Rooflights, Roof Windows and Part L Building Regulations for Thermal Performance

The UK’s commitment to carbon reduction has spurred the Ministry of Housing, Communities and Local Government (MHCLG) to tighten Building Regulations, with a focus on energy efficiency standards for homes. The introduction of ‘The Future Homes Standard’ aims to achieve a 30% improvement in energy efficiency standards for all new homes by 2025.

As of June 2022, the new Building Regulations include five Approved Documents, with a significant impact on rooflights and thermal performance. Notably, the Approved Document L, Conservation of fuel and power Vol 1: Dwellings, is of particular interest to manufacturers, architects, specifiers, and customers involved in installing rooflights and roof windows.

The regulations address thermal transmittance, measured as a U-value in W/m²K, where a lower value indicates better heat flow prevention. The new regulations set the maximum acceptable U-values at 2.2 W/(m2K) for rooflights and 1.6 W(m2K) for roof windows.

Rooflights, Roof Windows and Part L Building Regulations for Thermal Performance Stella Rooflights

Distinguishing between rooflights and roof windows is crucial for accurate U-value assessments. The definitions provided by the Approved Document categorise a rooflight as a glazed unit installed out of plane with the roof surface, while a roof window is installed in the same orientation as and in line with the surrounding roof.

The orientation of testing—horizontal or vertical—significantly affects the resulting U-value. Roof windows should be tested vertically, whereas rooflights are tested horizontally. The Building Research Establishment’s BR 443 further advises that U-values for roof windows and rooflights are generally quoted in the vertical plane for product comparison. However, for calculating heat losses from buildings, U-values should align with the component’s plane as installed in the building.

Despite the significance of U-values, there is ambiguity in how they are presented by manufacturers. Some misuse the figures, quoting the centre pane figure for the glass rather than the whole frame U-value, which is the legal requirement. Consumers should exercise caution and request evidence supporting thermal performance claims.

U-value calculations for roof windows and rooflights must be performed by an approved UKAS accredited product certification agency. This ensures accurate thermal performance figures based on the specific components in the product’s construction.

A crucial exemption exists for conservation properties, particularly those in traditional, barn conversions, and listed buildings. The Approved Document stipulates that energy efficiency improvements should only be pursued in historic and traditional dwellings if they won’t cause long-term deterioration.

New extensions to such properties should comply with energy efficiency standards, except when matching the external appearance or character of the host building is necessary. In these cases, compliance should be as practicable as possible, guided by the local authority’s conservation officer.

For further guidance, Historic England’s document, “Energy Efficiency and Historic Buildings: Application of Part L of the Building Regulations to Historic and Traditionally Constructed Buildings,” provides additional insights.

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