Why PHIUS? Because Climate Specific Design = Quality Assurance

Why do you build to PHIUS Standards?

Asked at the 2017 North American Passive House Conference in Seattle, WA.

Lindsey Elton, ECO Achievers:  …E-L-T-O-N like Elton John…

I believe in PHIUS because the organization has taken an extremely detailed look according to our climate zone of what it takes to build a net-zero home or a net-zero building.

Peter Marciano, Legacy Buildings, New York, NY: I’ve come to the conclusion based on what I’ve built that there’s a lot of information out here. There are several passive house programs available. And, for me, having come to the realization that it has to be climate-specific because that’s what works. That’s what works in this nation. That’s what works in this country.

Marc Rosenbaum, EnergySmiths, West Tisbury, MA: I’ve been doing this for almost 40 years, and one of the things that PHIUS brings to the table here that are so amazing to me is people are interested in the actual performance of the buildings. They’re measuring them. They’re comparing them to what they thought they should do, and it’s a really terrific community that is sharing the information to make better buildings.

Elton: They’ve taken all the guesswork out, they’re doing the calculations, and they were smart about it. And we can employ this time after time after time again.

Marciano: If it’s not climate specific, I have had definite problems with certain aspects of my enclosure and certain aspects of my building. And I wouldn’t make that mistake again. I would definitely use a climate specific standard to establish… To build my next passive house.

Rosenbaum: And I think we all know why we’re doing it. We’re doing it because we care about the climate, we care about the kids, we care about other species besides ourselves. And we don’t talk about that. We talk about BTUs, and thermal bridges, and solar heat gain coefficients.

Elton: We’re firmly behind it, our company’s firmly behind it. We believe in it, and that’s why we’re here.

Rosenbaum: PHIUS has really created this community of people, who I think, care about each other’s learning, and share our successes and our failures, and it makes all of us better.

PHIUS+: The path to positive energy

Become a PHIUS+ Professional and be a leader in the industry

Comparing ASHRAE 90.1 Appendix G / PHIUS+ / Passivhaus methods and standards

– James Ortega, PHIUS Certification Staff

The New York State Energy Research and Development Authority (NYSERDA) promotes energy efficiency and the use of renewable energy resources. NYSERDA has been at the forefront of promoting adoption of passive building in NY State. For example, over the past several years, NYSERDA has partnered with PHIUS and other organizations to offer qualified students substantial fee reductions for passive house professional training.

For the past year, PHIUS has collaborated with NYSERDA on a vital study of multifamily buildings in New York. The results of that study were released this past October, and we’re excited to share some of the summarized highlights that show we are on the right track with PHIUS+.

NYSERDA conducted the study to compare standards and methodologies to inform its development of the Multifamily New Construction Program (MF NCP). This program follows the goal of 40% reduction in greenhouse gas emissions that New York State set as part of the Clean Energy Fund (CEF), one of the pillars of the Reforming the Energy Vision (REV) program launched in 2014.

The study is part of NYSERDA’s exploration of alternative approaches and standards to promote high-performance buildings to supplement the ENERGY STAR® Multifamily High-Rise (MFHR) program, which NYSERDA has supported in the past.

The goal of the study is to create equivalent building performance targets for each certification program with the intent that the submitted projects achieve similar energy performance to qualify for incentives offered through NYSERDA’s new MF NCP, regardless of which energy modeling tool and protocol are followed.

Comparing ASHRAE 90.1 Appendix G_PHIUS+_Passivhaus methods and standardsStudy Description

This case study emulated a typical high-rise multifamily building designed and constructed in New York State based on the DOE/PNNL Prototype Models. The five design cases created and modeled using the three tools and protocols described above are listed below[i]. You can also review a fully detailed description and full PDF version of the NYSERDA report here.

Energy Modeling Tools

The study looks at and compares three different approaches and energy modeling tools and protocols.

  • The MFHR program uses the Quick Energy Simulation Tool (eQuest) in combination with ASHARE 90.1 Appendix G Performance Rating Method as the modeling tool and protocol
  • WUFI® Passive is used by the Passive House Institute US (PHIUS) to model performance
  • The Passive House Planning Package (PHPP) is used by the Passivhaus Institute (PHI)

Note: There neither was nor is an expectation that the three different methods would yield equal energy usage estimates because each protocol has different assumptions for operating conditions.


The Base Case and Packages A-C have prescriptive envelope and mechanical systems, but some assumptions such as occupancy, residential plug loads and residential lighting patterns vary based on the different modeling protocols from each organization. As shown in Figure 1 below, the difference in results between protocols is substantial.

For example, the PHI protocol estimate for source energy usage is nearly half of the Appendix G cases. This discrepancy stems directly from the PHI assumptions for residential plug loads and lighting.

Figure 1: Annual Source Energy by end use and Protocol (Using EPA Portfolio Manager Site-to-Source Conversions)

Figure 1: Annual Source Energy by end use and Protocol (Using EPA Portfolio Manager Site-to-Source Conversions)

The NYSERDA report explicitly calls out the plug loads category. First, while acknowledging that some difference in plug load assumptions were to be expected, the report finds the size of the difference troublesome, suggesting that the PHI assumptions are overly optimistic and warrant review.

From the report:

            Variations in this category [plug loads] are expected due to differences in the protocols’ assumptions, but the magnitude of the difference appears excessive, and warrants further investigation. Aside from affecting the total electricity consumption, plug loads interact with heating and cooling and significantly impact the total energy use of high-performance buildings.

The report summarizes that:

            PHI defaults for in-unit lighting and appliances energy use are significantly more optimistic than presented in references such as Building America and COMNET, which contributes to a much lower EUI projected by the protocol for equivalent design. It is recommended that PHI review those default assumptions made regarding multifamily buildings in the U.S.

Packages D-F designs include changes to envelope and mechanical systems by PHIUS and PHI to comply with their respective standards. The proposed design changes by PHIUS and the PHI for these cases are translated into eQuest to investigate the associated energy savings/difference that eQuest registers. There are some exclusions to these proposed changes that are left out of the eQuest model. For example: Both the PHIUS and PHI models incorporate interior blinds to incorporate passive cooling, but Appendix G protocol does not allow manual controls to contribute towards savings.

The difference in Source Energy by End Use between PHIUS and PHI improvements as modeled in eQuest are minimal as shown in Figure 2 below. However, the difference in envelope assumptions are not (particularly windows and airtightness) as shown in Figure 3.

The PHIUS Package (D) uses windows with overall U-value of ~0.28 Btu/hr.sf.F and an airtightness limit following the PHIUS protocol of 0.05 cfm/sf @50Pa. The equivalent infiltration at natural wind pressure is 0.017 ACH. The PHI Package (F) uses windows with overall U-value of 0.15 Btu/hr.sf.F and an airtightness limit following the PHI protocol of 0.6 ACH @50Pa. The equivalent infiltration at wind pressure is 0.05 ACH. The difference in required window U-value between these two models has significant cost implications in that the PHIUS modeled window performance could be met with high performing double-pane windows, while the PHI modeled window performance could only be met with triple-pane windows.

When it comes to airtightness, Appendix G protocol assumes an infiltration at natural wind pressure of 0.186 ACH. PHIUS protocol mandates an airtightness 10x tighter than the Appendix G Baseline for this building (0.017ACH). The airtightness is specified for this particular building because the PHIUS limit is per square foot of envelope area, which equates to varying ACH values with different building sizes. While the PHI target of 0.6ACH @50Pa may be comparable to the 0.05cfm/sf @50Pa target of PHIUS for small buildings, for large scale buildings such as used in this study, the PHIUS target is 3x tighter than the PHI requirement. (0.017ACH PHIUS vs 0.05ACH PHI). This difference in requirements has important ramifications for building durability as well as energy loss.

Figure 2: Energy by Protocol and End Use (Using EPA Portfolio Manager Site-to-Source Conversions)

Figure 2: Energy by Protocol and End Use (Using EPA Portfolio Manager Site-to-Source Conversions)

Figure 3: Modeled Configurations - Packages D-F window U-values and airtightness

Figure 3: Modeled Configurations – Packages D-F window U-values and airtightness

This comparison study is an important step for financing bodies and agencies to meaningfully evaluate performance and incentivize passive buildings. It also serves as a solid case study to build upon for other building types and sizes across the country. As larger passive projects continue to develop, monitored data will become an important assessment tool to verify the modeled results of comparison studies such as this one.


[i] Base Case: ASHRAE 90.1-2010 Appendix G Baseline Design

Package A: Base Case modified to include building components found in better performing MF NCP projects.

Package B: Package A with exhaust air heat recovery in corridors

Package C: Package B with Variable Refrigerant Flow (VRF) heat pumps in apartments

Package D: PHIUS design to meet PHIUS+ 2015

Package E: PHI Team 1 design to meet PHI Standard

Package F: PHI Team 2 design to meet PHI Standard