Is the default ductwork leakage class 2.5*A in the French energy proformance calculations mentioned in any CEN standard?

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It is not presently mentioned in CEN standards. Other countries might be using the same default level as France in their energy performance calculation standards, but there is no reason to harmonize it. The leakage class 2.5*A is very much leakier than ductwork systems in Scandinavia, for example. By the way, ASHRAE’s classification system for duct airtightness has higher classes near 2.5*A.

For more information you can check-out the Information Paper P187 “Duct System Air Leakage — How Scandinavia tackled the problem” by P.G.Schild & J.Railio, on the BUILDUP website