Principles of Passive House Design, Part II
A German Standard Dramatically Can Improve Energy Efficiency of Buildings
By Katrin Klingenberg and Mike Kernagis
Passive House design, a standard that has been used to certify buildings in Germany since 1996, focuses on balancing energy gains and losses to attain a level of energy efficiency that is far beyond the norm. But the norm is changing, and many people now recognize energy efficiency is profoundly important, economically and environmentally. It has been called a low-hanging fruit, an innovation that can and should be readily attained.
For images of commercial buildings using Passive House guidelines, visit the Media Center.
To attain up to 90 percent savings in heating and cooling costs compared with traditionally constructed buildings, Passive House designers and builders work together to systematically implement the following seven principles:
- Superinsulate
- Eliminate thermal bridges
- Make it airtight
- Specify energy- or heat-recovery ventilation
- Specify high-performance windows and doors
- Optimize passive solar and internal-heat gains
- Model energy gains and losses using the Passive House Planning Package, or PHPP
This article will address the final four principles of the Passive House standard—specify energy- or heat-recovery ventilation, specify high-performance windows and doors, optimize passive solar and internal-heat gains, and model energy gains and losses. (Read about the first three principles in “Principles of Passive House Design, Part I.”)
Specify Energy- or Heat-recovery Ventilation
Perhaps the most common misperception regarding Passive Houses concerns airflow. “A building needs to breathe,” contractors and builders might say disapprovingly when first presented with the idea of constructing very tight buildings. Rather than breathing unknown volumes of air through uncontrolled leaks, Passive Houses breathe controlled volumes of air by mechanical ventilation.
Mechanical ventilation circulates measured amounts of fresh air and exhausts known quantities of stale air, which dramatically improves IAQ. Needless to say, this ventilation system must be extremely energy efficient. Passive House designers specify energy-recovery ventilators or heat-recovery ventilators in cold, dry climates. These machines incorporate an air-to-air energy-recovery system, which conserves most of the energy in the exhaust air and transfers it to the incoming fresh air. This significantly reduces the energy needed to heat that incoming air.
State-of-the-art ventilation systems have heat-recovery rates of 75 to 95 percent. The ventilation system generally exhausts air from rooms that produce moisture and odors. Humidistats installed in these rooms monitor when moisture levels are elevated, initiating an increase in the ventilation flow rate. The exhaust air gets drawn through the ventilator on its way out of the building. There it passes through a heat exchanger that transfers the reusable heat energy to the incoming fresh air. It is important to note the exhaust air is not mixed with the incoming air; only heat is transferred. Although return air is circulated back to the furnace in a forced-air system, air is not recirculated with a mechanical ventilation system.
When operating, the ventilator provides a constant supply of fresh air. The incoming air is filtered and balanced. It is distributed at a generally low-flow rate through small, unobtrusive diffusers. The system is generally very quiet and draft free. The PHPP recommends an ACH of 0.3 to 0.4 times the volume of the building and a guideline ACH of 1,059 cubic feet (30 m³) per person.
The main difference between an HRV and ERV is the HRV conserves heat and cooling energy while the ERV transfers humidity, as well. In summer, an ERV helps keep humidity outside; in winter, it helps prevent indoor air from becoming too dry. For in-between seasons when no conditioning is needed, a bypass can be installed for either system to avoid heating the incoming air. Alternatively, the ventilation system can be turned off altogether, and windows can be opened to bring in fresh air.
Either system’s efficiency can be increased by prewarming or precooling the incoming air. This can be done by passing the incoming air through earth tubes. Because the ground maintains a more consistent temperature throughout the year than the outdoors, passing the air through tubes buried in the earth preheats or precools the air, depending on the season. Preheating and precooling can also be accomplished indirectly by circulating water in an underground pipe and using it to heat or cool the air with a water-to-air heat exchanger.
A word here about cooling and dehumidification: The Passive House concept was developed primarily in Central Europe, which has a relatively mild, primarily heating-oriented climate. Implementation of designs that meet the Passive House standard is more challenging in extremely cold, hot or humid climates. Nevertheless, many Passive Houses already have been built in very cold climates and now are starting to be built in hotter climates.
In cooling-load (as in heating-load) situations, the space-conditioning load must be minimized. This takes careful planning. The high levels of insulation in a Passive House help to keep indoor temperatures cool. In addition to the standard measures for preventing excess solar gain, convective venting behind siding and roofing and night cooling often will suffice to maintain indoor comfort. In humid climates, an additional cooling load may stem from the need to remove latent moisture from the air. A very small and efficient air-to-air heat pump—also known as a mini-split—can remove this moisture and provide adequate cooling.
Specify High-performance Windows and Doors
In modeling the energy of a building, the designers of Passive Houses choose windows and doors based largely on their insulating value. There have been extraordinary advances in window quality during the past 30 years, and thermal losses from windows have dropped dramatically. Many brands of windows and doors now are being made tighter, reducing losses through infiltration and exfiltration. Doors are being manufactured with appropriate thermal breaks and double gaskets. Overall, high-performance windows and doors are proving cost effective in Passive House applications.
One development that has significantly affected the heat conductivity of windows is the introduction of low-emissivity coatings, which are microscopically thin, transparent layers of metal or metallic oxide deposited on the surface of the glass. The coated side of the glass faces into the gap between the two panes of a double-glazed window. The gap is filled with low-conductivity argon or krypton gas, which greatly reduces the window’s radiant heat transfer. Different low-e coatings have been designed to allow for high, moderate or low solar gain. This provides a range of options for buildings in all climates. Today, builders can choose to install triple-pane, low-e-coated, argon-filled windows with special low-conductivity spacers and insulated, thermally broken frames. These windows eliminate any perceptible cold radiation or convective cold airflow even in periods of heavy frost.
Optimize Passive Solar and Internal-heat Gain
Not only must designers of Passive Houses minimize energy loss, they must also carefully manage energy gains. The first step in designing a Passive House is to consider how the orientation of the building—and its various parts—will affect its energy losses and gains. Where should the glazing be to allow for maximum sunlight when sunlight is wanted and minimize heat gain when heat gain is unwanted? The more direct natural lighting there is, the less energy will be needed to provide light. Designers can enhance occupants' enjoyment of available sunlight by orienting work spaces and living rooms to the south and putting utility rooms and closets where sunlight is not needed, to the north. However, it is not always possible to site a building in this ideal way. There may be buildings, trees or lanforms taht cast shadows during short winter days, blocking out much of the low sunlight. Or the designer may need to accommodate the owner's demand for a certain view not available with ideal orientation.
Windows are designed, oriented and installed to take advantage of the outstanding passive-solar energy that can be gained through them. But the goal is not simply to allow for as much solar gain as possible. Some early superinsulated buildings suffered from overheating because not enough consideration was given to the amount of solar gain the building would experience. A good design should balance solar gain within the building’s overall conditioning needs and within the glazing budget. Even very efficient windows can lose more heat during a year than they gain, depending on their location, and large windows are expensive.
In the Northern Hemisphere in climates dominated by heating loads, windows on the north do not allow direct solar-heat gain while those on the south allow for a great deal of it. In summertime in primarily cooling climates, it is very important to prevent excess solar-heat gain. This can be done by shading the windows. Roof eaves of the proper length can effectively shade south-facing windows when the sun is higher in the summer and still allow for maximum solar-heat gain in the winter when the sun is lower and days are colder.
Deciduous trees or vines on a trellis also can block out sunlight in summer and admit it in winter. In climates that have a significant cooling load, the designer should consider limiting unshaded east- and west-facing windows and specifying only windows that have low-solar-gain, low-e coatings. During the morning and late afternoon, low-angled sunlight can generate a great deal of heat in unshaded east- and west-facing windows.
Another, perhaps less obvious, source of heat gain is internal. Given the exceptionally low levels of heat loss in a Passive House, heat from internal sources can make quite a difference. Appliances, electronic equipment, artificial lighting, candles and people can have a significant effect on the heat gain in a Passive House. Although designers may not choose the appliances that are installed in a house, they often select the lighting sources, and they must take into account the heat gain from those sources when they calculate overall internal heat gain.
Use the PHPP for Energy Modeling
There are many elements of Passive House design that need to be integrated with one another. They include wall thickness, R- or U-values, thermal bridges, airtightness, ventilation sizing, windows, solar orientation, climate, and energy gains and losses. The PHPP is a powerful and accurate energy-modeling tool that helps a designer integrate each of these elements into the design, so the final design will meet the Passive House standard.
The PHPP starts with the whole building as one zone of energy calculation. The designer inputs the basic characteristics of the building—orientation, size, location of windows, insulation levels, etc. The PHPP also can be used to model solar-water heating for combined space and water heating or the contributions of natural ventilation to nighttime cooling. The PHPP then computes the energy balance of the design. If needed, the designer can change one or more elements—the size or location of a window, for example—within the PHPP and model the effect of those changes on the overall energy balance. Experienced Passive House designers often work with their drawing programs and the PHPP open. The Passive House standard is met when
- The space-heating requirement of the design is less than or equal to 4.8 kBtu per square foot per year (15 kWh/m²/year)
- The primary- (source-) energy use of the design is less than or equal to 38.1 kBtu/ft²/year (120 kWh/m²/year)
- The airtightness of the building is verified to be at or below 0.6 ACH at 50 Pa
Economic Sustainability
The focus on energy efficiency makes Passive Houses more expensive to build. Construction costs generally run 10 to 15 percent higher than construction costs for a conventional building. The additional upfront costs of more insulation, better windows and doors, and more labor for higher-quality installations are partially offset by the lower cost of the heating-and-cooling systems. Because Passive Houses have such small heating-and-cooling requirements, conventional heating-and-cooling systems can be replaced with miniaturized components and efficient mechanical ventilation. This is one example of the necessity of integrated planning. The additional construction cost is readily recuperated in savings on the energy bill. And the Passive House will generate a carbon footprint that is a fraction of the size of that of a conventional building. On-site renewable-energy sources can be added to create a true zero-energy or even plus-energy building, one which produces more power than it consumes.
In the past, most buildings were built with scant attention paid to their long-term energy consumption. This approach needs to change. We need to use our limited natural resources wisely and construct buildings with quality and durability in mind. The costs of energy consumption are high, and they continue to increase. The savings to be realized during the life of a Passive House are remarkable, economically and environmentally.
Katrin Klingenberg and Mike Kernagis are the founders of Passive House Institute US, Urbana, Ill. For more information, visit www.passivehouse.us.
This prefabricated panel is framed with 12-inch (305-mm) wooden I-joists set upright at 24 inches (610 mm) on center and sheathed with structural fiberboard on the exterior and oriented strand board on the interior. It is filled with a high-density fiberglass insulation.
Contractors prepare to apply fiber-cement-siding planks to Fairview I, a Passive House built for low-income, first-time home buyers. The furring strips create a cavity behind the siding that acts as a rainscreen and allows for convective cooling in the summer.
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