Occupant Behavior Makes a Difference
Or, when is a Passivhaus not a Passivhaus?
Energy experts often repeat the cliché, “There’s no such thing as a zero-energy home — just zero-energy homeowners.” Energy monitoring data from two well-publicized Massachusetts homes — the so-called Montague Urban Homestead house in Turners Falls and the home of Matt and Laura Beaton in Shrewsbury — prove the cliché to be true.
Energy-use data for the two homes were shared in a presentation by Mike Duclos and Paul Panish at the Better Buildings by Design conference in Burlington, Vermont, on February 8, 2012. Duclos and Panish are energy consultants at the DEAP Energy Group in Newton, Mass.
The first of the two houses, the Montague house, is owned by Tina Clarke and Doug Stephens. It has won at least two awards: the $25,000 Massachusetts Zero Energy Challenge, and NESEA’s $10,000 Zero Energy prize. Energy-use data from the Montague house show that the homeowners used far less energy than predicted by energy models.
The other house discussed by Duclos and Panish, Matt Beaton’s house in Shrewsbury, is the first certified Passivhaus in Massachusetts. Energy use data from the Beaton house show that the homeowners used far more energy than predicted.
The key factor in both cases was occupant behavior.
The Montague house
The Montague house was built by Bick Corsa with help from the homeowners. Construction was completed in 2009.
Here’s a snapshot of the house:
- Size: 1,152 square feet
- Number of bedrooms: 3
- Foundation: slab on grade
- Foundation insulation: 6 in. XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. (R-30) on 5 sides of the slab
- Wall construction: Two rows of 2x4 studs
- Wall insulation: R-42 dense-packed cellulose
- Attic construction: Vented unconditioned attic
- Attic floor insulation: R-100 cellulose
- Windows: Thermotech Fiberglass windows with orientation-specific triple glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill.; south glazing is U-0.23 and 0.44 SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.; interior R-6 insulated shutters
- Winter design temperatureReasonably expected minimum (or maximum) temperature for a particular area; used to size heating and cooling equipment. Often, design temperatures are further defined as the X% temperature, meaning that it is the temperature that is exceeded X% of the time (for example, the 1% design temperature is that temperature that is exceeded, on average, 1% of the time, or 87.6 hours of the year).: -5°F
- Design heating load: 7,500 Btu/h
- Space heat: Fujitsi 9RLQ ductless minisplit air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. rated at 9,000 Btu/h
- Domestic hot water: 2 4’x8’ flat-plate solar collectors with an 80-gallon storage tank; backup provided by a Tempra 24 on-demand electric resistance water heater.
- Mechanical ventilation: Lifebreath HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air.
- PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. system: 4.94 kW
- Refrigerator: Ultra-low-energy Sundanzer chest refrigerator uses only 34 kwh/year
- Cost: About $200,000
Energy modeling (using REMrate software) predicted that the homeowners would consume 5,479 kWh of electricity per year; in fact they used only 1,959 kWh — that is, only 36% of the projected usage. Average consumption was 163 kWh per month.
The home’s PV array produced 4,892 kWh during the first year — two and a half times as much electricity as the owners consumed.
Of course, there is no secret to low energy use. We all know how to do it: don’t use much hot water, only turn on a light when you really need to, and keep the thermostat low. (Clarke and Stephens reportedly kept the thermostat at 60°F when they were home, and turned off the heating system when they weren’t home.)
After all, the owners had a strong motivation to keep their electricity usage low: they were aiming to win two cash prizes worth $35,000. They succeeded — in part because their modest home has an excellent envelope and extremely efficient HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment, but mostly because they were focused like a laser on energy conservation.
Mike Duclos summed up the situation this way: “These are not normal users.”
Matt and Laura Beaton's house
Matt and Laura Beaton moved into their new home, the first certified Passivhaus in Massachusetts, in 2011. Since Matt Beaton owns a construction company, he is the builder as well as the owner. The architect was Mark Yanowitz, and the Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. consultant was Paul Panish.
Here’s a snapshot of the house:
- Size: 3,399 sq. ft.
- Basement slab insulation: R-41
- Basement wall insulation: R-51
- Above-grade wall construction: Double wall construction with 2x6 inner wall and 14-inch-thick I-stud outer wall
- Above-grade wall insulation: R-65 cellulose
- Ceiling insulation: R-126 cellulose
- Windows: Accurate Dorwin windows with orientation-specific triple glazing; center-of-glazing specs: south windows U-0.16, SHGC = 0.64; remaining windows U-0.13, SHGC = 0.55
- Space heat: Mitsubishi MXZ-3A30NA ductless minisplit air-source heat pump with 2 indoor heads
- Domestic hot water: 2 4’x8’ flat-plate solar collectors with an 80-gallon storage tank and electric resistance backup
- Mechanical ventilation: UltimateAir RecoupAerator 200DX ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.
- Air leakage rate: 0.44 ach50
According to Panish, who has been monitoring the home's energy use, the owners are using much more energy than predicted. “There is higher than expected cooling demand,” said Panish. “High internal loads are adding to the cooling loads. The default value for internal loads according to the PHPP software is 621 watts continuous, but the actual internal loads at this house are 1,061 watts continuous.”
The data were a surprise to Panish. “Something unexpected is going on,” he said. “Plug loads are sky high.”
It didn’t take long to figure out what was driving the high energy bills. “There is a very large plasma TV, plus a second TV on the porch,” said Panish. “There is a DVR. The two TVs and the DVR use 600 watts when they're on and 100 watts when they are off, and the TVs are on for an average of 6 hours per day. The loads for entertainment and computers are high. There is an old freezer in the basement. There is a basement dehumidifier. The lighting load is 600% of what was predicted. It seems as if all the lights in the house are left on all the time.”
After delving into the data, Panish reached a few conclusions. “You’ve probably seen a bell curve showing energy used by a range of Passivhaus homeowners,” said Panish. “Well, this family is at the high end of the bell curve. They are concerned about energy use, but their concern doesn’t get reflected in their behavior. The implication was that once they built the house with the good envelope, the job was done. But the job was not done.”
The bell curve will never go away
There are several points to be gleaned from the monitoring data from these two houses:
- Although both of these houses have excellent thermal envelopes and very efficient HVAC equipment, the occupants of the Beaton house used more than 7 times as much electricity as the occupants of the Montague Urban Homestead. (Of course, the Beaton house is 3 times as big as the Montague Urban Homestead).
- No matter what kind of house you live in, you can achieve significant energy savings by following your grandmother’s advice: turn down the thermostat, close the door, turn out the light, and don’t leave the faucet running.
- It’s possible for occupants of a Passivhaus to use a lot of energy; the Beaton family uses 14,744 kWh of electricity per year. (The national average residential use in 2009 was 11,040 kWh per household; of course, unlike the Beatons, most of these homes also used significant quantities of natural gas, propane, or fuel oil in addition to electricity.)
- When occupants of a Passivhaus use more energy for lighting and appliances than expected, there is one positive side effect: the waste heat from the humming appliances reduces the heating load.
- The default values for plug loads provided in the PHPP software may make sense for the typical German family, but they are probably too low for the typical American family.
Last week’s blog: “The High Cost of Deep-Energy Retrofits.”
- Matt Beaton
- Paul Panish
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