling' that can make a large roof light feel draughty even when the room is at the correct temperature.
Passivhaus or high-performance building standards
If the extension is being built to a high thermal performance standard and the roof light needs to meet a whole-building target, triple glazing may be required to achieve the specified U-value.
Large roof lights (2m² or more)
The larger the roof light, the more significant the total heat loss through the glass. A 3m² triple glazed roof light saves meaningfully more energy than a 0.9m² one, the larger the unit, the stronger the case for the upgrade.
When Double Glazing Is Sufficient
For most south and west-facing single-storey extensions, where solar gain through the roof light contributes meaningful passive heat in winter, quality double glazing with a low-e coating achieves Building Regulations compliance and delivers good year-round performance. The additional cost of triple glazing on a south-facing kitchen extension is harder to justify on energy grounds alone.
For summer-heavy uses, outdoor dining rooms, entertainment spaces, where the extension is rarely heated and warmth retention through winter isn't a priority, double glazing is entirely appropriate.
The Condensation Consideration
Triple glazed units maintain a warmer inner glass surface in cold weather, reducing the likelihood of condensation on the inner face of the glass. In a high-humidity environment (kitchen, bathroom), this is a practical benefit beyond the energy saving, condensation on the inner face of a roof light requires mopping up and can affect the ceiling finish below over time.
Contact SmartaView to Discuss Glazing Specification for Your Roof Light Project
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Author at SmartaView, writing about aluminium glazing, home improvements and the buying process.
