|Pressurization test with air flows due to leaks with negative pressure.||Measuring the pressure difference in the building.|
Building owners benefit from an airtight building envelope because it reduces:
- water problems
- a small air leak will allow a lot of moist air to get inside a wall and condense, risking structural damage and mold.
- heat transfer (heat loss in winter and heat gain in summer)
- environmental tobacco smoke (ETS) nuisance between apartment units
- likely to become even more of a liability for landlords in the future
- transfer of smoke and heated gases from a fire
- noise transmission
- use the airtight drywall approach (ADA) even for interior partitions
There are two main ways to document the airtightness of a building. One is to take the leakage in cubic meters per hour or cubic feet per minute and divide it by the area of the building envelope. The other method is to determine the number of air changes per hour (ACH) by measuring the leakage and dividing by the air volume. For example a 1000 m3 building with a measured air flow leakage rate of 1000 m3/h would have an airtightness rating of 1 h-1 since 1000 m3/h/1000 m3 = 1 h-1 . Using non-metric units: 583 CFM * (60 min/hr)/35,000 CF = 1 ACH. The Passive House standard requires testing at 50 Pascal while USACE requires 75 Pa. Testing at a higher pressure yields more accurate results, especially for larger buildings. (See the US Army Corps of Engineers Air Leakage Test Protocol PDF.) In order to distinguish between the natural leakage rate and the tested leakage rate at 50 Pa pressure, the airtightness number is noted as n50 or ACH50.
Why is air leakage for the Passive House standard measured in air changes per hour? Partly because it is unitless and understood in the US, but also because the leakiness of the envelope becomes less important in terms of energy efficiency as the volume increases.
A building 10 m x 10 m x 10 m = 1000 m3 in volume and 600 m2 in surface area. If n50 = 1.0 h-1, then air leakage in terms of the envelope is 1000 m3/h/600 m2 = 1.67 m3/(hm2 ).
A building 15 m x 15 m x 15 m = 3375m3 in volume and 6*(15 m * 15 m) = 1350 m2 in surface area. If n50 = 1.0 h-1, then air leakage in terms of the envelope is 3375 m3/h/1350 m2 = 2.5 m3/(hm2 ).
So, the same test result in terms of air changes per hour can have different values in terms of the envelope.
“Build tight and ventilate right” is a popular saying among green builders. Proper ventilation is essential for healthy indoor air quality, especially for airtight buildings. Building codes require mechanical ventilation if natural ventilation is not sufficient. Relying on open windows for natural ventilation can be problematic due to street noise, dust, rain, and extreme outdoor temperatures. Does a building need to breathe? That’s a confusing metaphor because there are two distinct issues. One is uncontrolled air leakage (bad) and the other is vapor permeability (usually good).
Use a blower door to find air leaks, not just to determine ACH50. Before the drywall goes up, pressurize the building with a blower door and use theatrical smoke, fingers (surprisingly effective), or an infrared camera to find leaks. The first test should be at a higher pressure to make it easier to find leaks. At this point, it might be interesting to determine the ACH50 using the Passive House methodology, but it isn’t really necessary until all the leaks have been fixed. In the US and the UK, count on three or four iterations of testing and sealing to get below the limit of 0.6 ACH50. In Austria and Germany, where Passive House construction methods are more standardized, they often just do one blower door test towards the end.