Home Energy — November December 2012
In 2007, project planning began on a house built to the Passive House (PH) standard, in Urbana, Illinois. The project was initiated by the Ecological Construction Laboratory (e-co lab), a nonprofit community housing development organization interested in bringing the PH standard to the United States. It became a joint research effort when IBACOS was brought in to lead instrumentation and monitoring through DOE’s Building America program. (For more information, see “About IBACOS.”)
At the time, the PH standard was far beyond what Building America was trying to achieve. The PH standard dictates very low energy consumption: 15 kWh per square meter per year (/m2/yr) thermal heating, 15 kWh/m2/yr thermal sensible cooling. It also requires that the house use no more than 120 kWh/ m2/yr for whole-house source energy, including lighting, heating water, and plug loads.
Passive Houses are typically well insulated and airtight, and they use passive solar and ventilation to keep temperatures comfortable without much assistance from a heating or cooling system. A key difference between a PH and a net zero energy home is that PHs cannot use on-site energy generation, such as PV systems, to off set that 120 kWh requirement.
The two-story, four-bedroom, 1,324 ft 2 home was designed by Katrin Klingenberg, a founder of e-co lab and the cofounder of the Passive House Institute U.S. (PHIUS). E-co lab completed construction of the house in early 2008 in an established neighborhood in Urbana, but due to the housing downturn, it was not finished and sold until early 2009. For the next two years, IBACOS monitored performance of the house under normal living conditions, with a family of three—two adults and one child.
At the time the project started, the PH standard (or PassivHaus standard, as it is called overseas) was not well known in the United States, although it is more common now and is growing in popularity with architects and small-scale builders. The implications of this research go well beyond verifying the PH standard for use in the United States. As code requirements become more stringent, the lessons learned from this project could help builders find solutions to many challenges presented by homes with highly efficient thermal enclosures, regardless of whether or not they are building to the PH standard.
PH Performance in U.S. Climate Conditions
The PH standard draws on housing research done in the United States in the 1970s and ’80s, but it was conceived and tested in central Europe, a more moderate climate than Illinois. Since the 1990s, tens of thousands of buildings have been certified to the standard in Europe, and there’s a wealth of data on their performance.
The Illinois project team, which along with Klingenberg included e-co lab cofounder Mike Kernagis and construction manager Tim Gibbs, decided that one of the two main goals of the PH construction would be to generate more performance data to support the use of the standard on this side of the Atlantic. Specifically, energy performance of the house would be documented and compared to initial modeling, to find out if the predictions were as accurate in U.S. climate conditions as they were in Europe. With the guidance of e-co lab, IBACOS created an intensive testing and monitoring program and installed more than 100 sensors in the house.
The second goal of the project was to look at the performance of the single-point ductless mini-split heat pump unit (DHU), the equipment that was selected to cool the home. Although this was not a major goal, the project also tracked the heating performance of the DHU, with the thought that this type of equipment might take care of both heating and cooling in PHs.
Specifications and Details
General PH design principles are well established, but details of the houses can vary. In the case of the Urbana home, design choices focused on the thermal enclosure and HVAC system, but also considered the Illinois climate and the cost and availability of materials. Table 1 provides an overview of the home’s features.
The Urbana house has a thick wall assembly made from 12-inch wooden I-joists used as studs, which help minimize thermal bridging. Cavity bays were filled with fiberglass insulation, resulting in an initial R-value of 49.2. The outside of the wall assembly is sheathed in fiberboard, while the inside is sheathed in oriented strand board, glued and fastened to the I-joist studs to create the primary air barrier.
To accommodate plumbing and wiring without penetrating the air barrier, a 2-foot x 3-foot wall was built on the interior of the wall assembly and filled with cellulose, raising the wall’s R-value to 60.2. The walls rest on an insulated, thickened-edge, slab-on-grade foundation.
Ceilings were insulated with loose-fill cellulose applied to the attic floor, to give an R-value of 87. Continuous polyethylene sheets were attached to the bottom chord of the trusses, and joints were taped to form the air barrier. As with the walls, wiring was routed to the interior of the air barrier by creating a cavity from 2 x 4 lumber. These and other air-sealing measures resulted in a tested leakage rate of 0.45 ACH50.
The windows used in the Urbana house were among the best-performing units available in the United States at the time. They have triple glazing with lowe coating, inert gas fill, warm edge spacers, and insulated fiberglass frames, to achieve U-values of 0.16 for the southern orientation and 0.15 for other orientations.
PH design dictates that southfacing windows have a higher solar heat gain coefficient (SHGC) than other windows, because passive use of low-angle southern sunlight in the winter helps to reduce the heating energy the house requires. But the south-facing second-floor windows in the Urbana house also feature overhangs that fully shade those windows in summer. Windows on the east- and west-facing sides of the house have a lower SHGC to reduce the cooling load in the summer.
Heating and Ventilation
The heating and cooling loads achieved by the house are very low, on par with those of a small apartment. Traditional ducted HVAC systems were too large, so to provide space heating, 500W electric resistance baseboard heaters were installed in each bedroom, the living room, and the dining area. A central thermostat controls the heaters.
A 9,000 Btu/h, 20 SEER DHU was chosen to provide cooling. It is located in the stairwell of the house between the first and second floors, about 10 feet above the first floor. Although it was installed primarily for the purpose of cooling the house, the unit also has the ability to heat, an additional factor in testing. Continuous balanced ventilation is provided by an energy recovery ventilator (ERV), which supplies fresh air into the living room and bedrooms while exhausting from the kitchen and bathrooms.
Data were collected about the home’s energy performance of the Urbana home in each sub-submetered area and compared the results to Building America (BA) benchmark-modeled predictions and PH performance criteria. Data for the period May 2009 to June 2011 were collected for
domestic hot water (DHW);
major appliances; and
miscellaneous electric loads (also known as plug loads).
Temperature measurements were also taken in each room to assess the ability of the DHU to provide uniform room air temperatures throughout the house.
To determine if the ERV had a significant impact in circulating cooled air through the house, the temperature of the fresh air supplied to the rooms by the ERV was also measured, as well as outdoor temperature and relative humidity.
All sensors were connected to a central data logger and were sampled every ten seconds, with the data averaged on both a minute and an hourly basis.
Modeled versus Actual Energy Performance
Initial modeling predicted that the Urbana house would achieve 48% source energy savings compared to the BA benchmark reference house. Actual performance of the home was better than predicted by the soft ware used for BA benchmark analysis, with source energy consumption less than what was modeled in every subcategory. Heating-source energy consumption was 33% less than predicted, and cooling was 20% less. In general, the home’s temperatures also stayed close to the modeled set point values of 71°F heating and 76°F cooling. Table 2 shows modeled and monitored source energy consumption compared to the BA benchmark.
As with other high-performance homes, results showed that when heating and cooling loads go down, domestic hot water (DHW), lighting, appliances and plug-in loads become a much higher proportion of energy consumption. For example, in the Urbana house, data showed that DHW, provided by an electric resistance tankless water heater, were about one-third of total house source energy consumption. A heat pump water heater or fuel-fired tankless water heater might prove to be a better option for this type of home. Additionally, the significance of the other electrical loads in the house should not be dismissed. Since the heating and cooling load due to the natural environment is mitigated significantly by the thermal enclosure, the waste heat from electrical devices and activities within the house plays a larger role in heating and cooling energy requirements in the house. The thermal energy impact of these devices was not studied specifically in this house.
The PH standard requires houses use no more than 15 kWh/ m2 (4,754 Btu/ft 2) heating thermal energy, and data show that the home came in at 18.7 kWh/m2 (5,930 Btu/ft 2), or 25% over the limit. It’s important to understand, though, that this is a small difference, equivalent to the heat generated by one adult person at rest in the home 12 hours a day.
Keeping in mind that the values are for the thermal energy required to maintain comfortable indoor temperatures, not electrical energy consumed, the home also appears to have exceeded annual cooling thermal energy requirements by 122%. PHs are not supposed to need more than 15 kWh/ m2yr, and the Urbana house required 33.3 kWh/m2yr (10,600 Btu/ft2yr). This is obviously a significant difference. So was a mistake made?
Not necessarily. In this case, the numbers tell only part of the story. A portion of the overage is due to latent loads, or humidity, that isn’t accounted for in the PH cooling thermal energy requirement. And unfortunately, separating sensible loads from latent loads was impossible with the instrumentation used in the house. Future studies on the thermal characteristics of PHs will require more-detailed methods of measuring sensible versus latent energy output of the cooling systems.
Space-Conditioning System Performance
Another goal of the project was to assess the performance of the DHU. No traditional ducted forced-air system with the right capacity was available, so the DHU was the best option. In the absence of ducts, the hypothesis was that in a PH, heating and cooling energy will be distributed by the house’s natural convective currents and by conduction through interior walls.
Results supported this theory, showing that the system is effective in maintaining fairly uniform temperatures during the months of active cooling. Even during peak cooling conditions, rooms were generally within 3°F of the temperature at the thermostat.
However, there is a clear need for controls integration when DHUs are installed alongside electric baseboard heating systems. In this house, the DHU and the electric baseboards were controlled by separate thermostats. During one day near the peak of summer, the occupant set the set point of the DHU thermostat below the set point of the thermostat for the baseboard heaters (which was centrally located in the first-floor living space). The result was a few hours of simultaneous heating and cooling. The problem was not immediately obvious, since the DHU maintained its set cooling temperature even while the heat was on.
The controls used, in conjunction with the physical location of the DHU indoor unit, also affected the winter operation of the DHU. In a typical forced-air system, the temperature sensor, which triggers operation, is located away from the unit itself, but in this case the temperature sensor is located directly on the DHU. Because the DHU is located in a relatively confined stairwell space, it tends to heat the stairwell quickly and turn off. This issue influenced the family’s decision to use the heat pump infrequently during the winter.
The stairwell location of the DHU turned out to be problematic in another way. Since it’s above the first-floor ceiling, not enough heated air reaches the downstairs living space, even when the DHU runs for a full ten minutes.
Another hypothesis—that the conditioned air delivered by the ERV to the living space and bedrooms would help distribute cool air in the house—was not supported by the data. Instead, results showed that during the summer, ERV-supplied ventilation air was typically between 2°F and 6°F above indoor ambient temperatures, which slightly increased the cooling load in each room.
The results of approximately two years of data collection (May 2009 to June 2011) in the Urbana PH revealed some aspects of the project that performed as expected, and some surprises.
The home performed better than the BA benchmark models of source energy consumption predicted and came close to the thermal energy use required to meet the PH standard in heating.
Because the PH standard does not take latent heat into account, and the thermal energy consumed by the DHU couldn’t be accurately measured in a way that would allow for the separation of sensible and latent energy, a true comparison with the PH standard could not be performed for cooling energy.
Also, although the DHU was effective in cooling the home during the summer, the placement of the DHU in the stairwell, between floors, reduced the unit’s ability to heat in the winter.
Many questions remain. Since the single-point DHU was so effective at uniformly cooling all rooms in the house, it may be that some of the unmeasured elements of the project contributed to its performance. For example, although the ERV did not provide any cooling energy to the rooms it supplied, data were not collected on the contribution of the ERV system to keeping air moving.
The use of a single DHU along with electric baseboard heaters has emerged as a strategy for PHs, as long as controls integration is included. However, in two-story houses, physical placement of the unit to enable effective cooling performance may compromise heating performance. More research needs to be done to determine technological solutions that enable a single-point DHU to both heat and cool a two-story home.
Using a single DHU may still be a good option for homes with low space-conditioning loads as a result of high-performance thermal enclosures, regardless of whether or not they meet the PH standard. But more research is needed to determine the climate-specific enclosure requirements that will make it possible to use such a system successfully.
The PH standard continues to grow in popularity in the United States. And the lessons learned from the Urbana house may help home builders to meet many of the challenges they face as more-stringent codes encourage more people to build high-performance homes.