016. An air-tight building envelope

We've already seen that the first step in making energy efficient buildings is a thick layer of insulation. Once the heat loss via conduction through the building shell is minimized, the amount of heat that is lost via air leaks in the shell becomes notable. Not only that, there's also increased risk of condensation around the areas of the leaks and consequently increased possibility of mold growth. With current technology, it turns out to be energetically favorable to make the shell 'air-tight' and incorporate a mechanical ventilation system equipped with a heat exchanger to replace the indoor air. More about this in the next post.

The air tightness of a building is quantified by measuring how many times in one hour the entire volume of air in it is replaced when the building is subjected to a pressure difference of 50 Pascals (1 Pascal ≡ 0.0003" of mercury). The test is called the Blower Door Test[1] and the quantity measured is called the air exchange rate and is denoted as n_L50. This test is mandatory for Minergie-P houses where n_L50 must be shown to be less than 0.6 [1/h]. In other words, it takes about 2 hours for the air in the house to be replaced by air from the outside. A mechanical ventilation system is required to refresh the air and I'll talk about in the next post. For plain Minergie houses this test is not required but it is expected that the level of air-tightness be less than 1 [1/h]. For comparison, consider that a 1989 survey of 35 Swiss timber homes done by EMPA[2] showed that they had an average air exchange rate of about 7 [1/h]. Current construction without special consideration to air tightness is somewhere around 3 [1/h].

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[1] About the Blower Door Test on Wikipedia → Blower Door Test

[2] Kropf F., et al. Luftdurchlässigkeit von Gebäudehüllen im Holzhausbau → EMPA Bericht 218, 1989