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Further Clarification From BFRC

The BFRC responds to questions raised following its statement clarifying the situation regarding measured thermal conductivity values for spacer bars.


Editor:  Why has the British Fenestration Ratings Council (BFRC) changed the way it deals with simulating spacer bars?

BFRC: The BFRC has adapted its requirements to reflect the changes to BS EN ISO 10077-2 2012. This standard requires measured thermal conductivity values to be derived according to a very specific set of criteria. Our investigations have determined that the only way to currently satisfy the standard is to use a measured conductivity value obtained from Bundesverband-Flachglas (BF) 2 box model (currently this is the only item of data taken from the datasheet). Only datasheets currently available on the BF website ( are deemed to be acceptable for new BFRC rating applications or for product substitutions. These changes came into place following numerous discussions with industry.

Editor : Can BFRC licence holders substitute components within a licenced window?

BFRC: BFRC licence holders can, of course, change the components within a licenced window. However, as these changes are likely to have a substantial effect on the window’s energy rating (WER) performance, in most cases a full re-simulation and re-evaluation of the window in question is required.  Nevertheless, under certain circumstances, it is possible to substitute certain components without requiring a re-simulation. The BFRC substitution rules, which were created in 2010 following consultation with the fenestration industry, are still applicable and available, and are designed to ensure that any simple substitution of components does not result in a lowering of the window’s energy rating performance. It is important to bear in mind that simple substitution (without re-simulation) is only permissible for one component at a time. For instance, substituting a spacer bar of equal or better thermal performance is only permissible providing the type and depth of secondary sealant is unchanged. If, when changing a spacer bar, a modification to the type or depth of secondary sealant is necessary, then re-simulation would be needed, as under these circumstances it is not possible to guarantee that the window’s energy rating will not decrease slightly. BFRC’s rules on substitution without re-simulation are designed to safeguard the homeowner in that the WER performance of a window will be the same (or slightly better) using these rules. Where the performance cannot be guaranteed to be the same or better, a full re-simulation and re-evaluation needs to take place.

Clause 4 of the BFRC Substitution Rules for Glazed Units states that: ‘Spacer bars (including the primary sealant and desiccant) may be substituted when the replacement has an equal or lower effective thermal conductivity (measured to 3 decimal places) than the spacer bar specified in the original licence. Effective thermal conductivity of the spacer bar (including the primary sealant and desiccant) shall be determined either by tabulated value from BS EN ISO 10077:2 2012 or if a measured value is required, thermal conductivity values shall be obtained from a currently listed 2 box model datasheet from the BF website.’ (Website address as above)

Editor: I’ve been told that my BFRC licence is valid for 5 years. Is this true?

BFRC: BFRC licences are created as a result of a BFRC simulation being undertaken for a given set of components, i.e. frame sections, reinforcement (if applicable), glass types, spacer bars and sealants. The existing BFRC licence is valid for five years provided the component parts remain commercially available. If a given component has been superseded by a replacement with equivalent, or better thermal performance, then simple substitution may be possible without full re-simulation. However, in cases where the replacement part has an inferior thermal performance, simple substitution is not permitted and a new licence will be required (supported by updated simulation data).

Editor: What depth of secondary seal is used for rating purposes?

BFRC: All spacers bars are simulating as being positioned with the top of the bar inset 12mm from the edge of the glass, as this is how the vast majority of IGUs are manufactured in the UK. Deviation from this depth is permitted, provided documentary evidence to support the compliant manufacture is provided and is deemed to be acceptable by BFRC’s Technical Committee. The BFRC is consulting with industry via the Glass and Glazing Federation (GGF) Technical Committee which is currently reviewing the sealant depth issue. More information will be made available in due course.

Editor: Does using the BF 2 box models prevent changing the type of secondary sealant?

BFRC: The BF website lists test reports in two categories. At the top of the page are results for spacer bars which can be simulated with both PU/PS and Hot Melt sealants, as they have demonstrated the ability to Pass EN 1279 parts 2 and 3 with all secondary sealant types. At the bottom of the page is a list of spacer bars that have only been tested with hot Melt sealant and can, therefore, only be simulated with this type of sealant.  BFRC will only accept simulation reports that have a listed combination of spacer bar and sealant.


For further information, contact the BFRC directly, tel: 020 7403 9200, or e-mail:


Caption 1         A spacer bar from Edgetech.


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