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Is U.S./RESNET ACH50 more lax than other countries ?

W Ramsay | Posted in Energy Efficiency and Durability on

My understanding has been that building volume for ACH50 is generally calculated as the volume where air is actually exchanged so basically floor to ceiling to walls of all conditioned space.

RESNET appears to specify a much larger volume and is the total volume to the exterior face of the building (The sum of the Conditioned Space Volume and Unconditioned Space Volume) minus the volume of: Floor cavities that have Unconditioned Space Volume both above and below, Unconditioned wall cavities, Vented crawlspaces, Garages, Basements (if the door between the basement and Conditioned Space Volume is closed during enclosure air leakage testing (Section 3.2.5), and, Thermally isolated sunrooms.

 
So, for instance, RESNET includes the space between floors which alone can add 20% to the volume.

For example, in our house when I calculated the volume from CFM50 based on my understanding of it from PHI (not PHIUS) and Minneapolis Blower Door manuals I get an ACH50 of 1.9 but when it’s done to RESNET standards by a RESNET rater it is 1.27 so a very significant difference.

Is a house that is 1.27 ACH50 in the U.S. actually only a 1.9 elsewhere?

Thanks,

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