2019 October 1 Build: How Airtight Do Houses Need To Be

This article doesn’t really come to a conclusion other than the old rule of thumb of divide the blower door ACHn50 by 20 to determine the average infiltration is not accurate.

I know how hard it is to estimate the site wind speed from weather station data and then the pressure across the envelope and of course the ACHn50 number that represents the blower door test result are a single point average and the flow rate at different pressures and pressurization or depressurization of the building all matter.

I’m not sure but I think the article writer is trying to make the point that the infiltration for the few measured buildings is less than that predicted by the old rule of thumb of divide the blower door ACHn50 by 20 and thus airtightness is not important. This implication that airtightness is not important is not correct. Any energy efficiency code or standard that ignores air infiltration and airtightness is not taking energy efficiency seriously.

To me, the more interesting line of thought is moisture control in new homes. I know these BRANZ building scientists and they are sharp and their measurements are to be trusted. If a ‘typical’ new home can be as low as 3-6 ACHn50 and the measured infiltration is potentially 0.03 ACH averaged over the year than the homeowner is nearly guaranteed to have VERY high Relative Humidity and mould unless they have windows open year-round or continuous mechanical ventilation.

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