Understanding skylights and H1 thermal performance

Skylights are part of the thermal envelope, and their performance needs to be considered accordingly.

The way we consider skylight thermal performance has changed significantly in recent years.

Rather than treating a skylight as having one generic R-value, H1 requires the construction R-value of the complete skylight to take account of both the glazing and the frame. For the glazing, the effect of its installed angle on heat transfer must also be considered.

There are therefore four particularly important factors when determining the thermal performance of a skylight:

Glazing selection

The thermal performance of double and triple glazing varies depending on the glass build-up, Low-E coatings, gas fill and spacer system.

Frame performance

The glass is only one part of the skylight. Frame performance also contributes to the overall thermal transmittance of the complete unit, making effective thermal breaks particularly important with aluminium roof windows.

Installed pitch

A glazing unit does not necessarily have the same thermal performance when installed horizontally or on a slope as it does vertically.

This happens because changing the angle of the glass affects convection within the cavity of the insulated glass unit.

H1 therefore requires the thermal transmittance of skylight glazing to be determined while considering the effects of horizontal or angled glazing.

Skylight size

Size also matters because it changes the proportion of glass to frame.

A large skylight may be dominated more by the performance of the glazing, while the frame can have a greater influence on a smaller skylight.

As a result, two skylights manufactured using the same frame and glazing specification can still have different overall construction R-values.

H1 has changed again

The Sixth Edition of H1/AS1 became effective on 27 November 2025. The previous Fifth Edition can continue to be used for building consent applications submitted before 27 November 2026, after which the Sixth Edition becomes the applicable Acceptable Solution.

One significant change is the removal of the Schedule Method. Under the Sixth Edition, thermal-envelope compliance within the scope of H1/AS1 is demonstrated using the Calculation Method, or through modelling under H1/VM1.

Under the Calculation Method, the skylight area and its construction R-value form part of the heat-loss calculation for the proposed building.

This makes accurate thermal information increasingly important at design and consent stage.

Adlux can help

This may sound complicated, but it doesn’t need to be.

For Harmony Roof Windows, Adlux can provide project-specific thermal information based on the proposed roof windows and their installation conditions.

To assist us, provide:

• roof window size
• installed roof pitch
• proposed glazing specification, where known
• project name or address
• building consent number, where available

We can then help provide the appropriate thermal information for the project.

As buildings become more energy efficient, roof glazing needs to be considered as part of the overall thermal envelope rather than simply as a source of daylight.

Better information at specification stage makes that process considerably easier.