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Principles of Passive House Design, Parts I and II

Credits: ©2009 Katrin Klingenberg and Mike Kermagis

The U.S. has many examples of superinsulated buildings that successfully harness the sun’s energy. However, while this country was enjoying low fuel prices during the past 15 to 20 years, the Europeans’ high fuel prices motivated them to advance the concept. Serious building science with respect to ventilation, air and moisture control, and thermal bridging, along with the development of energy modeling for very efficient buildings, has led to the rigorous German energy standard called Passive House. This voluntary standard has spawned an industry of high-performance windows and doors, super-efficient air handlers, miniaturized heating-and-cooling systems, thermally broken connections and fasteners, etc. In Europe, Passive House is a fully realized system of design and construction. Today, the Passive House system is finding an audience in a wide variety of U.S. climates.


Passive Solar Energy Research Facility

The exterior stepped clerestory of the Solar Energy Research Facility. ©2009 Solar Energy Research Facility

Principles of Passive House Design, Parts I and II 
A German Standard Dramatically Can Improve Energy Efficiency of Buildings
By Katrin Klingenberg and Mike Kernagis

Part I
The Passive House concept, which is viable for commercial and residential buildings, seeks to minimize energy losses and maximize passive energy gains. A Passive House uses up to 90 percent less energy for space heating and cooling than a conventionally constructed building.  To attain such outstanding energy savings, 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 addresses the first three principles of the Passive House standard—superinsulate, eliminate thermal bridges and make it airtight. The other four principles are addressed further down in “Passive House Principles, Part II.”

In a Passive House, the entire envelope of the building—walls, roof, and floor or basement—is well insulated. How well insulated? That depends, of course, on the climate. To achieve the Passive House standard, the Tahan House in Berkeley, Calif., required only 6 inches (152.4 mm) of blown-in cellulose insulation while the Skyline House in the far harsher climate of Duluth, Minn., needed 16 inches (406.4 mm) of the same insulation. Often the first feature of a Passive House to catch a visitor’s attention is the unusual thickness of the walls. This thickness is needed to accommodate the required level of insulation.

Even with this insulation requirement, Passive House designers have a wide range of choices for the materials used to create superinsulated building envelopes. Wall assemblies can be built using conventional lumber or masonry construction, double-stud construction, structural insulated panels, insulated concrete forms, truss joist I-beams, steel or strawbale construction.

Similarly, designers can choose from a number of types of insulation. These include cellulose, high-density blown-in fiberglass, polystyrene, spray foam and strawbale. Vacuum insulated panels, or VIPs, are a relatively new option with an exceptionally high R-value per inch. (Visit the ASTM standard for VIPs.) Using VIPs allows designers and builders to greatly decrease the thickness of the walls in buildings.

No matter which type of insulation gets chosen, Passive House builders must ensure the product is installed correctly. The application and performance of insulation can be directly measured using thermographic imaging. All objects emit infrared radiation, and the amount of radiation emitted increases with the temperature of the object. Variations in IR radiation, and therefore in temperature, can be observed using a thermographic camera. Because these cameras can readily detect heat loss, they usually can identify areas where insulation is insufficient, incomplete, damaged or settled.

Eliminate Thermal Bridges
Heat will pass very quickly through an element that has a higher thermal conductivity than the surrounding material, forming what is known as a thermal bridge. Thermal bridges can significantly increase heat loss, which can create areas in or on the walls that are cooler than their surroundings. In the worst-case scenario, this can cause warm, moist air to condense on a cooler surface.

Thermal bridges can occur at edges, corners, connections and penetrations. A bridge can be as simple as a single lintel that has a higher thermal conductivity than the surrounding wall. A balcony slab that is not insulated from and thus thermally isolated from an interior concrete floor can be a potent thermal bridge. An effective thermal isolation is called a thermal break. Without a thermal break, the balcony will act as a very large cooling fin in the wintertime.

In a Passive House, there are few or no thermal bridges. When the thermal-bridge coefficient, which is an indicator of the extra heat loss caused by a thermal bridge, is less than 0.01 watts per meter per Kelvin, the detail or wall assembly is said to be thermal-bridge free. Additional heat loss through this detail is negligible, and interior temperatures are sufficiently stabilized to eliminate moisture problems. It is critical for the Passive House designer and builder to plan to reduce or eliminate thermal bridges by limiting penetrations and by using heat-transfer-resistant materials. Thermographic imaging can be used to determine how effective the efforts to eliminate thermal bridges have been.

Make It Airtight
Airtight construction helps the performance of a building by reducing or eliminating drafts thereby reducing the need for space conditioning. Airtightness also helps prevent warm, moist air from penetrating the structure, condensing inside the wall and causing structural damage.

Airtight construction is achieved by wrapping an intact, continuous layer of airtight materials around the entire building envelope. These various membranes, tapes, plasters, glues, shields and gaskets are becoming increasingly durable, adherent, easy to apply and environmentally sound, which is making it easier for a builder to meet the stringent airtightness requirement of the Passive House standard. Special care must be taken to ensure the continuity of this layer around windows; doors; penetrations; and all joints between the roof, walls and floors.

The airtightness of a building provides a measurable dimension of the quality of construction. Testing airtightness requires the use of a blower door, which is essentially a large specialized fan. The blower door can be used to depressurize or pressurize a building to a designated pressure. With the fan set to maintain this designated pressure, a technician can assess how much air is infiltrating the building through its gaps and cracks. Specific leaks can be detected during the test by hand, by employing tracer smoke or by looking at thermographic images. It is best to conduct the blower-door test at a point in construction when the airtight layer still can be easily accessed and leaks can be readily addressed. At a standard test pressure of 50 Pa, a Passive House must allow no more than 0.6 air changes per hour to achieve certification through the Passive House Institute US, Urbana, Ill. Passive Houses built from timber, masonry, prefabricated elements and steel-framing members have met this standard. Airtightness does not mean you can’t open windows! Passive Houses have fully operable windows; most are designed to take full advantage of natural ventilation to help maintain comfortable temperatures. The final four principles are addressed in the following section, Part II.

Part II
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.

The following section addresses 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.