Thursday, April 1, 2010

That's tight

A few months ago I talked about the certification and testing process to become a Passive House.  In short, one of the biggest hurdles to clear is the air-tightness of the house.  A blower-door test must show less than 0.6 air-changes per hour at 50 Pascals pressure difference (ACH 50).  One air-change per hour means that an amount of air equivalent to the volume of your house is exchanged with outside air over the course of an hour.  All that new air must be re-conditioned, which uses a lot of energy.  0.6 ACH 50 is a very difficult level to reach -- a new house will commonly be ten times as leaky.

Last weekend, while giving a talk at the Passive House Northwest 2010 Regional Meeting, Blake announced the results of our latest blower-door test.  It's important to note that Passive House measures house volume differently than most other certifications (such as Earth Advantage).  PH does not count the inside of walls and similar areas that do not hold conditioned air, making the calculated volume smaller.  Therefore, a blower-door test will show a greater number of air changes per hour because there is less air to exchange.  We calculated numbers for both the PH method and the traditional method.

Ok -- the results.  Using the PH method, our house measures 0.26 ACH 50!  It is more than twice as tight as the standard requires.

Using traditional methods, we measure 0.23 ACH 50.  If you know other people who have tested their house for something like Earth Advantage (which requires 6.0 ACH 50), this is the number you would use to compare.

How did we get the house so tight?  Attention to detail.  Blake and Larry carefully taped all the seams in the plywood shell.  We ordered casement-style windows and doors with 5-point locks that close tightly on their seals.  Every penetration of the shell is taped or gasketed or somehow sealed.  We even ordered a special dog door that seals tightly with magnets all around its edge.

Some of you might be thinking, "Isn't that too tight?  You'll build up CO2 and radon and VOCs and stuff!"  Not to worry.  The reason we make the house airtight is so that we can carefully control how air moves in and out of the house.  We have a ventilation system that constantly provides fresh air from outside.  More than that, it takes air out of the "bad air" places (bathrooms, laundry, kitchen), and runs it through a heat exchanger that moves the heat into the fresh air that's being pulled into the house.  The fresh air is pumped into the living areas.  By recovering the heat from the air we are exhausting, we don't waste energy re-conditioning air over and over again.  And by being airtight we make sure that virtually all of the air coming in or out goes through the heat exchanger.

When combined with super insulation and plenty of solar heat gain through the windows, these features act to vastly lower the energy required to keep the house at a comfortable temperature.

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