A blower door test typically pressurises the home to +/-50Pa relative to outside, and measures the airflow required to achieve this in m^3/hour.  Using the total building surface area and total internal volume you can get the whole house air changes per hour @50Pa, and also the airflow per m^2 of building fabric - the two numbers usually come out quite similar when metres are used - hence often it's not clear which one is used.  Normally there isn't 50Pa difference between inside and outside, so the actual airflow through the building is a lot lower - by default I think a factor 20 is applied.  This magic factor of 20 is an approximation - in reality it depends on house shape (taller houses should have a lower factor applied) and how exposed the location is (more exposed will give a lower factor) and where the leaks are(encouraging stack ventillation again gives a lower ratio).

Building regs presently ask for 8m^3/hr/m^2 @50Pa or better(lower).  I'm not sure why 0.5 is picked for ventillation loss rather than the 8/20=0.4 that my maths gives, my guess is still thats where the value comes from.

Posted by: @robl

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A blower door test typically pressurises the home to +/-50Pa relative to outside, and measures the airflow required to achieve this in m^3/hour.  Using the total building surface area and total internal volume you can get the whole house air changes per hour @50Pa, and also the airflow per m^2 of building fabric - the two numbers usually come out quite similar when metres are used - hence often it's not clear which one is used.  Normally there isn't 50Pa difference between inside and outside, so the actual airflow through the building is a lot lower - by default I think a factor 20 is applied.  This magic factor of 20 is an approximation - in reality it depends on house shape (taller houses should have a lower factor applied) and how exposed the location is (more exposed will give a lower factor) and where the leaks are(encouraging stack ventillation again gives a lower ratio).

Building regs presently ask for 8m^3/hr/m^2 @50Pa or better(lower).  I'm not sure why 0.5 is picked for ventillation loss rather than the 8/20=0.4 that my maths gives, my guess is still thats where the value comes from.

My Air Permeability is : 2.35 m³/(h·m²) @ 50 Pa.  So if I use your magic factor: 2.35/20=0.1175.  According to my SAP report the overall Infiltration Rate for my building is 0.099.  So not far out.

FromAi:

Mick Wall has a very thorough measurement set of air changes per hour for his house, on his web site https://energy-stats.uk/air-changes-per-hour-heat-loss/

The generally accepted ratio from the 50Pa test to natural usage is 20. But I wanted to understand why and can’t now re-find the references I used to verify a factor of ~20 for us.

https://building-performance.org/bpa-journal/ach50-achnat/

This American web site gives a wider range of weather exposure conditions than the UK but that’s useful in itself because of how broad the factor is, from 9 to 30.

When evaluating SmartHome temperature sensors, I noticed many would also include hygrometers.

So that could provide a simple way to have targeted ventilation. In tropical locations, aircon operation is typically driven by a combination of temperature and dampness. To be clear, the sensors work in our range as well..