26 Jul Certified Passive House Vs The Pretty Good House – Part II

The PHIUS plan starts with a goal: to create buildings that use 40-60% less energy than built-to-code buildings, and to do so while making these buildings more comfortable and healthy for occupants, more resilient to an increasingly hostile and unpredictable environment, and more durable over time. The goal includes the further ambition of doing all of this as economically as possible. There is an ordered list of fundamental principles through which the plan is enacted.  First among these is the requirement to minimize the energy use of a building (no erecting of enormous solar arrays on a built-to-code house just to reach net-zero energy use.).  This is followed by the requirement to maximize passive energy gains, for example, heat from the sun and throw-away heat from appliances and electronics.  The third requirement, a measure of last resort, is to make use of the most energy-efficient mechanical systems available to bridge the gap between the energy demand and passive energy production of a building, and to sufficiently vent the building in order to meet the goals of occupant comfort and building durability.

At the core of PHIUS certification plan is an exacting set of metrics.[1]  The flexibility of the PHIUS system lies in the fact that PHIUS is not concerned with how these metrics are met, only that they not be shirked.  The rigor of the PHIUS system, on the other hand, lies in the fact that there is zero room to nuance, work around, or in any way dodge the meeting of any of these metrics.  Every choice of design and layout, detail and assembly must be made while keeping in mind the impact that choice will have on the metrics. 

So, what are those metrics?  No matter the climate, the size, the use, or the construction type of a building, the following metrics must be met:[2]

1. Strict limit on net source energy demand:  3840 kWh/person/year. 

This is the mother of all metrics, a well-defined limit on the net energy per person a building can consume in its operation.  Notice here that PHIUS is not expecting net zero energy demand.  That is the eventual goal, but PHIUS, in a flash of realism, recognizes that society needs some time to get there.  So Passive House-Certified buildings today can be net consumers of energy.  Does that make this standard less rigorous than a Net Zero Energy (NZE) home?  Has PHIUS lost its rigorous German pluck?  Not necessarily, and here is why.  There is no standard for NZE house.  The generally accepted definition of a NZE home is that it generates as much energy as it consumes.  Logical enough!  But energy consumption is a complex dynamic and that can obscure the actual meaning of NZE.  Key to unpacking that complexity and overcoming the obscurity is the crucial distinction between site energy and source energy.  Simply put, site energy is the quantity of energy used on site, i.e., the energy demand of the building.  Source energy, sometimes referred to as primary energy, is the total amount of energy required to meet that demand; this is almost always a higher number than site energy, and it is a really significant higher number when looking at energy coming from the electric grid, especially when that grid is being fed primarily by coal-fired plants.  Organizations that study energy costs publish continuously updated lists of ratios that define the relationship between site and source energy for different fuels and energy delivery systems.  Over the past ten years the site to source ratio for grid electricity has fluctuated between just under and just over 1:3, in other words, it takes roughly three units of source energy to generate and deliver one unit of site energy.[3]

What this means for the comparison between a NZE building and a Passive House certified building is that a NZE building, based on site energy consumption, could be using more than double the amount of source energy than a Passive House building that is net-positive consumer of energy.  In the minds of those developing the PHIUS standards, the crucial goal is to reduce total, society-wide energy use, and the source energy metric more precisely measures exactly that. PHIUS is to be applauded for employing the more accurate and honest means of measuring energy use.

2. Air Tightness of building envelope:  <­­­ 0.06 cfm/ft² of envelope @50pa.

The mathematically gifted among you may have already calculated to the conclusion that this standard is less exacting than the .6 ACH₅₀ typically associated with Passive House.  And that is indeed the case, at least for buildings that come in under 10,000 square feet.  But for larger buildings this is a more rigorous standard requiring a more tightly sealed enclosure.  The researches at PHIUS found that larger buildings required this tighter standard in order to properly manage moisture issues and safeguard the buildings’ long-term durability. These kinds of revisions speak volumes about the seriousness of the people involved with developing PHIUS.  Their research does not stop with setting strict standards.  They study the impact of these standards and make appropriate changes when real-world, empirical findings indicate a need for change.   

3. Limits on heating/cooling demand and loads.

The final four standards specify the heating and cooling energy demand, and the specifications are broken into four sub-categories, or four different ways of measuring the energy needed for space conditioning.  These four categories are,

• Specified Annual Heating Demand (kBTU/ft².yr)
• Specified Annual Cooling Demand (kBTU/ft².yr)
• Specified Peak Heating Load (kBTU/ft².hr)
• Specified Peak Heating Load (kBTU/ft².hr)

These four standards are complementary and are climate-specific.  This fine degree of both climate and demand/load specificity, helps PHIUS to balance its goals of making buildings as energy-efficient as possible on the one hand, but as easily and economically as possible on the other.  This is all so smart, so well-conceived. 

In the final analysis, PHIUS shows promise because it is precise and demanding and yet also quite accommodating: like Elizabeth Warren in that grainy campaign video of her at home, drinking a beer, accommodating her voters, all the while speaking in great detail about one of her extremely well-thought-out plans to fix medicare or limit corporate power, or even resolve the climate crisis. 

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[1]For the current Energy Star calculation of this ratio go to:  https://energystar-mesa.force.com/PortfolioManager/s/article/What-are-the-Site-to-Source-Conversion-Factors-1600088532330

For further description of source vs. site energy and a calculation of the ratio from 2010 go to:  https://www.buildingscience.com/documents/digests/bsd151-understanding-primary-source-site-energy

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[2] For a further elucidation of these metrics see the PHIUS Passive Building Standard Guidebook, which can be viewed at the following link.  https://www.phius.org/phius-certification-guidebook

[3] There is some climate-based variability to the standards depending on building use and construction type, but the type and degree of variability have been carefully justified according to most recent developments in building science and understanding.  For details on the variations in metrics see the PHIUS Passive Building Standard Guidebook cited above, p. 19.