#Relatively Small Investments Can Add up to Big Gains with the Right Design Strategies
With R-60 insulation and a south-facing wall punctuated by triple-glazed windows, this Passivhaus home stays comfortable during cold Illinois winters with a heater the size of a hair dryer.
There’s actually a bit more to the story: The tall wall of glass brings in a good deal of solar heat; an exposed concrete floor absorbs and slowly releases that heat; a 100-foot-long earth tube warms incoming fresh air; and an HRV equipped with a 1,000-watt electric resistance heating element scavenges heat from outgoing stale air. The earth tube and HRV also help keep the home cool during warmer months.
Following a good example
The Smith House is the modest, comfortable home of German-born architect Katrin Klingenberg. More important, it’s an example of what she believes could and should be done with residential construction in this country. She and her late husband, Nic Smith, were inspired to build the house after visiting a Passivhaus residence in Hanover, Germany.
Katrin and Nic were disheartened that most energy-efficient building projects fall short of the environmentally responsible benchmarks being set around the world. They hoped to create a building envelope that would achieve a tenfold reduction in energy use compared to a typical home. They were guided by the standards established by the Passivhaus Institut and the Institut’s Passive House Planning Package (PHPP) software.
In 2004, not long after Katrin’s new home was completed, she had the chance to meet Dr. Wolfgang Feist, director of the Passivhaus Institut. Dr. Feist performed energy modeling that confirmed that Katrin had more than met the Passivhaus heating load standards, prompting him to include the Smith House as the first sample home in an English translation of the PHPP.
Keeping the heat in
The house incorporates construction details that might be considered extreme by many American builders: The insulation layer under the concrete slab — 14 inches of expanded polystyrene (EPS) foam — is unusually thick; 12- and 16-inch I-joists create deep wall and roof cavities, making room for plenty of blown-in fiberglass insulation; the exterior walls and foundation are wrapped in 4 and 6 inches, respectively, of EPS; to reduce the chance of infiltration, utilities and HRV ducts enter from deep beneath the slab, and interior wiring and electrical devices (with the exception of wireless light switches) are confined to floors and interior walls.
Sustainability extends beyond the site
Building this house was about more than just energy savings. Katrin’s commitment to environmental responsibility demanded that she scrutinize every material that went into the home. She also considered how sustainably the components were produced and transported and what would happen to them at the end of their lifespan. Proximity to work and public transportation influenced the choice of where to build the home. Katrin planned for future reduction of her environmental impact by roughing in electric and plumbing lines for future photovoltaic (PV) and rainwater collection systems.
Cost: The bottom line
At $94 per square foot, the house topped the highest averages for new construction in the region, although not by much. With Katrin’s modest budget and her goal of using the home as a model for affordable housing, the cost was more than she would have liked. But she points out that this was a prototype that would likely cost less on a production scale. Besides, in her opinion, the successes in the area of sustainability, efficiency, and comfort were well worth the investment.
The good news is that the design worked well. The house is always very comfortable. As for the heating load, recent tests put the annual demand around 11 kWh per square meter. That's a bit above the expectation of 8 kWh, but it's a full 4 kWh below the maximum prescribed by the Passivhaus Institut. Possible explanations for the discrepancy might be higher-than-assumed thermal bridging through the I-joist framing and poorer-than-expected performance of the exterior door seals.
Tight homes can still have fresh air
Katrin often keeps the windows closed even in warm months because the HRV and earth tube do a great job of keeping the humidity down. Natural building materials and an effective ventilation system are the keys to the home's good indoor air quality. In fact it was one of only four test homes in the region that met the EPA's strict indoor air quality standards. At least one visitor to the Smith House felt the benefit of this, noting that his chronic allergy symptoms subsided when he was inside the home.
Solar heat is only good when it's cold outside
Although the house passed the test in the heating department, Dr. Feist voiced concern that the large amount of unshaded south glazing might cause overheating in summer. He was right. "The first couple of years, it was tough," Katrin admitted. She constructed a grape trellis, and when the vines finally reached the top of the structure, the home stayed comfortable on even the hottest days. Katrin pointed out that it's reasonably common knowledge that deciduous plants are a great automatic source of seasonal shade, "but when you live with it, and see it, it's totally amazing." She says that with the right shading, there's no need for an active cooling system in Illinois.
General Specs and Team
Builder: Edward Sindelar, Chicago
Architects: Urbana, Ill.
Energy consultant: Duluth, Minn.
Foundation: 4-in. slab over 14 in. EPS foam (R-56), surrounded by concrete-block frost wall covered in 6 in. of EPS foam (R-24)
Walls: vertical 12-in. I-joists (Trus Joist); 12 in. blown-in fiberglass, plus 4 in. EPS foam over exterior sheathing (R-60 total)
Roof: 16-in. I-joists with vent channels above the sheathing; 16 in. of blown-in fiberglass (R-60)
Windows: triple-pane, argon-filled, low-e, fiberglass frames with XPS insulation (average overall U-factor, .19)
- Superinsulation (R-56) and supertight thermal envelope
- I-joist framing for minimal thermal bridging
- Oriented for maximum passive solar heating and cooling
- Triple-glazed windows positioned for winter solar gain
- Instantaneous electric water heater
- Prewired for future PV system
Cooling: HRV (Westaflex) with 1,000-watt electric resistance heating element coupled to 8-in.-diameter, 100-ft.-long earth tube buried 6 ft. deep
Water heating: tankless electric water heater with spare conduit for future solar thermal system (Stiebel Eltron)
Annual energy use: 14.8 MMBtu
Indoor Air Quality
- HRV constantly exchanges air
- Linseed oil floor finish
- Water-based sealant on concrete floors
- No-VOC wall paint
- Solid wood countertops
Green Materials and Resource Efficiency
- Durable, recyclable galvanized-steel roof
- CMU frost wall/on-grade slab are material- and labor-efficient design
- Recycled tub, fixtures, and medicine cabinet
- 100% regionally sourced wood (fir, pine, and cedar)
- Recycled slate cladding on foundation
- Recyclable blown-in fiberglass insulation