Spacer bars and measured thermal conductivity values

bfrcThe British Fenestration Rating Council (BFRC) has responded to questions raised following its recent statement clarifying the situation regarding measured thermal conductivity values for spacer bars (Bullseye issue 254, 23 September):

,• Why has BFRC changed the way it deals with simulating spacer bars?

,Answer:  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. BFRC 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.

,• Can BFRC licence holders substitute components within a licensed window?

,Answer: BFRC licence holders can of course change the components within a licensed window. However, as these changes are likely to have a substantial effect on the window’s energy rating performance, in most cases a full re-simulation and re-evaluation of the window in question is required. However, 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 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 three 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’.

,• I’ve been told that my BFRC licence is valid for five years. Is this true?

,Answer: BFRC licences are created as a result of a BFRC simulation being undertaken for a given

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