This article discussing ASHRAE's ventilation requirement brought up some new viewpoints I hadn't heard before (looking towards the end of the article). I know there is some contention about the CFM number the standard generates being too high or too low. I do like that the article mentioned the wide variety of indoor activities that can make 'too high' and 'too low' arguments both valid.
I'm not installing or sizing fans on a daily basis, so I don't have a feel for what the average spec'd flow rate is for a post-weatherization house. I'd guess a large number of post-weatherization houses come in under 55 cfm continuous. This leads me to think that the ability to vary a fan's hourly flow (either directly from the fan or with a timer) is a pretty important detail. Joe Lstiburek is mentioned in the article recommending a fan be sized to 150% of the sized flow rate, but initially setting them at 50% of the flow rate. I don't know if that procedure would be able to fit in weatherization's policies. But in a high performance private sector situation, that would make lots of sense along with a little customer education. The house wouldn't be over ventilated, but there would be additional capacity to ramp up with additional loads (humidity, indoor pets, or plain old bad smells) if the situation should warrant it.
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