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HERS Rating a Passive House

An Innovation Design Request filed with RESNET is helping this Iowa DOE Zero Energy Ready Home and passive house project acquire a more accurate HERS Index.

By Celeste Yeager Karan, George Sullivan and Bob Chomko

A NEW BUILDING IS TAKING shape at Full Revolution Farm, a “micro eco-farm” located just outside of Des Moines, Iowa. This project, which will include bed-and-breakfast accommodations, is being built to the standards of Passive House International, ENERGY STAR 3.1 and DOE Zero Energy Ready Home (ZERH), which includes the standards of the EPA’s WaterSense and Indoor AirPLUS programs. This ambitious 9,527-square-foot project represents a collaboration between motivated homeowner Celeste Yeager Karan, Bob Chomko, RESNET QAD for the Building Science Institute, and George Sullivan, principal at Eco Smart Building. Since the project broke ground, Sullivan’s company has since been reorganized into Net Zero Analysis & Design Corp. to better reflect the company’s revised focus and capabilities.

A Net-Positive Solar Home

The project at Full Revolution Farm is designed as a passive solar home, but because of its mechanical system, in actuality the entire structure operates as an active solar thermal collector.

The home has no annual heat load, but rather a continuous cooling load throughout the year. This was achieved through an envelope that has no thermal bridging, thorough air-sealing, a significant amount of insulation, strategically placed windows and an advanced mechanical system.

Full Revolution Farm

I’VE LONG WANTED TO build the most energy-efficient and sustainable home possible. I wanted a home that could function off grid, even if that wasn’t allowed by zoning code, and used relatively little electricity and water, yet provided a high level of comfort. I became fascinated with the subject 20 years ago while taking an environmental science class in college, and it was something I had planned to do with my husband at “some time in the future.” When I found myself widowed, I knew I needed to rebuild my life around caring for my young children and creating a home-based business near supportive family. I bought an acreage at the edge of the Des Moines, Iowa, metro area that was an ideal site for a small bed and breakfast, and the idea for Full Revolution Farm was born. I put together a team of friends and acquaintances I had gained over a decade of involvement in the Chicago real estate industry and organizations dedicated to “green” building. I had peripherally worked with George Sullivan and Michael Realmuto of Eco Smart Building and knew they had the experience and knowledge to create a cutting-edge building design. The design process began with a discussion of the multi-use nature of the building. The home is meant to accommodate a multi-generational family and paying guests by organizing public and private space on different levels. The design takes into account the building orientation (south and slightly east), soil type (heavy clay), microclimate (Zone 5B Marine) and shading factors, such as trees and the water tower. We were able to finish the full design and some necessary redesigns during construction before Mr. Realmuto’s untimely passing. Bob Chomko of the Building Science Institute came on board to provide testing, additional modeling and third-party verification of building performance. By Celeste Yeager Karan

Properly sized and shaded south-facing windows allow for a large amount of solar radiation into the home. Heat is also captured from normal human activity, mechanical equipment and appliance operation. This energy is collected via the multi-zone Mitsubishi CITY MULTI Variable Refrigerant Flow (VRF) geothermal system, which uses two 125-gallon solar thermal domestic hot water tanks as primary heat storage; the mass of the lower-level slab floor functions as secondary heat storage. After both primary and secondary heat storage is met, the CITY MULTI system will use the exterior glycol-filled geothermal field as a heat sink. An earth tube system (open-loop ground-to-air geothermal system) coupled with an energy recovery ventilator (ERV) provides ventilation and primary cooling.

We installed a 12-inch layer of high-density insulation under the basement slab and between the inner and outer walls of the double foundation. Twelve-inch-thick custom designed and engineered panels (CDEPS), manufactured without internal framing members, were applied as a curtain wall to the exterior of the home’s wood frame. Spray foam was used for air-sealing after the CDEPS were installed.

This design allows for net-positive energy production and therefore, a negative HERS rating.

Pushing the Envelope

Building performance modeling is an integral part of the design process, as it helps design professionals simulate design decisions and analyze the strengths and weaknesses of various choices. Building models are used to estimate a preliminary HERS Index, which offers a projects’ decision-makers better tools for cost/benefit analysis.

When we sat down at the dining room table in 2010 to begin the design, we had no idea how difficult it would be to get a HERS rating that accurately matched the performance of the house. There is no suitable group of mechanisms in most approved software programs to model a house that uses solar thermal gains and internal heat exchange for nearly 100 percent of its heating and supplies its cooling needs with an ERV/earth tube system. After discovering the limitations of the RESNET-approved modeling programs, we tried to work around them. We sought out more obscure software, contacted manufacturers for help and wrote our own in-house models for domestic water supply systems, cold water sprinkler systems and the earth tube system, which is based on actual data we collected. Finally, we were ready to submit an Innovative Design Request (IDR) to RESNET and ask for the house to be assigned a HERS Index that reflects actual calculations.

Why File an IDR?

As energy codes evolve, homes are built to higher efficiency standards. We are discovering many points of energy consumption that were not previously considered significant enough to quantify and include in the Rated Features of a Home. As homes’ overall energy use is reduced, the previously “insignificant” energy use now comprises a significant percentage of the homes’ overall energy consumption. In addition, new building technologies and innovative systems have emerged that are not yet assigned inputs in the RESNET-approved building models.

The Innovative Design Request (IDR) process is an attempt to refine the HERS Index where new technologies, design innovation and features of energy consumption previously were not quantified as a rated feature of the home.

An IDR must be submitted by “an approved RESNET-accredited quality assurance provider who is listed in good standing in the National RESNET Registry.” Net Zero Analysis & Design Corp., a RESNET-accredited Quality Assurance Provider, and Building Science Institute Inc., our RESNET quality assurance and training provider, submitted the IDR for Full Revolution Farm.

We looked at several of the major components that were and were not covered in the RESNET Standard:

  1. Live cold water sprinkler system
  2. WaterSense-designed domestic hot water system
  3. WaterSense-designed domestic cold water system
  4. Windows installed in the insulation layer with over-insulation on the window frame
  5. Solar thermal greenhouse
  6. ERV model in REM/Rate
  7. Earth tube system model
  8. Variable refrigerant flow (VRF) mechanical and domestic hot water system model

All of the above systems were found to be contributing factors in reducing the HERS Index of the home.

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Power House

The IDR process could adjust Full Revolution Farm’s HERS Index from 23 to -2 before the addition of photovoltaics.

EPA CORRECTION:The chart above shows that a ENERGY STAR certified home earns a HERS index of 85. This is actually incorrect. Under the current ENERGY STAR program requirements, there is no single HERS index associated with a certified home. Instead, the ENERGY STAR HERS index target is determined for each individual certified home. However, for homes certified in most states, the target tends to fall between a HERS index of 65 and 75. In states with the more stringent 2012 or 2015 IECC, the target tends to fall between 55 and 65. Both of these ranges are significantly more stringent than the value of 85 indicated.

Custom Models

The IDR process is set up to allow for unique building elements or systems that reduce the project’s energy use to be recognized in the home’s HERS Index. When analyzing Full Revolution Farm, we looked at all of the building systems compared to the RESNET Standard for rated features of the home, DOE ZERH, ENERGY STAR 3.1, Passive House International Standard, WaterSense, Indoor airPLUS and local building code requirements. The modeling of Full Revolution Farm incorporated THERM models, EnergyPro models, REM/Rate models, “Manual J” heating and cooling load calculations and proprietary models that we developed in-house.

We reviewed local building code for systems that are required and modeled their energy impact to ensure we met the Passive House Standard. The modeled impacts were then analyzed to mitigate any increase in energy use over the Passive House Institute Standard.

We then reviewed the RESNET Standard for rated features of the home and the RESNET-approved modeling software against the Passive House International Standard and the Passive House Planning Package (PHPP) model to identify differences between the two standards in the way they calculated energy use and balance. The comparison led us to a number of differences. We discovered that an IDR would be necessary to account for the previously mentioned eight systems, because they were not covered in the RESNET Standards or in RESNET-approved modeling software.

We also discovered that, although several of the home’s systems are included in modeling software as rated features of the home, applicable modeling inputs for the systems were not available, and using pre-set inputs resulted in an obviously skewed HERS Index.

Innovative Systems and Uncharted Territory

Each of the eight systems required different modeling strategies to calculate the loads of the system or the reduction in energy use. The REM/Rate model was then adjusted to account for the load or energy use reduction.

Domestic Water Systems The domestic hot and cold water systems are normally not accounted for when calculating energy use in a building. We discovered that there is actually quite a large energy loss associated with them, and if that energy loss is added to the HERS Index, it has a significant negative impact on the rating. If all water pipes are insulated to R-6, however, there is no negative impact on the HERS Index in cold climates.

Window Installed U-Values Window installation has a major effect on the labeled U-value of the window. Windows installed in contact with exterior building finishes such as brick, for example, will have a U-value of 1.0, regardless of the stated value from the manufacturer. The project’s windows are wood-framed argon-filled triple pane with a stated window U-value of 0.177. By using an innovative window installation technique that we developed, the window’s stated U-value was decreased to 0.067—in other words, it was cut in half. Windows are installed mid-wall per Passive House Standards and then over-insulated to thermally break the frames from the exterior. THERM model results were reviewed by Passive House International for accuracy. This window installation method decreased the HERS Index score by 3 points.

Solar Thermal Collector Full Revolution Farm has a solar thermal greenhouse which is conditioned by a Mitsubishi air-to-water head. This was modeled as a solar thermal vacuum tube, per manufacturer specifications, for sizing the vacuum tube collector to the two 125-gallon solar thermal DHW tanks. This design was reviewed by a solar thermal vacuum tube manufacturer, who also suggested an adjustment based on heating degree days to reflect the actual performance of the Mitsubishi air-to-water head in the space. The adjusted solar thermal system was modeled in REM/Rate and the HERS Index decreased by 5 points.

ERV System Impact We asked the ERV manufacturer UltimateAir to review the ERV energy input into REM/Rate for their RecoupAerator 200DX. The modeled performance in REM/Rate did not match the manufacturer’s performance curves. The manufacturer suggested a calculation that would correct the inaccuracy of the performance. We performed the calculations and adjusted the energy inputs in the REM/Rate model. The adjusted ERV system was modeled in REM/Rate and the HERS Index decreased 3 points.

Earth Tube System Model The project has an open-loop air-to-soil geothermal system before the ERV; this system heats or cools intake air to 55° F. The modeling of the ERV/earth tube system was reviewed by the ERV manufacturer UltimateAir, which has a number of these system in the field, and they have approved the calculations and energy impacts. Our proprietary earth tube system model was modeled in REM/Rate and the HERS Index decreased 8 points.

Variable Refrigerant Flow (VRF) HVAC-Modeled System Neither REM/Rate nor EnergyGauge have the ability to model VRF mechanical systems. Mitsubishi Electric Company recommended that the system be modeled in EnergyPro to determine an accurate system performance in the modeled project building. Mitsubishi Electric Company reviewed the EnergyPro model for accurate VRF input and function, and found that their equipment is correctly modeled in the Full Revolution Farm EnergyPro model. The only exception is that the domestic hot water system cannot be modeled in any of the current energy modeling software.

The EnergyPro model revealed that the building has a negative heating load. The building’s as-built shading features were input into the Manual J model, which was uploaded into both models (REM/Rate and EnergyPro) for this example and comparison. Next, the energy use of the VRF system, as modeled in the EnergyPro model, was used to adjust the REM/Rate model. When the VRF HVAC system was modeled in REM/Rate with the adjusted values from the EnergyPro model, the HERS Index decreased 6 points.

Impact and Next Steps

The overall impact of our analysis was a 25-point decrease in the project’s HERS Index. The HERS Index of this project is negative without using photovoltaic or small wind (5 kW systems) for onsite power generation (see Chart). There are a number of additional systems in this project which, once analyzed, are likely to lower the HERS Index even further.

But an approved IDR has a greater impact than adjusting one project’s HERS Index; it allows us to adjust the HERS Index of other projects that use any of the eight systems that are outlined in it. The IDR allows us more tools to accurately model low-energy projects in general.

It includes a request for three measures:

  1. “As-installed windows” vs. NFRC ratings
  2. Earth tube pre-heater installed with ERV
  3. The Mitsubishi ground-source heat pump with variable refrigerant flow

Once an IDR is submitted to RESNET, the committee has six weeks to review all of the models and supporting paperwork and to respond to the request. During that time, they may ask questions and request additional supporting documents. The current set of eight systems that we have reviewed all have an impact on the RESNET standard for the rated feature of the home and on the current set of RESNET-approved models. We will be submitting these items to the RESNET Standards Committee to allow passive homes and zero-energy-ready homes to be properly modeled and assigned correct HERS Indexes.

We are in the final steps of preparing several other innovative design requests. To date, there has not been a detailed analysis of a similar project submitted as an IDR to RESNET. In fact, since 2013, only seven IDRs have been submitted, and two of those submitted were not approved. The time commitment of hundreds of man-hours required to thoroughly investigate building modeling issues is often an impediment to teams who have valid concerns about the accuracy of a building’s HERS rating.


Progress on Full Revolution Farm can be followed on the project blog.

George D. Sullivan is the Sr. Principal and CEO of Net Zero Analysis & Design Corp., a commercial & residential design build energy consulting company. Mr. Sullivan started his company in 2015 after realizing that his former company Eco Smart Building LLC could no longer meet the market challenges and needs of the Building Market after 24 years in business. He has been working in the area of energy efficiency in buildings since 1992. He has a Bachelor’s degree in General Engineering and Biology as well as a Masters in Engineering Physics. Mr. Sullivan is passionate about energy efficiency, sustainable building practices, renewable energy systems, passive solar building design, product development, and helping companies in the home energy efficiency industry grow and prosper.

George has also published articles and has been extensively quoted in numerous publications: the Wall Street Journal, US Green Building Council’s newsletter Green Bytes, Chicago Tribune, Chicago Sun Times, Illinois Real Estate Review, Lerner Newspaper, The Chicago Reader and Rogers Park 2000. He has been selected by the Chicago Chapter of the American Institute of Architects to participate on the Review Board for the City of Chicago Building Department’s Green Building Code and Green Permit Process.

He has been involved with a number of lecture tours in Japan and USAID series in Jordan, covering energy efficiency, renewable energy system design, Passive House Design, and RESNET Testing and Certification of Energy Efficient Buildings.

Recent awards George has received: City of Chicago’s Department of Environment Award for Rooftop Gardening, United States Environmental Protection Agency and Chicago Wilderness’ Conservation and Native Landscaping award, and Rogers Park Community Council’s Spirit of Rogers Park Award for Environmentalism.  

Bob Chomko, President and Director of Building Science Institute, has amassed a wealth of experience in construction and remodeling over the past three decades and earned a stellar reputation as an energy efficiency and building science consultant.

Widely known as an expert energy efficiency consultant, energy modeler, and instructor. Bob’s passion is to promote energy efficient building science concepts and technologies that are cost effective, durable and provide high levels of indoor air quality. In 2011, he implemented his vision by forming Building Science Institute Inc., a not for profit 501(c)(3) teaching institution.

After earning his master’s degree in business administration from the University of Illinois, Bob got his start working in residential and commercial property development. His first project was in 1985 developing an early 1900 furniture store into a contemporary restaurant/banquet facility. He has also worked as an insurance general agent for 25 years and more recently as a general contractor.

In 2002, Bob launched BSC Ltd. dba Home Services and Consulting, a Hinsdale-based company that provides general contracting, home and small business energy audits, weatherization services, contractor consulting and training and educational seminars. Most recently Bob served as the exclusive energy rater for energy audit programs of the Village of Schaumburg and City of Wheaton and partner with the City of Chicago in their Home Energy Efficiency Training Program. He’s also taught at Moraine Valley Community College as an associate professor teaching building science.

Bob has been trained as a NAHB (National Association of Home Builders) Green Verifier, and ACCA (Air Conditioning Contractors of America) EPIC Instructor (HVAC system and duct design modeling software). He was certified as a Passive House Consultant, and Residential Energy Services Network (RESNET) energy rater trainer and quality assurance designee, EPA’s Energy Star for Homes v3 trainer, DOE Zero Energy Ready Homes trainer, and the Department of Energy’s BECP Train-the Trainer – 2009-2012 IECC and ASHRAE 90.1-2007. In December 2010 Bob’s application for RESNET Rater Training Provider was approved under the name of Building Science Institute Inc. Other training includes: Trace 700 HVAC design software, load design and ASHRAE 90.1/LEED; Mitsubishi Electric ‘City Multi’ Service Course.

Bob served on the Board of Directors of the Passive House Alliance Chicago, the Illinois Association of Energy Raters (IAER), AUYA a not-for-profit youth organization and First Security Federal Savings Bank.

His most current project is the construction of the first Net Zero Ready and Passive House building specifically built to be opened to the public in order to highlight high performance building design concepts. The project is located on the grounds of Sandburg High School in Orland Park, IL. During the day the facility will be used by students and after school hours the building will be open to the public.