The PassivHaus Concept and European Residential Design

In my research for this book, I found many interesting innovations, both cultural and technical, in the European approach to green buildings. Perhaps the most interesting and potentially most meaningful in terms of its impact on energy use and climate change is the approach taken in Central Europe to building design: the Passive House (PassivHaus in German).

For a moment, suppose you were assigned to design an energy-efficient home for a client in the United States or Canada. That appears easy enough. After all, you just add more insulation, upgrade the windows, maybe use some innovative technology such as structural insulated panels or insulated concrete forms, install a more efficient heating and hot water system, upgrade the Seasonal Energy Efficiency Ratio of the air conditioner, and maybe even add a solar water heater, and you're done. For the most part, you would save about 50 percent of the energy use of a conventional home. But now suppose your client wanted to save 90 percent, without sacrificing indoor air quality or reducing the number of windows. That would be a very difficult task, particularly without adding many solar panels for heating and hot water. Well, it turns out that the PassivHaus standard addresses exactly that 90 percent savings assignment.

European governments take far more seriously the problem of global warming than the U.S. government. But while the United States has taken some steps to lower commercial building energy use, Germans and Austrians have tackled the housing sector. Like the United States and Canada, they're still struggling with the more important issue of what to do with the existing housing stock. This is a more critical problem because the turnover of housing stock is much lower in Europe, because of slower population growth, less workforce mobility, and a large number of historically significant buildings, which makes renovating them to be more energy efficient more challenging. The Germans and Austrians are not alone. Each of the twenty-seven countries in the European Union must adopt national standards to achieve carbon dioxide reduction goals. However, according to one expert, only four countries currently have both government and private sector programs to achieve this goal, even though the European Union's 2002 energy directive for building performance is supposed to be fully implemented by 2010.

In April 2008, I attended the annual PassivHaus conference in 2008 in Nürnberg, a delightful medieval city in Bavaria of some 180,000 people, known to most Americans (if at all) only as the place where Nazi war crimes trials took place after World War II, memorialized in the well-known movie Judgment at Nuremberg.

The conference is the brainchild of Professor Wolfgang Feist, an academic impresario of residential and commercial green building who has managed over 12 years not only to popularize the PassivHaus concept but also to have it accepted by the German authorities as the national standard. Attracting more than 1,000 serious researchers, businesspeople, students, and foreign visitors, the Passivhaustagung is a great place to begin considering what we can learn from the green building movement in Europe.

WHAT'S A PASSIVHAUS?

The basic concept behind the PassivHaus is quite simple. In a continental climate dominated by energy use for heating during a longer cold season, one should build a house like a thermos bottle that recovers most of the heat in the outgoing air to warm the incoming air. That's it. It is quite simple in concept but very hard to implement in the field because it requires rigorous attention to construction detail, basically sealing up all potential outside leak points.

Look at Figure 1.1. The opposite of the thermos bottle is the coffee cup, which cools off quickly and needs reheating to stay warm, as might be the case in a typical American home.

The PassivHaus standard is quite exacting because it's aimed at achieving an incredibly low heating energy use of 15 kilowatt-hours (of primary energy) per square meter per year. Consider that the average new home in the United States is about 2,400 square feet, or about 220 square meters. To meet the standard, you shouldn't use more than 3,300 kilowatt-hours per year, considering both gas and electricity for heating. Now, look at your own electricity and gas bills. In the United States, in which natural gas is the dominant fuel for heating, cooking, and hot water outside the Northeast, the average annual use of natural gas in single-family homes is 800 therms, or 23,000 kilowatt-hours, seven times the PassivHaus standard, not counting electricity use, which averages nearly 12,000 kilowatt-hours. (In California, with its mild climate, the numbers are closer to 6,000 kilowatt-hours for electricity and 460 therms [13,000 kilowatt-hours] for gas.)

Passive houses are superinsulated, walls, roof, and below, using the best windows in the world. It's hard to beat a German window manufacturer for quality. Consider what a well-insulated window will do for you. The German design standard for windows in passive homes is that when it's 20°F outside and 72°F inside the home, the surface temperature of the window in the room won't be below 67°F (a 3°C difference), so your body won't sense a cold surface when you're in the room. You can do this only with triple-glazed windows that have no thermal bridges, or "heat freeways," between inside and outside. Feel a standard U.S. builder-grade aluminum frame window on a cold winter night, and you'll instantly feel a surface that's as cold as a refrigerator, so the room is losing heat even while the double-glazed window with "low-e" coating is keeping out some of the cold. The cold surface you feel is a thermal bridge that is steadily transporting nice, expensive heat from inside the house to the outside.

What gives the PassivHaus its importance is a strong commitment by the German central government to reduce the entire country's carbon dioxide emissions significantly by the year 2020. There's no way to do this without the PassivHaus being adopted on a massive scale. Consider that all of Germany lies at the latitude of Canada and that heating energy use can represent more than 80 percent of all energy use for heating, cooling, and hot water in the home. You can see why the Germans (and the Swiss and Austrians also, by the way) focus on improving residential design and energy performance.

What's the goal of the PassivHaus movement in Europe? To reduce energy use in residential homes and apartments (considering only heating energy use) by 95 percent of today's average home, which uses about 300 kilowatt-hours per square meter per year.

To meet these energy targets Professor Feist and his colleagues are determined to revolutionize the way homes are built and operated.

Passive Homes, the German and Austrian Way

Most of the world's estimated ten thousand passive houses are in Germany and Austria, where winter design temperatures are about 9°F to 16°F. These homes are built to standards that require approximately R-50 walls (equal to about 15 inches of fiberglass batts or 8 inches of sprayed polyurethane foam) and triple-glazed U-0.14 (R-7) windows, with maximum air leakage rates of 0.6 air changes per hour. As a result, these homes have the most efficient building envelopes in the world.

In a typical PassivHaus in Central Europe there is a heating coil in the ventilation duct. Most of these heating coils circulate hot water produced by a gas-fired water heater or a heat pump water heater. So they're not truly passive, but they certainly are low-energy.

What you should get is a home that is very comfortable, with quite low energy bills. Because Central European PassivHaus designers deliver heat mainly through ventilation ducts, heat recovery ventilators act as the key residential heating appliance. In some very cold climates of the United States, such as Minnesota and Wisconsin, there will still be a need for supplemental heat beyond that delivered through the ventilation system.

PassivHaus technical specifications are strictly established in Germany and Austria, and they are generally well understood by builders. The annual energy consumption for space heating must be no more than 15 kilowatt-hours per square meter. (Note that this level of energy use would equate to an annual energy use for heating of 2,800 kilowatt-hours—about 100 therms—in a 2,000–square-foot house, pretty low by American standards in cold climates.)

For an entire residence, the PassivHaus standard specifies that the maximum annual energy budget for all purposes (including space heat, domestic hot water, lighting, appliances, and all other electrical loads) must be no more than 120 kilowatt-hours per square meter (11 kilowatt-hours per square foot, or about 760 therms per year for a 2,000–square-foot house in the United States). Although this is a small energy budget, it is not zero energy by any means. If you want a net-zero-energy house, you'll have to supply the balance with solar thermal and photovoltaic panels for both heating and hot water. In a mild climate without a significant air-conditioning need, these systems would be quite affordable in most parts of the United States and Canada, taking into account energy prices along with current local, federal, and utility subsidies.

The Innovator, Wolfgang Feist

Unlike many academics, Professor Feist emphasizes the practical nature of the PassivHaus standard and its usefulness in North America. He says 15 kilowatt-hours per square meter (1.4 kilowatt-hours per square foot) is not arbitrary but a good benchmark.

The definition of a PassivHaus doesn't need any [particular] number. As long as you build a house in a way that you can use the heat-recovery ventilation system—a system that you need anyway for indoor air requirements—to provide the heat and cooling, it can be considered a PassivHaus. Since you need a house to be tight, you need a supply of fresh air. If you need that anyhow, the idea is to do everything else—the heating and cooling and dehumidification—with the ventilation system. To do that, the peak load for heating and cooling has to be quite low, including appliances.

Delivering heating by methods other than through the ventilation system has adverse impacts, according to Feist. For example, direct electrical heating is inexpensive to install, but the primary energy use (for electric power production) is extremely high, so he thinks that's not a good idea, in most cases. Woodstoves are okay with him, but a good one is quite expensive, so you should use just one stove. If you do that, and you still want to have good thermal comfort all around the building, you will need quite good insulation. Because biomass is limited, if you burn biomass (e.g., in the form of wood pellets) in your house, the house should be well insulated—in the range of what is required for a PassivHaus. Keeping the heat source in the ventilation system (in the form of a hot water coil) is not necessary, but in Feist's opinion it's the cheapest way.

Feist believes that in many of the milder climates of the United States, low-energy houses can indeed include standard exhaust-only or supply-only ventilation systems and not use heat recovery ventilators.

If you can meet the requirements for a very low amount of additional energy, in summer and in winter, without a heat-recovery ventilator, why not? In San Francisco, for example, you don't need a heat-recovery ventilator; just build the house with operable windows. [In colder places], I think it is important to install a heat-recovery ventilator before any other system in the house, such as a forced-air heating system.

The PassivHaus standard has very exacting requirements for window performance, specifications that would be hard to meet with standard double-pane, low-e windows that are the current U.S. definition of an energy-efficient window, with or without thermal breaks.

The window specification depends on the climate. In Central Europe, we need an R-7 [U-0. 14] window. You would not need the same window in Florida or California. The reason for the U-value that we now require in Europe is the comfort of the occupants. It is a functional definition. During the winter, the coldest surface temperature in the room will be the window. If you don't have a radiator in your room, the difference between the surface temperature of the window and the average surface temperature of the room should not be more than 3°C (5°F); that's for comfort reasons. [For this, you need very efficient windows.]

This is not as complex as it sounds. In essence, Feist believes that a home design should be kept simple, by improving insulation and windows and by installing a heat recovery ventilator. In North American climates, the biggest problem may be changing mindsets about how to get started. For example, Feist says, "Most builders I have talked with in North America still think that increasing insulation is an expensive thing. I'm surprised, because insulation is the cheapest thing you can do." The main issue is doing away with the notion that energy efficiency upgrades must "pay back" in terms of current energy prices. He says, "I think it is important to do away with this idea of the payback calculation. We should do advertising to say that a payback calculation is not important for determining energy efficiency, because a house will be there for more than five years—it will be there for maybe 70 years."

Explained this way, do you think now that you could use the PassivHaus standard to design a 90 percent energy-efficient home just about anywhere in North America?

The Role of Government Regulation

The larger issue in adopting a standard such as PassivHaus may be in our own approach to regulation and energy codes. Germans have a long history of government regulation, including the building sector. The first laws regulating quality in buildings came about at the same time (1882) as the lawless Arizona Territory saw the gunfight at the O.K. Corral, where Wyatt Earp, Doc Holliday, and other Earp brothers gunned down the notorious Clanton gang in the aptly named town of Tombstone. In much of the United States, we're still fighting battles over the role of government regulation that the Europeans settled long ago, in favor of the government.

Germans tend to regulate everything, including buildings, in a way that most Americans do not understand and probably would not tolerate. For example, it's socially unacceptable to jaywalk in Germany. After visiting a fantastic green building near the Frankfurt Airport, I wanted to cross a completely empty street against the light. My German guide wouldn't hear of it. Why? It would set a bad example for the children, she said, and they might dash out into the street and get run over. (Good reason, except that there were no children anywhere near this busy commercial area; Germans just like "Alles in Ordnung.") What this means is that if the German government puts something in the building code, such as the PassivHaus standard, it's likely to be taken up and followed by one and all.

Building Physics

Building physics underlies the PassivHaus standard. The Germans are quite taken with the notion and language of building physics, a topic and field of study mostly unknown in North America and certainly not part of the common language and thinking of most homebuilders, architects, or engineers. Simply stated, building physics looks at heating, cooling, and moisture management in buildings from a scientific viewpoint. Simple physics principles most of us learned in high school can be used to design better buildings; for example, heat flows from warm to cold, moisture flows from wet to dry, moisture condenses on a cold surface, and heated air rises.

While in Munich, the home of Bier und Bratwurst, I visited the world-famous Fraunhofer Institute for Building Physics, a research station employing about three hundred people who are looking for new ways of heating, cooling, and dehumidifying buildings, using scientific research methods. I saw hundreds of mold and algae cultures being tested to see under what environmental conditions they thrive (and therefore how to prevent them from growing, by eliminating those conditions). I also went inside a three-story research building with twelve different types of building façades being tested for environmental performance. The researchers at Fraunhofer are developing new types of external and internal paints that will inhibit mold growth, even in severe wet winter weather. The institute's work will continue to form the backbone of many building regulations in future years in Germany.

WHAT CAN WE LEARN FROM THE PASSIVHAUS APPROACH?

There is no way to stop global warming over the next 40 years without significantly reducing household carbon dioxide emissions to well below 1990 levels. In the housing sector, the PassivHaus concept shows that it is possible to maintain a high-quality standard of thermal comfort with only 10 percent of the current average energy use—if we learn to build our homes better.

The U.S. Department of Energy has been laboring for the past 20 years, with a dedicated group of researchers, to accomplish the same results with their Building America program. But I don't think it has had the impact and acceptance among practical builders that the PassivHaus standard has had in Central Europe. Part of the reason is that the United States has greater diversity in climate zones than Germany. If you live in Miami, you've got a tropical climate (wet–humid), and if you live in St. Louis or Dallas, you have cold winters and hot, humid summers (mixed–humid). If you live in Phoenix, you have desert (hot–dry), and if you live in Los Angeles, you have a Mediterranean climate. So in the United States, we need more diverse approaches that take all these different climate zones into account and then translate those differences into simple standards that every builder can use. (In fact, the federal government's Building America program has done this to some degree.) We also have to return to better skill in homebuilding, something that may have been lacking in the rush to overbuild in the first half of the present decade, but a skill of which German homebuilders are rightly very proud.