As mentioned in the previous posts, airtightness is a key factor in the passivhaus standard and the Chilean CEV Standard. This refers to the uncontrolled flow of air through unwanted “openings” (gaps) in the building envelope. This leakage has an important impact on the energy performance of the house. The concept used to measure airtightness is called “air renovations/hour” which measures the number of times the air in the building changes in the space of one hour. The average in Chile for this indicator is 12,9 renovations/hour across all types of houses (concrete, bricks, bricks and timber, timber). And if we zoom in on the specific building type that we are using (timber), the rate is 24,6 renovations/hour. The Passivehaus standard requires an infiltration level 98% less than Chile’s average rate. These high levels of air infiltration in Chile are responsible for between 40 and 60% of heating energy costs. See this previous post for more details.
So how exactly is airtightness measured? A Blower Door Test…
There are three primary components to a blower door: (1) a calibrated, variable-speed fan, capable of inducing a range of airflows sufficient to pressurize and depressurize a variety of building sizes, (2) a pressure measurement instrument, called a manometer, to simultaneously measure the pressure differential induced across the face of the fan and across the building envelope, as a result of fan airflow, and (3) a mounting system, used to mount the fan in a building opening, such as a door or a window.
The blower door fan is used to blow air into or out of the building, creating either a positive or negative pressure differential between inside and outside. This pressure difference forces air through all holes and penetrations in the building enclosure. The tighter the building (e.g. fewer holes), the less air is needed from the blower door fan to create a change in building pressure.
The procedure consists of both depressurization and pressurization tests. Different values for blower door metrics are to be expected for pressurizing and depressurizing, due to the building envelope’s response to directional airflow.
We have always been very cautious and conservative about the results we would be able to achieve in this test, as we are by no means construction or energy efficiency experts, this standard of building is very new in Chile and we are trying out new technology such as the exterior airtight membrane. Rodrigo has said from the beginning of this project that he would be happy with a result of around 2,5 air renovations per hour. This is nowhere close to the 0,64 required for the Passivhaus Standard, but it is still an extremely good level of airtightness. Remember that the average infiltration for a wooden house in Chile is 90% higher (24,6 renovations/hour).
In the end though, we achieved a result that far exceeded our expectations. After performing the pressurizing and depressurizing tests 4 times to be completely sure, the average result was 0,68 air renovations/hour. Up until now we have heard that the best infiltration rate achieved in Chile was around 1 renovations/hour, but we have managed to exceed that quite significantly. Needless to say, we are extremely happy with the outcome.
How did we achieve this result: 1) our investment in the Wraptite membrane was indispensable in achieving this level of airtightness. It really is incredible to understand what a difference the membrane makes in sealing all the miniscule gaps that exist, and like I said in a previous post, it is incredibly easy to install, and 2) our investment in high quality windows, especially the sliding doors in the living and dining areas, which are normally notorious for air infiltrations (see this post for more on our windows).
So, finally, what does this number really mean for us?
We modelled the energy consumption of the house (the passivhaus model – PHPP) using an air infiltration level of 3 air renovations/hour (conservative as mentioned above). But lowering the infiltration level from the estimated 3 to the real 0,68 means a reduction of nearly 30% in the heating energy demand of the house.