air changes per hour

Ventilation

Mill Creek NetZero Home Heat Recovery Ventilator

I heard a story once about a man who built a house using insulated concrete forms (ICFs). While I don't advocate their use in general, ICFs have some distinct advantages (certainly over conventional construction). The primary of these advantages is supreme air tightness. ICF homes (those that are built with ICFs from top to bottom) can achieve hourly air change rates of 0.2. In contrast, the Mill Creek NetZero Home has an airtightness measurement of 0.36 ACH, which is really amazing for a wood-framed house.

So back to this guy who built the ICF house. He apparently wasn't aware of how air tight his house was, or at least the consequence of that air tightness. Six months after he moved in, he had to rip all of the drywall out because it had rotted. He had built a house as tight as a plastic bag, and forgotten a critical aspect: ventilation.  read more... »

Airtight

The importance of air tightness in building construction cannot be overstated. Very few factors affect energy performance in a cold climate more than air moving in and out of a building.

The Mill Creek Net Zero Home (MCNZ) has achieved an air tightness test result of 0.36 air changes per hour (ACH) at a pressure of 50 Pascal. In other words, when it’s really cold out, which creates a big pressure difference between the inside and the outside of a house, the 0.36 of the air in the MCNZH would leak out and be replaced with cold air over the course of an hour. It has the equivalent of a 13.8 square inch hole in it leaking air all of the time.

To put the number (0.36 ACH @ 50 Pa) into perspective, here are some average numbers:

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Source: Biggs et. al. 1987

The 0.36 number is extremely small. In fact, the MCNZH is probably one of the most air tight wooden buildings in the world.  read more... »