Passive House Construction

Posted on November 20, 2010

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I’ve been trying to understand some of the differences between the various “branded” approaches to reducing the energy footprint of a house: LEED-certified, Zero Net Energy house and Passive House.  LEED certification goes beyond energy consumption to also look at other impacts on the environment (water use, indoor air quality, material usage, construction practices, etc), but some critics have also argued that it doesn’t go far enough in reducing the energy usage of the typical American home.  I’ll take a look at that debate at some other time.

For now, I’ve been reading about the construction of the first Passive House in Seattle by Dan Whitmore of Blackbird Builders.  You can read about it at Linda Whaley’s Existing Resources blog.  There is also an excellent introduction to the Passive House standard in Building Green Magazine.

As I followed along on Linda’s blog the construction process of Dan’s Passive House, the first thing that struck me as different than the construction of a more typical house was the installation of a thermal barrier underneath the foundation to eliminate a potential thermal bridge between the house and the ground.  A thermal bridge provides a pathway for the conduction of heat out of a house, in this case from the radiant heated floor down into the ground.  Dan used polystyrene foam from Insulfoam to insulate the foundation and prevent heat from escaping at this point.  When he first proposed using this foam underneath load-bearing sections of the foundation, the city inspectors wouldn’t grant his building permit.  They were concerned that the foam would compress under load, resulting in excessive settling of the house.  Dan, with the help of the manufacturer and his structural engineer, was able to convince the city that the particular foam they would be using is dense enough not to compress excessively under load.  This may be an issue for anyone trying new construction techniques to build a more energy efficient home.

One of the key milestones in the building of a Passive House is the blower test.  To be certified as a Passive House, the home must be sealed to a level of .6 ACH (Air changes per hour) at a pressure of 50 Pascals.  The blower test measures this number.  In Linda’s blog, she makes the point that Dan left the walls open so that he could seal leaks if the blower test wasn’t passed the first time around.

Because a passive house is so tight, they have to ensure that they’re getting enough air circulation to ensure good indoor air quality.  Typically, that means installing a ventilation system.  Here’s a description from the Artisan’s group blog:

There are two types of ventilation systems available that are being used in Passive Houses.  One is a heat recovery ventilation system (HRV), the other is an energy recovery ventilation system (ERV).   The difference between the two is that an ERV manages moisture as well as heat while delivering fresh air into the home.  With both systems, incoming fresh air receives heat from the stale air that is being exhausted.  With an ERV, moisture in the air is exchanged as well.”

This distinction between a HRV and an ERV is important as control of moisture is particularly important in a well sealed house like a Passive House.

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