Image Credit: Matt Beaton The south elevation of Matt and Laura Beaton's house. This bar graph compared projected energy use at the home of Matt and Laura Beaton with the actual energy use from May 2011 to January 2012.
Image Credit: Paul Panish
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. XPS (R-30) on 5 sides of the slab
- Wall construction: Two rows of 2×4 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 glazing; south glazing is U-0.23 and 0.44 SHGC; interior R-6 insulated shutters
- Winter design temperature: -5°F
- Design heating load: 7,500 Btu/h
- Space heat: Fujitsi 9RLQ ductless minisplit air-source heat pump 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
- PV 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 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 House 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 2×6 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 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.”