Wood is an attractive material for passive design because of how it combines thermal mass with a number of performance merits, including water resistance, structural integrity and finish quality.
What Is Passive Design?
Passive design is an exacting, voluntary design and construction standard for new and existing buildings that was popularized in Europe and is gaining traction in the U.S. Passive House buildings have highly efficient envelopes to maintain consistent energy performance and indoor comfort year-round.
Passive House gets its name from the design strategies it employs, including building orientation, an air-tight and highly insulated envelope, solar shading and the use of thermal mass. High-efficiency heat exchangers are often used to bring fresh air into the building and ensure that heat from the ground is dispersed throughout the space.
According to the International Passive House Association (iPHA), Passive buildings use as much as 90 percent less energy than typical buildings, and many Passive projects strive for net-zero energy.
According to the iPHA, there are more than 60,000 buildings around the globe built to Passive House standards, though far fewer are certified. Today, more commercial buildings are seeking to meet the Passive House standard, helped along by wood products in structural and finish applications.
Several organizations promote Passive House globally. Those include the International Passive House Association, the Passive House Institute US and the Passive House Institute in Germany.
Passive House History
To understand the role of wood in Passive House construction today, it’s helpful to look back at how the material has historically been used in buildings. Heavy timber construction is an age-old structural solution that can withstand high stress levels while maintaining a minimal environmental footprint. According to the American Wood Council, heavy timber is one of the oldest building types in the U.S. It has typically been used for multistory industrial and storage buildings due to timber’s strength-to-weight ratio.
Wood’s Role in Passive House
Both light-frame solid wood product are a natural option for helping to meet Passive House’s space heating and cooling requirements.
Softwood lumber products in particular are ideal for this. Thermal capacity is determined by its thickness and is measured in R-value (insulating capability) and U-value (heat transfer coefficient). The thicker the panel, the lower the U-value and, as a result, the higher the R-value, which is measured at 1.25 per inch thick. Softwood lumber is one-third as insulating as an equal amount of fiberglass batt insulation, 10 times as insulating as concrete and 400 times as insulating as steel, according to the American Wood Council.
Mass timber components are fabricated to meet the exact specifications of a project, achieving a tighter fit in application and reducing airflow between components. Compared with concrete and steel, wood is more easily routed for mechanical, electrical and plumbing systems, offering project teams more capacity for insulating walls and roofs.
Other uses for wood in Passive House projects include:
- Framing high-performance windows
- Wood fiber insulation
- Advanced framing systems that allow for more insulation
- Structural insulated panels with insulated foam between layers of oriented strand board
The following case studies show how wood can be used to meet Passive House standards.
Case Study: Kiln Apartments; Portland, Oregon
Completed in June 2014, this project was the first market-rate multifamily building in the U.S. to achieve Passive House certification. The 16,000-square-foot, five-story project uses wood in several ways to ensure Passive House performance and evoke the feel of a smaller-scale, single-family home. Those include a highly insulated wood frame, high-performance wood windows and wood finishes throughout the interior and exterior. Read more.
Case Study: Rocky Mountain Institute Innovation Center; Basalt, Colorado
The new headquarters for the nonprofit Rocky Mountain Institute used wood to help achieve net-zero energy performance and Passive House certification. A CLT structure, exposed glulam beams and columns, a wood-slat ceiling and Douglas fir stair treads are a few of the ways Portland, Oregon–based ZGF Architects incorporated the natural building material. Read more.
Case Study: Orchards at Orenco; Hillsboro, Oregon
Wood was critical to allowing this affordable housing project to achieve Passive House certification. The three-story, 58,000-square-foot project features a 2×10 wood frame with plywood exterior sheathing and prefabricated wood roof trusses, creating an air-tight and highly insulated envelope. Read more.