Building airtightness : how tight is tight enough ?

The author explains that too tight building envelopes can cause bad operation of atmosperically vented combustion systems (e.g. gas water heaters) in case of of unintended depressurization of the building, for example with large exhaust fans and dirt filters. He considers that airtightness requirements of standards are often too severe. He proposes a building airtightness of 2 to 6 air changes per hour at 50 Pa for warm climates and 1.5 to 4 for cold climates, buildings with atmospherically vented combustion appliances being at the high end of the range or higher. He explains that these values allow reduced infiltrations and control of desired pressure, while avoiding excessive levels of depressurization in case of unbalanced air flows.