If you design a “pretty good house” with R-20 basement walls, R-31 above-grade walls, an R-49 ceiling, triple-glazed windows, a minisplit heat pump, and an HRV, what should you do next to reduce your energy bills? Maybe aim for the Passivhaus standard?
According to Marc Rosenbaum, that wouldn’t make any sense — in part because this type of house uses more energy for domestic hot water and miscellaneous electric loads (lights, appliances, and plug loads) than for space heating and cooling. “Maybe install a heat-pump water heater or a solar water heater,” Rosenbaum advised at the recent passive house conference in Portland, Maine. “That’s what you need to do. But don’t make it a passive house. Look at all energy use instead of putting 12 inches of foam under the slab.”
Monitoring data from 12 homes
Rosenbaum’s session was called “Getting to Net Zero.” He began his presentation by explaining his aim: to design net-zero all-electric homes heated by minisplit heat pumps. (Rosenbaum defines a net-zero house as one that produces as much PV electricity on site as it uses on an annual basis. In other words, this is a site energy calculation; it has nothing to do with source energy.)
Rosenbaum is the director of engineering at South Mountain Company on the island of Martha’s Vineyard in Massachusetts. He has helped install monitoring equipment at 12 all-electric superinsulated homes in New England, and he presented the monitoring data from these homes at the Portland conference.
The homes varied in size from 1,200 to 1,600 square feet; each house had between 2 and 4 occupants.
These homes don’t meet the Passivhaus standard
The houses that have been monitored for the longest period are eight homes that are part of a small development on Martha’s Vineyard called Eliakim’s Way. (Green Building Advisor reported on these houses in an article called Superinsulated House Specs; the founder of South Mountain Company, John Abrams, wrote a JLC article about the houses called High-Performance Homes on a Budget.)
The energy-related specifications for these homes were listed in the opening paragraph of this article. The homes are quite tight — a few of them passed the Passivhaus airtightness goal of 0.6 ach50 or less — and have more insulation and better windows than most new homes. However, they fall short of meeting the Passivhaus standard. Domestic hot water is provided by electric-resistance water heaters. Each house has a 5-kW roof-mounted photovoltaic (PV) array.
Rosenbaum has monitored the homes’ energy use patterns for four years, with submeters that record the electricity used for space heating and cooling, domestic hot water, ventilation, and miscellaneous electrical loads (lighting, appliances, and plug loads).
Here’s the data on the average annual electricity use per household at the Eliakim’s Way project:
|Lights, appliances, and miscellaneous plug loads||3,913 kWh|
|Domestic hot water||3,051 kWh|
|Space heating and cooling||1,790 kWh|
|Total annual energy use per household||9,061 kWh|
If you want to whittle down these loads, what do you focus on? Not space heating and cooling. Here are some of Rosenbaum’s observations:
- “For most of these houses, lights, plug loads, and appliances is highest category of energy use.”
- “When it comes to plug loads, people use stuff that you can’t predict.”
- “In almost all cases, energy used for domestic hot water is greater than energy used for space heating.”
- “You have to deal with hot water, lights, plug loads, and appliances or you will not get to net zero.”
- “Once you stop making boutique passive houses, you have to start thinking about real people. If this is a movement, you have to think about how people really live in their houses.”
To reduce energy use in these houses, the next step would probably be to install a heat-pump water heater. “My Steibel Eltron heat-pump water heater keeps my basement dehumidified,” Rosenbaum said. “The peak relative humidity in the basement during the summer was 60%. Yes, a heat-pump water heater steals energy in the winter, but it’s good in the summer.”
Rosenbaum likes LED lighting. “Every net-zero energy home needs LED or CFL lighting,” said Rosenbaum. “These days, the LED light quality is so much better. I bought four of those Cree things things from Home Depot. They really make everything look sparkly.”
A few more remarks about the Passivhaus standard
Although Rosenbaum’s presentation was focused on net-zero-energy buildings, he occasionally commented on the Passivhaus standard.
He noted, “It’s harder to make small houses meet the Passivhaus standard, although it is not hard to make small houses use less energy.”
One of the houses that Rosenbaum has helped monitor is Ted and Andrea Lemon’s Passivhaus in southern Vermont. At the Lemons’ house, “Heating and cooling use twice as much energy as PHPP predicts.”
Rosenbaum questioned the usefulness of the PHPP default assumptions for plug loads and domestic hot water (both of which are unrealistically low). He said, “PHPP assumes 6.6 gallons [25 liters] of hot water a day per person, but that’s not enough for normal Americans.”
Of course, missing the target sometimes has side benefits. Rosenbaum said, “If people are using three times as much electricity for plug loads as the default value set by PHPP, that helps you hit your heating number.”
[Author’s postscript: After this story was published, Marc Rosenbaum posted a comment (Comment #2, below). Because it’s important not to give the wrong impression, I’d like to highlight portions of three of Rosenbaum’s sentences: “Martin’s write-up could lead people to think that I’m disdainful of the Passive House movement, which is not accurate. I’ve critiqued certain major shortcomings… PHIUS is taking this on, and I’m excited about the future of the Passive House movement in the U.S. because of this.” I appreciate this correction.]
Martin Holladay’s previous blog: “Wolfgang Feist Defends Thick Insulation.”