Go to Forum Home Building Design Calculating AECB Standard u-values

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    • #30552

      I have just set a piece of coursework for some BSc students which involves the preparation of a building regulation application to AECB silver standard. My question is this:Is there a standard methodology for calculating the 'real' u-values stipulated?

    • #33159

      If you read the web site you will see that AECB is seeking resources (from numerous different sources) to refine and publish the Standards in a comprehensive (and legally enforceable) way (as has been done in several other countries whose work is well-renowned in this field includiing Germany, Canada, Switzerland et al).

      Pending that, the Passive House Institute, Darmstadt state in their publications that if thermal bridge-free construction (defined) is used, and external dimensions (defined) are used in calculating elemental areas, then no further consideration of (the non-repeating) thermal bridges is needed. This procedure would be acceptable here too, indeed it encourages designers to adopt construction details which are substantially free of thermal bridges.

      Incidentally, I regard BFF articles as a bit off-topic here. As far as I'm aware, the editor of BFF, not AECB, is responsible for what appears in that journal.

      Also, as far as I know from discussions with Andy, a publication on detailing to meet the Silver Standard in masonry and timber-frame will be appearing on the web site soon. I hope this helps.


    • #33160

      The bre one is £20 and allows for thermal bridging.


    • #33161

      About ten years ago, I was told, Germany adopted a default correction of +0.15 W/m2K unless certain precautions were taken to reduce the number & severity of non-repeating thermal bridges. I think the UK correction of +0.08 for all the non-repeating thermal bridges is often too low if internal areas are used for the calculations. Also many “robust details” such as the ones for steel lintels aren't thermally robust as small changes in the as-built detail can greatly raise the psi-value.

      The rule I've followed for years in the UK is not to believe official reassurances on this subject but to check it oneself. For some reason we've followed a path of rosy overoptimism when calculating heat loss. German calcs. gave predictions closer to the truth or even a touch pessimistic – but slight pessimism is not a real problem when sizing heating systems and overoptimism is a *big* problem; people will be cold.

      The thermal bridge catalogues avalable in some continental countries give the psi-values of common details, esp. in solid masonry and could be used so long as in cases of discrepancy the designer must use the psi-values given for slightly worse details (thicker steel plate, wider concrete inner leaf, etc) than his/her own proposals. Alles ist auf Deutsch but could be translated in time.


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