Insulating Rooflight Builder's Kerbs

Clarification of the Bauder approach to preventing a thermal bridge through builder's kerbs supporting rooflights.

What Do the Regulations Say?

The amount of insulation required for builder's kerbs supporting rooflights has never been clearly defined within the Building Regulations. However, it has been recognised for many years that thermal bridging should be minimised wherever possible and that the maximum U-value on any part of a flat roof should be 0.35W/m²K.

The most recent version of BR443 (Conventions for U-value Calculations) states the method of calculation to be used for builder's kerbs within Clause 11.5.3, but does not prescribe a specific U-value, as this remains a Building Regulations requirement.

Early indications suggest that clarification of the required U-value for rooflight builder's kerbs will be addressed when Building Regulations across the UK are next revised.

The Interim Solution for Bauder Projects

Bauder has decided that all insulated specifications will include 60mm insulation applied to the external face of builder's kerbs wherever these occur within a project.

When using BauderPIR insulation, this achieves a U-value of 0.35W/m²K. For practical purposes, the same insulation thickness will currently be specified for other insulation types.

Bauder standard drawings and specification clauses will include this requirement with immediate effect, and contractors should take this into consideration when pricing and installing Bauder systems.

It is important to note that if the insulation thickness is increased for any reason, Bauder must be notified, as this may affect both the size and potentially the design of the rooflight supplied.

Please Note

This requirement applies solely to builder's kerbs supporting rooflights.

Rooflights supplied complete with integral upstands do not require any additional insulation.

Any questions regarding this guidance should be directed to: