Getting to Zero Part II: PHIUS+ 2018

Graham S. Wright, Senior Scientist & Product Program Manager

PHIUS took a leap a couple of years ago with the launch of the PHIUS+ 2015 Passive Building Standard: North America. It’s been great to see how well the standard was received, and the number of projects seeking certification is way up.

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Back in 2015, we committed to updating our standard periodically. Earlier this year, the Technical Committee began to discuss a PHIUS+ 2018 update, and we are now gearing up for some number crunching. In this post I’ll give an overview of what’s likely to change and what’s not.


PHIUS+ is a “performance based” energy standard mainly concerned with the operating energy of buildings, meaning compliance is largely determined by energy modeling and thus emphasizes designing for low energy use. As such, it’s a great compliment to other green building rating systems and standards such as LEED, NGBS, Living Building etc. In addition to the performance modeling requirements there also a number of “prescriptive” requirements having to do with quality assurance that have to be met as well. Compliance-wise, it’s a pass/fail standard rather than a “rating system.”


What makes PHIUS+ a passive building standard is that it sets limits not just on overall predicted energy use, but also specifically on the heating and cooling loads. Those targets guide the designer to use passive measures first, upgrading the envelope and limiting ventilation losses to reduce the annual heating and cooling bills and the size of the required heating and cooling systems— passive measures are a formal/structural priority for the standard.


Thus the standard has three pillars:

  • Limits on heating and cooling loads (peak/design and annual)
  • Limit on overall operating energy (source not site)
  • Air-tightness and other prescriptive quality requirements


As mentioned in the PHIUS+ 2015 development report published by the DoE, the limits on heating and cooling loads and the overall source energy limit are based on different underlying principles. The annual heating and cooling limits are based on economic optimization, minimizing the total energy-related costs (utility bills plus the annualized cost of the energy-saving upgrades). The annual criteria are a reasonable indication of where to stop investing in passive measures, denoting the economic sweet spot between conservation and generation. The peak heating and cooling loads assure thermal comfort and limit the size of active systems needed to maintain comfort. The resulting targets for both annual and peak loads are climate-dependent. The limits are absolute numbers in terms of energy/power use intensity per square foot of floor area. The targets are not relative to a baseline, and do not follow a “percent better than code” approach .


The overall source energy use, however, represents the building’s impact on society, the national environment and the global climate. Therefore, source energy use is allocated on a fair-share principle: a certain amount per person for residential and per square foot for nonresidential, plus allowance for industrial/process loads. This does not vary by climate, and is not subject to economic analysis. When it comes to the environment, we are under a global emissions cap and must do what is necessary to meet the reduction goals.


Most of the meta-structure for PHIUS+ described above will stay the same for 2018, but there will be changes that will touch all three pillars. Here are the possible/probable changes, pending input from members and final Tech Committee voting:


On the heating and cooling front, the criteria will become more nuanced and will vary by building size and occupant density. The adjustments for size might be categorical, i.e. mid-rise versus high-rise, and/or key on a continuous variable such as envelope to floor area ratio. Because the heating and cooling criteria are lifecycle cost-optimization based, they need to be updated periodically as the cost and performance of energy-saving technologies changes. A lot of the development effort will go into the heating/cooling criteria because many cases need to be set up and run in different climates. In terms of comfort constraints, the criteria will again be predicated on strict limits on window U-value, and the assumed humidity ratio setpoint might be lowered from its current value of 0.012. The analysis period may be increased from its previous value of 30 years, but a discount rate will still apply.


We also aim to implement new calculation methods in WUFI Passive for solar shading and non-residential peak cooling load. This will likely mean only WUFI Passive energy models can be used for certification under PHIUS+ 2018.


On the source energy front, the limits will begin tapering to zero over the coming decades. In PHIUS+ 2015, only onsite renewables with storage or coincident production-and-use count as offsets to the energy usage; therefore, tightening the criterion to zero would effectively force all buildings to go off-grid. This is not a practical route to Zero Emissions or 100% renewable for the building sector, particularly in cities where larger buildings with limited onsite potential prevail. Therefore, for PHIUS+ 2018, off-site renewables will also be allowed to offset source energy use.  


Architecture 2030’s Zero Net Carbon (ZNC) building definition allows for procurement of off-site renewable energy, though it doesn’t go into any hard-and-fast rules. The US Dept. of Energy has a definition for both a Zero Energy Building (ZEB), which is source net zero with only onsite renewables, and a Renewable Energy Certificate Zero Energy Building (REC-ZEB), which also allows renewable energy obtained via traded certificates. Both of these definitions recognize that, at some point, the building designers have done all they can to reduce energy use with conservation and on-site renewables and the responsibility to get to zero emission or 100% renewable energy shifts to the building’s energy providers. This now involves building occupants who can choose their energy providers.


Renewable energy certificates are currently quite cheap. For PHIUS+ 2018, the idea is to impose a limitation on their use so on-site efficiency measures do not go by the wayside. This will probably take the form of an REC allowance with several terms: a fixed amount available to any residential project to get from 6200 to 4200 kWh/person, an amount proportional to the number of stories (that is, the ratio of floor area to roof area), plus RECs could be used for any process loads.


Also, with respect to the use of local source energy factors for grid electricity and other fuels, national averaging will likely remain the rule but will align with Energy Star Portfolio Manager, which has different values for the US and Canada.


On the prescriptive requirements and quality assurance front, we aim to publish a new protocol to cover non-residential buildings in addition to the current single-family and multi-family inspection workbooks. There are a number of existing non-residential building commissioning standards and guidelines that already cover  many items. These are being reviewed to create more focused synthesis that will cover items that bear on the energy performance.


Also, spray foam with low-global-warming-potential HFO blowing agent has become more available, and the use of high-GWP foam may be restricted.


As we did last time, the intent is to have PHIUS+ 2018 ready for pilot projects starting in the 4th quarter of this year, with a formal launch in March 2018.


Looking forward to your comments!