Principles of Passive House Design, Part I

A German Standard Dramatically Can Improve Energy Efficiency of Buildings

By Katrin Klingenberg and Mike Kernagis

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.

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.

For images of commercial buildings using Passive House guidelines, visit the Media Center.

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 will address the first three principles of the Passive House standard—superinsulate, eliminate thermal bridges and make it airtight. The other four principles are addressed in “Passive House Principles, Part II.”

Superinsulate

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 “Passive House Principles, Part II.”

Katrin Klingenberg and Mike Kernagis are the founders of Passive House Institute US, Urbana, Ill. For more information, visit www.passivehouse.us.

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.
James Anderson of the University of Illinois’ Building Research Council, Champaign-Urbana, performs a blower-door test to measure the airtightness of a Passive House.
Completed in 2003, the Smith House, Urbana, Ill., was the first house built in the U.S. to the Passive House standard.

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