Bill Bordass – illuminating notes on the Importance of Airtight Construction


(These notes are selected from an email discussion with Bill’s permission)

“In practice air infiltration tends not to be uniform, particularly in modern component-built construction where there can be labyrinthine air leakage paths.  In particular, infiltration can easily be focused on the parts of the building with the more complex detailing, e.g. near corners, re-entrants, balconies and so on.  The result is often that some rooms (and often these more heavily-detailed rooms have the more senior staff in them) get cold and the FM response is to turn up the wick - extending times and temperatures of heating, and increasing the risk of heating, cooling and ventilation systems fighting each other.  If this happens, energy use can really scream up.”

“So to aim for a low level of air permeability is essentially QA on the fabric, which then allows the services to be sized and operated in closer accordance with the design intent and the theoretical model.  Otherwise, local infiltration problems can easily exacerbate the design-performance gap.”

"in ... ordinary buildings it is possible to feel simultaneously hot and cold.  Partly because of something the British Gas research labs found out from their comfort research in the 1970s "cold feet = cold everything".  Cold feet and hot head is about the worst combination!"

JB Comment: I am aware of a current situation where a services engineer on a project is saying that the difference between a large building tested under 50pa to 1 air change according to their calculation only achieves 3% performance improvement over a building with a result of 5ac. Bill's comments make it clear that if the leakier building is un-evenly leaky, eg there are cold drafts in some complex details in the directors' suite, the winter temperature settings for the whole building may be increased and this may go some way to explain why buildings without really high performance fabric seem so often to perform much worse than their design, with the resulting familiar call to 'blame the occupants'.

Justin Bere

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