U-What?! Understanding Rooflight U-Values
Updated October 2026
When choosing rooflights for a new build, renovation or heritage project, thermal performance is an important consideration. However, understanding the figures quoted by different manufacturers is not always straightforward.
U-values are widely used throughout the construction industry to measure heat loss, but not all U-values are calculated or presented in the same way. This can make comparing rooflights particularly challenging, especially when products are designed for different roof pitches or glazing configurations.
So, what exactly is a U-value, why does it matter, and what should architects and homeowners look for when comparing conservation rooflights?
What is a U-value?
A U-value measures the rate at which heat passes through a building element, such as a wall, window, door or rooflight.
It is expressed in watts per square metre kelvin (W/m²K) and indicates how much heat is transferred through one square metre of a building element for every degree of temperature difference between inside and outside.
Put simply, the lower the U-value, the better the thermal insulation.
For example, a rooflight with a U-value of 1.0 W/m²K will generally lose less heat than an equivalent rooflight with a U-value of 1.5 W/m²K under the same conditions.
Good thermal performance can help reduce heating demand, improve indoor comfort and contribute to the overall energy efficiency of a building.
However, the U-value of the rooflight itself is only part of the picture. Correct installation, effective insulation and careful detailing around the roof opening are also essential to minimise unwanted heat loss.
Centre-pane versus whole-unit U-values
One of the most important distinctions when comparing rooflights is the difference between centre-pane and whole-unit U-values.
A centre-pane U-value, often referred to as the Ug-value, measures the thermal performance of the glazing itself, excluding the frame and edge effects.
A whole-unit U-value, commonly referred to as the Uw-value for windows, considers the complete product, including the glazing, frame and thermal effects around the glazing edges.
This distinction is significant because a rooflight with excellent glazing performance may not necessarily deliver the same level of efficiency once the complete frame assembly is taken into account.
For example, a manufacturer might advertise a glazing U-value of 0.5 W/m²K, but the complete rooflight could have a considerably higher U-value depending on its construction.
For meaningful comparisons and Building Regulations assessments, architects and specifiers should therefore request the appropriate whole-unit performance figure, rather than relying solely on the glazing specification.
Why roof pitch affects thermal performance
Unlike conventional windows, rooflights are installed at a variety of angles, from almost horizontal to steeply pitched roofs.
This can affect their calculated thermal performance.
The orientation of an insulating glass unit influences heat transfer within the glazing cavity, meaning that U-values can vary depending on the angle at which the product is assessed.
A rooflight tested or calculated in a vertical position may therefore achieve a different result from the same product assessed horizontally.
This is particularly relevant when comparing performance figures from different manufacturers.
Where possible, specifiers should request supporting thermal performance calculations that identify the assessment conditions, including the product configuration, glazing specification and orientation.
A headline U-value is only useful when it can be related to the product being installed.
How are rooflight U-values calculated?
Thermal performance can be established through recognised calculation and testing methods, including standards such as BS EN ISO 10077-1 and associated glazing assessment standards.
These methods consider the contribution of different components to the overall thermal performance of the product.
The glazing, frame material, edge spacers and construction details can all influence the final result.
For architects and specifiers, independently assessed performance data provides greater confidence that published figures are reliable and suitable for the intended application.
It is also important to establish whether a quoted figure applies to a particular size or configuration, as changes to a rooflight’s dimensions or design can affect its overall thermal performance.
Do glazing bars affect rooflight U-values?
Traditional conservation rooflights often incorporate genuine glazing bars, particularly where larger openings are divided into multiple panes to replicate historic designs.
Unlike decorative bars applied to the surface of a single glazed unit, genuine glazing bars form part of the rooflight’s construction.
While these provide a more authentic appearance, they can also influence thermal performance by introducing additional frame elements and potential thermal bridges.
Consequently, a rooflight incorporating genuine glazing bars may have a different whole-unit U-value from an otherwise similar product without them.
This is why specifiers should be cautious when comparing headline thermal performance figures, particularly if the proposed rooflight includes architectural features that are not represented in the manufacturer’s standard calculations.
Balancing thermal performance with traditional design
Modern rooflight manufacturers can achieve impressive thermal performance through advanced glazing, insulated frames and sophisticated construction techniques.
However, improving insulation by increasing glazing thickness or frame dimensions can introduce other considerations, including additional weight, deeper profiles and more prominent sightlines.
For conservation projects, these details are particularly important.
Traditional rooflights were designed to sit discreetly within the roof covering, often featuring slim metal frames and minimal external projection.
Replacing these with bulky modern alternatives can significantly change the appearance of a historic roof.
The challenge is therefore to achieve excellent thermal performance while retaining the proportions and architectural details appropriate to the building.
At Stella Rooflight, our bespoke conservation rooflights are manufactured using slimline 316L stainless steel frames, high-performance glazing and carefully engineered components that preserve traditional aesthetics.
Independent thermal performance assessments through the British Fenestration Rating Council (BFRC) demonstrate whole-unit U-values as low as 0.92 W/m²K for selected triple-glazed fixed configurations.
Our double-glazed rooflights incorporating genuine glazing bars can achieve 1.4 W/m²K, providing excellent thermal performance without sacrificing authentic architectural detailing.
Actual performance will depend on the rooflight’s dimensions, glazing specification, opening mechanism and configuration.
What do Building Regulations say about rooflight U-values?
Part L of the Building Regulations in England establishes requirements for the conservation of fuel and power, including the thermal performance of roof windows and rooflights.
The requirements vary depending on the building type, the nature of the work and whether the product is classified as a roof window or rooflight.
This distinction is important because conservation rooflights designed to sit flush with the surrounding roof covering may be classified as roof windows for regulatory purposes.
A new edition of Approved Document L was published in 2026 and is scheduled to take effect from 24 March 2027, subject to transitional arrangements.
For detailed information on the applicable requirements, read our guide to Part L Building Regulations for rooflights or consult the 2026 edition of Approved Document L.
What about listed buildings and conservation areas?
Historic buildings can present particular challenges when improving thermal efficiency.
In certain circumstances, Approved Document L allows special consideration where meeting standard energy efficiency requirements would unacceptably alter the character or appearance of a historic building, or risk damaging its fabric.
However, listed buildings and properties within conservation areas are not automatically exempt from all Building Regulations requirements.
The appropriate specification will depend on the individual building and the proposed work, and advice should be sought from the relevant building control body and conservation officer where necessary.
For sensitive heritage projects, bespoke conservation rooflights can help achieve a balance between preserving architectural character and improving thermal performance.
Choosing a thermally efficient conservation rooflight
Understanding U-values is an important part of selecting the right rooflight, but the lowest advertised figure does not necessarily tell the whole story.
Architects and homeowners should consider whether the U-value represents the complete product, how it has been calculated, and whether it applies to the rooflight configuration being specified.
Frame design, genuine glazing bars, roof pitch and installation details all deserve consideration alongside the glazing specification.
By choosing conservation rooflights with independently assessed whole-unit thermal performance, specifiers can make informed decisions that support energy efficiency, regulatory compliance and architectural authenticity.
For further information about Stella’s thermal performance figures, or to discuss the requirements of a particular project, contact the Stella Rooflight team for technical guidance and bespoke specification advice.
