PassivHaus

So, why not just go with a more traditional, above ground, passive solar house?

There is quite a range of what is considered a passive house.  On the “uber” end of the spectrum is the “Passivhaus” (or “Passive house” in English), which is really a performance standard for high-efficiency housing commonly applied in Europe where high energy prices (necessity) have focused a lot more interest and invention towards this sort of housing…  There are tens of thousands of these built in northern Europe and some here in the USA also (a greater challenge due to our tougher climate).  To be certified as a “Passivehaus”, the home needs to meet a few strict requirements including low annual heating demand, less than 15kWh/m2 (4746btu/ft2) per year, and be as airtight as possible, the building must not leak more than 0.6 times the house volume per hour, as tested by a blower door.

Basically these are just standards of energy efficiency, and “Passivhaus” owners typically claim a 90% reduction in their heating bills.   I guess it is a good thing to have standards and metrics, but the more I thought about it, the more hollow it sounded.  While I am interested in reduced energy bills, I am also interested in increasing my robustness against interruptions to the grid.  I am even more interested in climate comfort, temperature stability and a healthy environment.  Don’t get me wrong, when I am done, I will check to see if I met the “passivhaus” standards, they are just not my focus.

When I studied examples of Passivhaus construction, particularly the American examples, I did not think they were taking the correct approach.  For instance, they spent a fortune on R60 insulation all the way around…  The insulation its self is only part of the cost, they first had to balloon frame the walls with trusses instead of 2x4s, and then sheath both sides for stiffness, then they created an airspace and a second wall (double envelope house).  Most also used a “cold roof”, which is a double roof with an airspace between (and costs about as much as two roofs).  Most use triple glazed casement windows which needed to be imported because they just don’t make those here.  In some cases, they put 14 inches of rigid insulation under the slab and 6 inches around the foundation.  A lot of expense is also tied up in making sure that the envelope is well sealed against infiltration, yet permeable enough to allow trapped moisture to escape.  In terms of performance, they typically get large temperature swings on a daily cycle, the passive solar heating is also not uniform within the space.  In one example, I read about plastic toys melting in the living room.  Because they are trying to keep the construction costs manageable and the volume to surface ratio as high as possible, most passivhaus examples I have seen are very simple boxes ( upper left) although many can still make those boxes interesting like this one (right).

It reminds me of those automotive “fuel sippers”.  These are people who spend thousands more for a Prius or other hybrid, then drive it very slowly (coasting when ever possible) and risking their lives as big rig trucks over take them, in order to save a few dollars in gasoline.  I had one such colleague tell me that she is saving so much money on gas that she “drive[s] all the time now”, she didn’t understand why I laughed.

 

Leakage is the main obstacle to keeping a home comfortable in a challenging environment.   Sealing a home above ground is a difficult challenge.  Think of it this way, the strict PassivHaus inspectors are impresses when you only leak 59% of your homes air every hour! The average home has much higher infiltration rates.  While it costs a lot to seal a regular stick frame home above ground, below ground construction is naturally air tight (instead we worry about bringing in enough fresh air).  Also, an earth sheltered house has insulation and thermal storage.  Thermal storage works as a sort of dynamic insulation.   Our particular earth shelter plans also call for some cellular concrete (R~1/inch) and a rigid insulation umbrella.  It should give an average roof R value of 47, but at a relatively low cost (more on that later).