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Passive Design Strategies

Expert tips on achieving an energy-efficient building

Twin Rivers Aug18 2
Eave overhangs at Twin Rivers Charter School in Yuba City, Calif., shade windows during the summer, while allowing solar heating during the winter. Photo: Jerry Gregg, Swift Lee Office

To increase the energy efficiency of a building, a variety of active and passive design strategies can be incorporated. Active strategies usually consist of heating and cooling systems, while passive design measures include building orientation, air sealing, continuous insulation, windows and daylighting, and designing a building to take advantage of natural ventilation opportunities.

“Passive measures find ways to reduce the size of the heating and cooling system by keeping the heat (or cooled air) inside the building,” explains James Ortega, who is part of the certification staff at the Chicago-based Passive House Institute US Inc. (PHIUS).

As Vuk Vujovic, Assoc. AIA, LEED AP BD+C, WELL AP, principal and director of sustainability and energy at Chicago-based Legat Architects, explains, the key to passive design is to minimize the energy used by the building, including eliminating plug loads and specifying Energy Star equipment. He recommends doing an inventory of everything that uses electricity in the building, so even the plug loads can be included in design calculations.

“Passive design strategies take advantage of natural energy opportunities as they relate to the location of the building’s site, the local climate (and the site’s microclimate, if relevant), and the properties of building materials,” notes Christine Robbins-Elrod, AIA, LEED AP BD+C, WELL AP, GACP, associate principal at 5G Studio Collaborative in Dallas. “Active design strategies would then become part of the design process when mechanical and electrical systems are integrated into the building design.”

Passive design strategies are a designer’s first opportunity to increase a building’s energy efficiency before going into more advanced building systems. As Robbins-Elrod explains, “These are typically going to add much less front-end cost to a project as compared to active design strategies, and they also have the added benefit of reducing the cost of active design strategies by reducing heating and cooling loads so that a building’s mechanical system to be downsized and sometimes reducing a building’s electrical lighting through the use of daylighting design strategies.”

The dual-glazed prismatic, acrylic-domed skylights at Twin Rivers Charter School integrate seamlessly with Butler's MR-24 standing seam roof.

BUILDING ORIENTATION

Finding the right combination of active and passive strategies to incorporate into a project comes down to balancing the budget, program and the specifics of the site and microclimate. “Passive design depends a lot on the way the building is orientated,” Vujovic says. “So, the most successful energy-efficient designs are facing south or north to allow better solar energy management.”

Orientating a building to take advantage of how the sun moves across the sky is the easiest and most effective passive design strategy. “We know that the same free heat streaming through windows in the winter can sabotage comfort in summer—unless it is considered in the design,” Ortega explains. “Overhangs, exterior shades and deciduous trees can help keep summer sun out while thermal mass—like a concrete floor or wall—inside the building can store winter heat.”

An elongated and narrow building footprint allows for more of the building to be daylit. “This is usually the best form for passive design in terms of massing. The building can be designed as a single or multi-story, but the long, south or north facing façade facing is the key element of good passive design,” Vujovic says.

“It is typically best for the longest façades to face north and south so that a building can take advantage of indirect sunlight (without glare and direct solar heat gain) from the north, and controlled direct solar heat gain from the south,” explains Robbins-Elrod. “While direct solar heat gain at east and west-facing glazing can be minimized with exterior shading during the months that a building is mechanically cooled, is it easiest to control direct solar heat gain at south-facing glazing where exterior horizontal shading elements can protect the building from direct solar heat gain in the summer months when the sun is highest in the sky and allow the building to take advantage of direct solar heat gain in the winter months when the sun is lowest in the sky.”

Gloria D. Lee, principal of Swift Lee Office, Pasadena, Calif., notes that for the Twin Rivers Charter School in Yuba City, Calif., the design team looked at a variety of different orientations and settled on taking advantage of the prevailing east-west wind. They also designed a courtyard between the two buildings, which helped cool the buildings by providing shade and oxygen, and controlling the winds.

The building envelope of Legacy ER in Allen, Texas, was optimized for energy efficiency in the hot Texas climate.

DAYLIGHTING

The climate in which a building is located may dictate the type of windows needed. Ortega notes that in hot climates, the goal is to keep heat out of the building, so windows may have low-E coatings that exclude radiant heat and/or lower solar heat gain coefficients (SHGC). On the flip side, he goes on to say that cold and mixed climates can be a bit trickier, since part of the year is hot and the other is cold. “Some faces of a building are more prone to heat gain or loss,” he explains. “In general, windows in colder climates should be specified depending on the wall they sit in.”

Building orientation and exterior shading options are important considerations when locating windows and glazing. “It can be more difficult to control direct solar heat gain and glare at skylights, but it helps to orient skylights to maximize daylighting from the north,” Robbins-Elrod says. “Typically north-facing glazing is best for quality daylighting, but daylighting strategies can also be very effective for south-facing glazing (and can help for west and east-facing glazing as well).”

“It’s very important to use advanced window systems and technologies, including cutting-edge glazing and coatings that are available today,” Vujovic notes. It’s also important to understand the impact of natural light, heat gain and glare on building systems design. By coordinating the massing of the building, location of windows and daylight harvesting with mechanical and electrical design, engineers can downsize the building systems and match the high efficiency goals of the project.

And, Robbins-Elrod notes that louvers and grilles can help protect the building envelope openings from unwanted debris, while allowing air to flow in and out of the building. “Louvers and grilles can also be integrated into exterior shading elements, which can help significantly in reducing solar heat gain during warmer months of the year.”

“Exterior shading, either operable or fixed, can block unwanted sun from entering the living space during summer, while allowing heat to enter during winter,” explains Ortega. “Because the sun is at a different angle in the sky, these exterior shades and grilles can be tailored to the path of the sun for that particular location.”

Metal panels frame a south-facing extension at Joliet Junior College’s Natural Sciences Building in Joliet, Ill. Additionally, randomly arranged metal sun shelves extend 8 inches from the curtainwall to add visual interest and shade the glass at a nominal cost.

NATURAL VENTILATION

According to Robbins-Elrod, natural ventilation is most effective in climates where there is a comfortable ambient air temperature outside for a number of months of the year. Using natural ventilation as a passive design strategy is less common in climates where there are fewer days out of the year that a building can be comfortably occupied without being mechanically cooled or mechanically heated.

“When using natural ventilation as a passive design strategy, it is important to consider how air will move throughout the space given a number of factors such as the orientation of windows and other openings to be used for natural ventilation and the physics of how air moves (e.g., cold air sinks and hot air rises),” she says.

According to Lee, the balance between active and passive systems can be achieved by relying on the passive systems before the set point for the active system is reached. “You can reach the set point by having natural ventilation work for you with the windows open, so rooms stay cool and the active system doesn’t kick in,” she says.

INSULATION

Adding insulation to a building is another passive design strategy. “What is required by energy code in terms of insulation is usually just the bare minimum,” Vujovic explains. “Designing a building envelope above the mere code requirement in terms of adding extra insulation is always a good passive design strategy. If a building is designed properly, the extra money invested in better insulation, better windows and a better roof, will result in a reduction of the cost of the mechanical and electrical systems and a passive design structure.”

Insulation is particularly important for buildings in colder climates. “Insulation helps the building envelope to resist the conductive flow of heat, and it is typically most effective when installed as continuous insulation (which significantly reduces thermal bridging as compared to cavity insulation),” says Robbins-Elrod.

Continuous insulation means the building is essentially wrapped with a blanket of insulation outside the structure to thermally separate the inside from the outside with no thermal bridges. “This is important not just for energy performance and comfort,” Ortega says, “but for indoor air quality and mold elimination. Thermal bridges tend to become cold spots, which will condense moisture out of the indoor air, just like a glass of lemonade will. The cold spots are often hidden corners where mold can grow, rot can develop and bugs can move in without being noticed.”

Curtainwalls, light shelves, a slim building design, and an east-west orientation bring daylight to 95 percent of interior spaces at Mundelein High School, Mundelein, Ill., science and classroom expansion. A skin of insulated metal panels provides a thermal envelope, while complementing an adjacent addition.

METAL BUILDING PRODUCTS

Metal building systems and metal construction products can be used to aid in passive design strategies. For Twin Rivers, Lee notes the project used pre-manufactured metal building systems from Butler Manufacturing, Kansas City, Mo. “The systems are very quick to assemble, and they can also be disassembled easily,” she says. “The engineer took the solution with the least amount of material to achieve the most optimal structural design.”

And, Lee says insulated metal panels provided ease of construction, while also giving high R-values on the walls and roof.

According to Robbins-Elrod, metal’s reflectivity can increase a product’s solar reflectance index (SRI) value, aiding in the reducing the cooling loads through passive design. Metal also lends itself to providing attractive exterior shading elements that can block or minimize direct solar heat gain while preserving some transparency, such as using louvers or perforated metal, for an exterior building element.

“Metal building systems can be particularly effective when integrated with the building envelope to create strategically placed exterior shading,” Robbins-Elrod says. “Also, when used as roofing or as cladding for exterior walls, metal with a high SRI can help to reduce the building’s cooling loads.”

Ortega adds, “Metal roofs can act as radiant barriers, which can significantly cut the amount of work that the insulation needs to do. Because most of the heat that hits a building hits the roof, this can effect the amount of insulation needed and can free up some budget for upgrades in other places.”

Photovoltaics can also be installed easily on metal roofs, making it easier for projects to operate off the grid. At Twin Rivers, Lee notes that the roof was structurally engineered so it could support photovoltaics, which are being added over time, with the goal of achieving net zero energy.

“Passive building targets are an energy balance and performance-based set of criteria to reduce heating and cooling as the primary energy saving strategy,” Ortega says. “Because the entire building needs to be evaluated as a whole, there is no silver bullet for passive design strategies.”

And, as Robbins-Elrod notes, using 3-D modeling early in the design process can be very helpful in informing the design team on what passive design strategies will likely have the most impact on creating a highly energy-efficient building envelope.