Thermal vs non-thermal roof lights

Thermal performance is one of the most important — and most misunderstood — aspects of roof light specification. A thermal roof light uses a modern insulated glass unit to limit heat loss through the roof aperture. A non-thermal unit — single-skin plastic, single glazing or an uninsulated legacy dome — offers little resistance to heat transfer and is rarely appropriate over heated habitable rooms.

What makes a roof light thermal?

A thermal roof light incorporates a sealed insulated glass unit (IGU):

• Two or three panes of glass separated by a spacer bar and hermetically sealed cavity.
• Low-emissivity (low-E) coatings on internal glass faces to reflect radiant heat back into the room.
• Argon or krypton gas fill in the cavity to reduce conductive heat transfer.
• Warm-edge spacer to limit cold bridging at the glass perimeter.

The manufacturer declares a U-value (W/m²K) for the complete unit — lower numbers mean less heat loss. Always compare whole-unit declared values calculated to a recognised standard, not informal glass-centre figures.

What counts as non-thermal?

Non-thermal roof glazing includes:

• Single-skin polycarbonate domes with no insulated cavity.
• Single-pane glass without a sealed insulating layer.
• Older double-skin plastic units with minimal thermal resistance.
• Unsealed or failed IGUs where the cavity has lost its gas fill and misting is visible.

These products may still admit daylight but behave thermally like a hole in the insulation layer.

Performance comparison

• U-value — thermal double-glazed unit: typically in the region of 1.0–1.4 W/m²K for whole-unit declared values (exact figure depends on build); non-thermal single-skin: effectively no meaningful insulation.
• Condensation — thermal: inner pane stays closer to room temperature, reducing cold-surface condensation; non-thermal: high risk of dripping in winter months.
• Comfort — thermal: occupants can sit beneath the aperture without noticeable radiant cold; non-thermal: perceptible cold downdraught and discomfort.
• Energy use — thermal: limits additional heating load from the roof opening; non-thermal: increases annual energy demand in heated spaces.
• Regulations — thermal: can comply with Part L requirements for dwellings when correctly specified; non-thermal: unlikely to comply in habitable extensions.

Building regulations context

Approved Document L in England sets maximum U-values for glazing in new dwellings and extensions. A non-thermal roof light over a kitchen or living area will almost certainly fail to meet the notional building or the limiting standard. Building control may require replacement with a compliant insulated unit.

Commercial and refurbishment projects have their own targets — confirm with the project energy assessor or building control officer.

Specifying for your project

• Habitable heated rooms — always specify a thermal insulated glass unit with declared U-value suitable for your compliance route.
• Utility and unheated spaces — non-thermal products may be acceptable where regulations permit; confirm before ordering.
• Replacement projects — upgrading a failed polycarbonate dome to a thermal glass unit typically improves comfort and compliance simultaneously.

Standard thermal units are available as custom frameless and custom framed products. Oversized openings, triple-glazed builds or specialist coatings may be discussed through a bespoke roof light enquiry.

Summary

Thermal roof lights are insulated sealed units that limit heat loss and condensation — they are the correct specification for extensions and loft conversions. Non-thermal units belong in unheated or non-habitable applications where regulations allow. For any room you heat, specify a declared U-value and glazing build that meets your project’s energy requirements.

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.