For years, the English-language website of the Passivhaus Institut in Germany provided this definition: “A passive house is a building in which a comfortable interior climate can be maintained without active heating and cooling systems. The house heats and cools itself, hence ‘passive.’”
The idea of a house that “heats and cools itself” has a certain PR value, of course. The only problem is that it doesn’t exist. Excluding a few buildings in areas with a very mild climate (for example, in San Diego or Honolulu), no one has yet managed to build a house that heats and cools itself. That’s why Passivhaus buildings always need an active heating system, an active cooling system, or both.
The chickens come home to roost
Until recently, most of the people complaining about exaggerations from the Passivhaus Institut were cynics and curmudgeons like me. Now, however, even Passivhaus fans have begun grumbling about the fallout from the false claim that Passivhaus buildings don’t need heating or cooling systems.
The myth is responsible for a great many misunderstandings — misunderstandings that can in some cases lead to bad building designs.
“Beware the Passivhaus Kool Aid”
Allen Gilliland is the founder of One Sky Homes, a design/build firm in San Jose, California. His company specializes in Passivhaus construction, and Gilliland designs all of his projects’ HVAC systems. In a recent PowerPoint presentation that he shared with a monthly meeting of a group called Passive House California, Gilliland expressed his frustration with the idea that a Passivhaus building “heats and cools itself.” In one of his slides, Gilliland warned, “Beware the Passivhaus Kool Aid: Hyperbole, careless references, and hearsay are everywhere.”
It is often said that a building with a low rate of air leakage and above-average amounts of insulation is inherently more comfortable for occupants than an average building. While this is usually true, there are exceptions. An unfortunate corollary to this belief is the idea that designing a heating system for such a building is relatively easy.
In fact, Passivhaus buildings can easily be uncomfortable. As Gilliland explained in his presentation, Passivhaus buildings offer an “opportunity for dismal performance.” And designing a good HVAC system for a Passivhaus building is more challenging than designing an HVAC system for an average building.
Unmet comfort expectations
In his presentation, Gilliland listed some of the possible comfort problems that can occur in Passivhaus buildings. These include uneven temperatures — for example, room-to-room temperature differences that exceed 5°F.
According to Gilliland, it’s also possible to end up with “routine overheating, inadequate cooling capability, unresponsive heating and cooling systems, noisy mechanical systems, [and] unmet occupant expectations for superior comfort.”
A new generation of designers learns about overheating
Back in the 1970s, many designers of passive solar homes fell in love with big expanses of south-facing glass. As a result, almost every solar designer built at least one house that suffered from overheating. By 1977, when Donald Watson wrote Designing and Building a Solar House, the problem was well understood. The book has a section, “Controlling the Overheating Effect of Sun-Oriented Windows,” in which Watson wrote, “Without some way of evenly distributing the heat that is trapped by a sun-oriented window, even during the winter months an overheating effect is created in the spaces next to the window. When that much heat builds up in a room, a homeowner might then wish to close the draperies to keep the sun out or even open the window to let the heat out.”
In his presentation, Gilliland chose to quote from the 1985 classic, The Superinsulated Home Book: “Superinsulated houses are highly sensitive to relatively small energy flows within the thermal envelope. … Intrinsic heat from people, lights, and appliances is an asset in cold climates but a liability in warm climates.” The title on Gilliland’s slide declares, “Overheating — We were warned 30+ years ago!”
When I called Gilliland to discuss his presentation, he elaborated. “These buildings are so well insulated that they don’t give up their heat,” he told me. “You must pay attention to shading.” Several Passivhaus buildings in the U.S. suffer from overheating. “Where people have made mistakes, they haven’t paid attention to shading or internal electrical loads. Everyone is obsessed with keeping heat in, and now everyone is having overheating problems,” said Gilliland.
Designers are discovering that it’s possible to build a Passivhaus building that is comfortable in winter but uncomfortable during the summer. “A superinsulated building, built to a rigorous standard for the envelope, is in my experience inherently more comfortable because you have dramatically reduced the load and reduced unwanted infiltration,” said Gilliland. “But because you have built a superinsulated shell, it is vastly more sensitive to internal and solar loads. We know that when you turn on a big screen TV and cook dinner, your room is going to gain a few degrees. Internal loads make a difference. And you have to shade the windows properly. So the superinsulated shell is a double-edged sword. You have an opportunity for amazing performance, but you also have an opportunity for dismal performance. You can have a mess.”
Poor Passivhaus training
Bronwyn Barry is a certified Passive House consultant who works regularly with Gilliland. When I asked Barry about overheating, she blamed poorly trained designers. “It’s a training issue,” said Barry. “When I was trained, I was told that when PHPP says ‘10% overheating,’ that’s OK. But that is total nonsense, because 36.5 days a year is too may days to be uncomfortable in your house. We were not trained in shading. Overheating is a physics problem. You have all these additional loads, and this well-sealed superinsulated shell, so you get heat build-up over the day, and it can keep building and building from day to day.”
There are still a few designers who confuse the Passivhaus standard with passive solar design. “There are hangovers from the old days of passive solar,” Barry told me. “There was a Passivhaus project in New York where the architect included a Trombe wall. I thought that was insane for a Passivhaus. The project is on Long Island, and it has all sorts of bells and whistles. Apparently they managed to make it work. They had to actively shade the exterior of the Trombe wall, making a simple thing really complicated and difficult. That is the nature of architects wanting to experiment. Unless you can really control the heat from that thermal mass, you are asking for overheating problems.”
Flawed Passivhaus designs?
In his PowerPoint presentation, Gilliland discussed Passivhaus buildings that, according to Gilliland, had problematic HVAC systems. Although he didn’t identify the buildings, one is instantly recognizable to anyone who pays attention to Passivhaus buildings in Western states: the photo appears to show the Evans-Rue Passivhaus in Salem, Oregon.
In his presentation, Gilliland noted that the unidentified house that looks like the Evans-Rue house in Oregon suffers from “serious overheating” and a “flawed cooling system.” He also noted that the house has the following problems:
- Flawed shading design
- Solar HW storage inside; condensing dryer
- No distribution of cooling
- Noisy ventilation system
Gilliland told me, “The designers of the house didn’t pay attention to shading, and the second floor overheated. The southern exposure doesn’t have enough of an overhang for the latitude. If they had done the math and shaded it properly, it would have worked. The house has a condensing clothes dryer, and they located the solar thermal storage tank inside the home’s thermal envelope. The homeowners told me, ‘We can’t use our dryer for most of the year because it gets too hot inside.’ It’s a classic overheating situation. All that kind of stuff the creates heat — the condensing dryer, the solar thermal tank — you need to get it it out of your building envelope.”
[Author’s note: some of Gilliland’s contentions are disputed by the owners of the house in question. See Comment #5 from Sarah Evans, below.]
Myths to bust
Gilliland summed up some of his main points by listing five “myths to bust”:
- Passivhaus buildings don’t need mechanical systems design
- Passivhaus buildings don’t need heating systems
- Passivhaus buildings don’t need cooling
- HRVs inherently distribute heating and cooling
- A single wall-mounted minisplit can properly heat and cool a typical American Passivhaus.
In fact, as Gilliland understands well, an HRV can’t help with heating, cooling, or correcting room-to-room temperature differences. After all, during the winter, an HRV will be exhausting air from some rooms — and we all know that exhausting air doesn’t help heat a house — and supplying fresh air to other rooms — and we all know that the fresh air enters the house at a lower temperature than the indoor temperature. Neither of these processes — exhausting stale air or introducing cool air — helps heat the house.
Moreover, the air flow rates required for ventilation are so low that even when fresh air is delivered near a heat source and exhaust air is pulled from the bedrooms, there is little hope that the meager airflow — 10 or 15 cfm of air that is 5°F or 8°F warmer than the bedroom air — will be enough to raise the bedroom temperature in winter. (John Straube explains this fact in a recent GBA article, Choosing HVAC Equipment for an Energy-Efficient Home.)
It’s not easy to design an HVAC system for a Passivhaus
Clearly, Gilliland is battling the perception that it’s easy to design a heating and cooling system for a Passivhaus building.
“You need good mechanical design now more than ever,” Gilliland told me. “With a Passivhaus building, you have the ventilation component, so it is more complicated. There are more things to worry about. We have lower loads, and in the current context, you have to ask, ‘Where is the equipment?’ My 12,000 btuh furnace doesn’t exist. The loads are 90% lower than a typical house, so we have fewer options in the marketplace, and that becomes challenging. The minisplit on the wall is simple and cost-effective, but you still need to get heating and cooling to where you want it to be. It’s a distribution problem.”
Bronwyn Barry echoes Gilliland’s analysis. “The equipment we have available is overkill, so people are cobbling together some very strange systems,” Barry told me. “You’ve reduced the heating and cooling load dramatically, but we haven’t really got the equipment that is specifically designed for those super-low loads. The equipment hasn’t caught up. It requires a lot more skill to create this mechanical system. The system has to be finely tuned and really well designed.”
Unfortunately, this country doesn’t have enough experts who are qualified to design HVAC systems for low-load buildings. “There is a lack of good mechanical designers,” said Barry. “A lot of architects get terrible training on mechanical systems. If you give them a load calculation they want to run screaming from the room.”
In search of low-cost equipment
By now, most Passivhaus designers know that the cheap solutions to these problems aren’t very effective, and effective solutions aren’t cheap.
“A lot of projects don’t have a budget that can afford a Zehnder HRV, which costs $7,000 or $8,000 by the time it is installed,” Gilliland told me. “One of our biggest challenges concerns the famous dip in the Passivhaus cost curve. In some ways, the cost curve for Passivhaus projects is going the other way. It is a hump instead of a dip.”
One of Gilliland’s latest projects is a 1,600-square-foot house with a single ductless minisplit on the living room wall. Gilliland’s HVAC design includes a continuously operating 110-cfm Panasonic fan that will move air from the living room to a duct that will deliver between 15 cfm and 60 cfm to each of the three bedrooms. The project will be monitored to see if it attains Gilliland’s design goal of a maximum room-to-room temperature difference of 3°F.
It should go without saying that Gilliland and Barry are strongly committed to the Passivhaus standard. Both are more interested in coming up with good design solutions than dwelling on problems or mistakes. Barry told me, “There are plenty of idiots doing the wrong thing, and I think trying to fight that is a waste of good energy. Instead, I’m a big fan of encouraging the other people who are headed in the right direction.”
Two homeowners speak
As it turns out, Sarah Evans, one of the owners of the Passivhaus in Salem, Oregon, seems to be satisfied with the performance of her home, in spite of occasional overheating episodes. In an interview with Mary James published on the Low Carbon Productions website, Evans said, “There were probably three times during the summer when it [the outdoor temperature] reached 100° for a couple of days. The first time we didn’t close our blinds, and we did a bunch of laundry on a really hot day. It got really hot inside, and we had to turn on our air conditioning. (We have a ductless heat pump for heating and cooling.) We realized maybe we shouldn’t do all those heat-generating activities when it is really hot. The next time [a heat wave hit] we were better prepared. We closed all our curtains and blinds to cut off the solar gain during the day. At night we opened all our windows to let in cooler air and turned off the heat recovery portion of our energy recovery ventilator (ERV) at night.”
I had a chance to speak to Sarah’s husband, Stuart Rue. “There are times when we can solve the heating issue by just opening the windows,” Rue told me. “We open the windows at night and it is fine. But for a week or two weeks out of the year, it doesn’t really cool down at night, and we’ll run the heat pump more often during that time. Since the minisplit is on the first floor, a little stratification will occur. The minisplit will cool the first floor very quickly, but it takes a while for the cool air to reach upstairs, and it can be uncomfortable upstairs for a week or two. If we can anticipate the hot weather, we might cool the house a little earlier. So it requires a little bit of adaptation.”
I asked Rue, “If you had to do it again, would you include two minisplits instead of one?” He answered, “My wife and I differ on this. I would put in two minisplits, one upstairs and one downstairs, but she thinks it’s not worth it for one or two weeks a year.”
I asked whether he thought that locating the solar thermal tank and the clothes dryer indoors was a mistake. “We heat a lot more than we cool, and we would rather have the extra heat indoors,” he said. “We pay more attention to how we use the dryer in the summer. We often run the loads at night, when the windows are open. On a really hot day, we will just delay using the dryer. We do notice that when the dryer is going, the laundry room will heat up quite a bit. But it hasn’t been a problem for us.”
Rue said that he doesn’t mind adjusting the curtains or opening the windows to fine-tune his home’s performance. “At our old house, we would just set the thermostat and forget about it, which was really nice. You didn’t have to do anything. And we could do that same thing here, but we like the idea of spending a little bit of extra effort, like opening the windows at night or adjusting the blinds so we don’t need any cooling. We don’t have to do these things, but we want to do those things.”
Putting these issues in perspective
While the approach adopted by Gilliland and Barry — designing an HVAC system that will keep room-to-room temperature differences at 3°F or less — is admirable, it is not the only reasonable approach to HVAC system design. Some homeowners may prefer a less expensive HVAC system, even at the risk of occasional overheating.
The common-sense approach adopted by Sarah Evans and Stuart Rue — an approach which occasionally requires them to take active measures to adjust curtains and blinds, and to accept a greater range of indoor temperatures than their more finicky neighbors — shouldn’t be dismissed out of hand. If this approach saves the owners thousands of dollars in HVAC equipment costs, it’s an option worth considering.
Martin Holladay’s previous blog: “All About Thermal Mass.”