Passivhaus (Passive House) Minimum Performance and Typical Physical Characteristics

So, what are the efficiency performance characteristics needed to reach Passivhaus (Passive House)?

The minimum performance characteristics of a Passivhaus (Passive House) are:

• Airtight building shell ≤ 0.6 ACH @ 50 pascal pressure, measured by blower-door test.
• Annual heat requirement ≤ 15 kWh/m2/year  (4.75 kBtu/sf/yr)
• Primary Energy ≤ 120 kWh/m2/year (38.1 kBtu/sf/yr)

In addition, the following are recommendations, depending upon the climate of building site:

• Window u-values ≤ 0.8 W/m2/K  
• Mechanical ventilation system with heat recovery of ≥ 75% efficiency using low electric consumption @  0.45 Wh/m3 
• Thermal bridge-free construction of ≤ 0.01 W/mK

Typical physical characteristics of a Passivhaus (Passive House) in a heat-load climate within the northern hemisphere:

• Proper building orientation (south) for maximum solar heat-gain during the winter
• Large amounts of window glazings with high SHGC values on the south-facing side of the building
• Roof overhang, trellis or thermal-shutter use to provide shading to minimize heat-gain during summer months
• Minimal amounts of window glazings on the north side of the building
• Windows with very low U-values in all instances
• Windows and doors which have very low air leakage 
• Consideration of increased thermal-mass use
• Extreme air-tightness (not to be confused with insulation)
• Extreme use of insulation on all exterior wall and ceiling surfaces
• Insulated and thermally "broken" concrete floor slab
• Minimal thermal bridging construction
• Exterior and interior dimensions for maximizing heating and cooling efficiency
• Mechanically ventilated
• Use of smaller HVAC systems, such as ductless mini-splits
• Minimal external penetrations to unconditioned spaces
• Most efficient domestic hot water heating and/or solar thermal system
• Use of efficient appliances (Energy Star) and ceiling fans
• Use of most efficient lighting (ie; LED or CFL)
• A small PV (photo-voltaic) system

Less typical physical characteristics of a Passivhaus (Passive House) in a heat-load climate within the northern hemisphere:

• Earth air tube ground-source fresh-air intake system

As part of the design and certification process PHI utilizes a sophisticated PHPP (Passive House Planning Package)--a very intense MS Excel spreadsheet/program--which takes data, in great detail, of the physical characteristics and components of the building.  PHPP, to high accuracy, predicts the performance characteristics of the building through extensive calculation and its determinations have been empirically verified as being highly accurate.  I believe no other energy modeling tool is as thorough as PHPP.

The one performance characteristic that is not calculated, but actually measured, is the air-tightness of the building.  This number is absolute.  If you don't empirically meet the target, while your building may be very energy efficient, it can not be classified or certified as a Passivhaus.

With respect to the calculated performance characteristics, since the performance numbers are a result of an aggregate of individual physical performance characteristics, we had a fair amount of flexibility over the design of the building structure.

For example, if you choose, as we did, to deviate from some of the recommended or typical approaches that could hurt the overall performance values, you can make up the performance deficits in other areas.  To use just one example: in our own project, we decided that we wanted an especially "bright" naturally lit home.

One of the ways of accomplishing this was to increase the amount of window glazings on the north ("dark") side (a big no no with PH in the Northern Hemisphere).  To make up the performance "penalty" for that decision, we could increase the insulation of certain walls or ceilings, improve air-tightness performance beyond the PH requirement, and/or employ a higher efficiency HRV or ERV ventilation system

Since the Passive House standard doesn't specifically mandate any one building approach, Lisa and I were free to pick our "battles," so to speak, allowing us to choose aesthetic appeal over optimal energy efficiency in certain aspects of our home providing we made up for those "losses" in efficiency, elsewhere.

As we wanted a luxury home, first and foremost, one where the energy efficient designs would be "invisible" or hidden from plain view, we prioritized what we wanted and where.  The PHPP program easily allowed for alternative modeling, simply by plugging in different physical performance values.  This way, we were able to determine, before actual construction where we would land at the project's completion.

We will explore each of these above listed items in greater detail, in future articles.