Constructed in 2002–2003, the Smith House was the first house built in the United States using the specific practices, technologies, and energy-modeling tools developed by the Passive House Institute.
Led by Katrin Klingenberg, architect and founder of e-colab, an Urbana, Illinois, nonprofit specializing in energy-efficient design, the Smith House design tackled several goals. First was to reduce its operational energy use to one-tenth of a conventional home of the same size in the region. Second was that the design needed to consider lifetime consequences of product choices: all the house’s components had to be high in recycled content, and they needed to be reusable or recyclable at the end of the home’s useful life. Third, building materials were considered based on their environmental impact. This meant purchasing local, sustainably harvested wood products and locally manufactured building products whenever possible. Finally, since she would occupy the house, Klingenberg wanted to ensure that the house would help minimize her personal environmental impact. To reduce travel-related emissions, Klingenberg chose a site near a bus stop and that was close to the city where she works.
The Smith House’s simple, compact shape reduces energy losses through its roof and walls because of its small surface-to-volume ratio.
Although the home’s shed roof slopes toward the south, which is contrary to traditional passive-solar design, this orientation was chosen to provide the maximum solar exposure for a future PV system. And this shape resulted in additional useful interior space—a loft above the living room.
For optimal solar heat gain in winter, the Smith House is oriented due south (although up to 30° away from true south is acceptable for a Passive House—see page 70 in this issue), and all windows on the south façade have a solar heat gain coefficient (SHGC) of 0.61. In the summer, overhangs and a trellis provide exterior shading to prevent unwanted solar gain from increasing the cooling load.
Windows facing east and west are more important to shade in the summer than windows facing north and south, because they are exposed to hours of low-angled sun. Overhangs cannot block sunlight coming in at a low angle; only vertical exterior shading will do so. To minimize unwanted solar gain and overheating in the summer, the windows on the east and west sides of the Smith House are kept to a minimum, and those windows have a low SHGC.
Most of the windows in the Smith House are operable to take full advantage of the wind for natural ventilation in the warmer months. Windows placed high up on the north side of the house give warm air an easy exit, when the mechanical ventilation is turned off.
Insulation. Climate and location determine the appropriate amount of insulation for a Passive House. First to consider is an area’s winter design temperature—the lowest temperature that an area generally experiences. Urbana has a winter design temperature of -3°F, considerably lower than that for Berlin, Germany, at 7°F, and Paris at 22°F, for example—climates for which many Passive Homes have been designed and built. However, with abundant solar radiation, Urbana has a climatic advantage that makes the design of a Passive House a bit easier compared to cloudier locales. With these climate factors, Klingenberg determined that the Smith House required a superinsulated envelope on all six sides of at least R-56.
Foundation. The foundation of the Smith House is a concrete-block frost wall surrounding a floating 4-inch concrete slab, which was left exposed on the interior for thermal storage. The frost wall is insulated around the perimeter with 6 inches of expanded polystyrene (EPS), with 14 inches of EPS under the slab. EPS was chosen over extruded polystyrene (XPS)—the type of expansion agent used at that time in XPS depleted the ozone layer and contributed to global warming. All available research showed that EPS would perform appropriately below grade.
Walls. The walls of the envelope are balloon framed, using 12-inch engineered I-joists. The cavity is filled with high-density blown-in fiberglass insulation. An interior, structurally required sheathing of oriented strand board (OSB) serves as an airtight layer and a vapor barrier, since it has a permeability (perm) rating below 1. In addition, the Smith House is wrapped on four sides in two 2-inch layers of EPS, with the joints staggered. Another layer of 1- by 4-inch pine creates a vent space and doubles as a rain screen façade underneath the final layer of cedar lap siding. The roof framing is 16-inch engineered joists filled with high-density fiberglass insulation, topped by a vented metal roof.
Thermal bridging has largely been avoided due to the minimal thickness of the OSB that connects both flanges from the inside to the outside. Additionally, the TJIs are thermally broken on the exterior by 0.5-inch-thick structural fiberboard and 4 inches of EPS insulation. Penetrations through the exterior envelope are minimized to reduce air infiltration. Utilities and ducts enter the house from under the slab. Electrical installations, switches, and outlets along exterior walls are all surface-mounted or located in the floor to avoid penetrations through the exterior airtight layer. Conduit for a future PV system also enters the house from underneath the slab.
The plumbing vent stack is capped in the attic with an air admittance valve, a small vacuum cap that makes it unnecessary to install a vent stack above the roof, which would require a penetration. (Note that not all state plumbing codes allow using this valve.)
Windows. Windows are triple-pane and argon-filled, with low-e coatings specific to their location in the house. The insulated fiberglass window frames contribute to the overall thermal performance of the walls. All windows and doors have multipoint locks to ensure that they seal tightly when they are closed. Their overall airtightness was measured with a blower-door test at 0.52 ACH compared to 5.0 of an average conventional house. With an R-value between 6 and 8, all the windows, even the ceiling-height, south-facing ones, have a sufficiently warm inner surface to eliminate drafts caused by convection—so there is no need to place a heat source or supply vent directly under the window.
The central component of the mechanical system is a 90%-efficient heat-recovery ventilator. This European HRV has a computer-controlled summer bypass for the shoulder seasons when heat recovery isn’t desired. It exchanges the air in the house at a constant low flow, delivering air to the bedrooms and living rooms and exhausting air from the kitchen and bathrooms. The ventilation system has an integrated 1,000 W electric heater so no centralized, separate heating system is needed.
In winter, when the windows in the Smith House remain closed and the mechanical ventilation is working, stratification is noticeably absent. Temperatures on the second floor of the Smith House were measured in the winter and found to be lower than temperatures on the first floor. This is the opposite of what happens in most conventional homes and what one would expect, given that hot air rises. A core principle of a Passive House is insulating the walls, floors, and windows to a level that the surface temperature difference of exterior components is no less than 4°F from the temperature of the interior walls. But in the Smith House, and in Passive Houses generally, air mixes very slowly and evenly, keeping surface temperatures even, which helps to reduce stratification.
The fresh air intake for the ventilation system is a 100-foot-long, 8-inch-diameter PVC “earth tube” buried 6 feet underground, sloping away from the house to drain condensation. A filter at the intake keeps out mold, mildew, and other organic matter. The earth tube enters the house from under the slab, and so does not act as a thermal bridge. In the winter, the earth tube prewarms the incoming air to above-freezing temperatures; in the summer, it precools and dehumidifies the incoming air, keeping the house comfortable without any mechanical air conditioning.
Construction cost for the Smith House, using exterior dimensions, was $94 per square foot—at the time, 17.5% more than a conventionally constructed home—but with 10 times the energy performance. Simple payback for the Smith House would be about 17.5 years—the home’s average utility bill is $50 and a house of comparable size in the same climate averages $150 per month, yielding a monthly savings of $100. But this distorts the picture, since simple payback does not consider interest or energy price increases. The initial project budget did not include the installation of a PV system or rainwater catchment and distribution system that would have made the home more self-sufficient. But preparations were made for adding these features in the future.
Katrin Klingenberg is the cofounder and executive director of the Passive House Institute US, which promotes the Passive House standard. She designs and consults on Passive House projects across North America, and is a licensed architect in Germany.
Mike Kernagis is the cofounder and program director of the Passive House Institute US. He was one of the first builders to adopt and build to Passive House standards, including the nonprofit Ecological Construction Laboratory building.