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Case Studies: The Tofte Cabin

GENERAL INFORMATION

Project name: The Tofte Cabin

Location: Tofte, MN

Web site: www.tofteproject.info

Architect: Sarah Nettleton Architect (www.sarah-architects.com)

Builder: General Contractor Tofte Construction, Greg Tofte

Building size: 950 sf

Building use: Residential (remodeled cabin)

Completion date: 2000

Ratings and awards:

  • TOP TEN GREEN PROJECTS AWARD 2002 - American Institute of Architects Committee on the Environment
  • HONOR AWARD 2001 - American Institute of Architects/Minnesota Chapter

OVERVIEW

Cabin owner Medura Woods approached Sarah Nettleton and the design team with a challenge in 1997 to transform a 950 sf 1947 cabin into a model of sustainable design. The list of goals and strategies included sustainability issues addressing waste, materials selection, orientation, daylighting, and energy production. The challenges at that time included scarce resources and information for solving the design problem. According to Nettleton, the process was educational as they pioneered a new approach to design. The results included a passive solar design with applications of both solar and wind systems grid inter-tied with battery back-up.

BUILDING PERFORMANCE

Effective energy use solutions: In shaping the design of the building, several concepts fed into the programming. At the top of the list was the health and comfort of the client. From this came the strategies of indoor air quality, natural ventilation, and daylighting. As the inventory of goals began to grow, a theme that has driven Nettleton's work over the years emerged: simplicity.

The footprint of the cabin grew only by 3%, but remained in the same position. This left the cabin surrounded by trees while still enabling it to connect with the sun, wind, and the lake. The orientation of the building's views, still focused on the lake, were matched with the sun's seasonal path across the sky. The winds from the lake moving up the slope enabled natural ventilation to mesh with the orientation and decision to maintain the placement of the cabin.

In order to let the light into the cabin Nettleton chose to lift the roof off, turn, and split it. This choice allowed daylighting and shade to convert the space into a warmly lit, but carefully modeled entry of light. Nettleton described the outside to be like a wave pattern while the curved interior ceiling connects to the historic character of the region, and recalls the inside of a boat. The walls were super-insulated and made tight to yield a wall depth that enhanced the quality of light. The window placement provided connectivity and a sense of place while enhancing the views and the feeling of being out in nature. Each step of the design was carefully conceived, effectively connecting the building with the site and the natural elements.

Orientation: South

Daylighting strategies: Nettleton developed daylighting concepts in the Daylighting Lab with Mary Guzowski from the Architecture Department at the University of Minnesota. The modeling for daylighting scenarios helped Nettleton guide the design to integrate window and wall placement for effective daylighting as well as natural ventilation. The clerestory windows on the south side allowed the living areas of the public space to gain light as well as let light into the bedroom space over a lowered wall. The placement of large windows at eye level allowed the rooms to be filled with light as well as provide views of Lake Superior.

Passive heating and cooling strategies: Super-insulation and a tight building combined with appropriately placed windows and overhangs allowed the building to retain the heat gained during the day from the sun. Mass was incorporated in the radiant floor for heat storage. The overall net zero energy effect is less like traditional passive solar homes with large amounts of mass, but more like the Passive Haus standards of tight well insulated approaches, used in Germany, mentioned in the Case Studies for the Bio Haus and Eco Home. The window placement allowed for natural cooling, both from cross ventilation as well as the chimney effect from the vertical exhausting of warm air out of the clerestory windows.

Shading of structure: Overhangs and existing tree canopy

Envelope:

  • Insulation values:
    • Ceiling uses extruded insulation: U-value 0.026, R-value 38.5
    • Walls use blown in cellulose: U-value 0.048, R-value 20.8
    • Foundation: U-value 0.05, R-value 20
  • Building tightness: Extensive attention to building details ensured a tighter envelope.
  • Windows: U-value 0.27, R-value 3.7; Solar Heat Gain Coefficient of 0.27

Climate control systems: The heating system consists of a ground source heat pump connected with the radiant floor, which also provides the domestic hot water, and a heat recovery ventilator exchanges heat with fresh air in the ventilation system.

Backup heating/power: wood burning stove, propane, utility grid inter-tied

Total building energy use: 11,000kWh, 43% better than a code base building

RENEWABLE ENERGY SYSTEM INFORMATION

Solar system description and size: 8.2kW, 415 sf. Building Integrated PV on garage at 35 degrees

Wind system description and size: 3kW Jacobs Longcase on a 90 ft. tower, 14 ft. rotor diameter

Date installation was completed: 2000

System designer: Robert Erb, Solar Design Associates

Engineer: Jim Keller, PE, Gausman & Moore

Installer: Kenny Pohjola, Electric Contractors Inc.

Daylighting consultant: Mary Guzowski, College of Design, University of Minnesota

Estimated amount of energy delivered by system: 11.2kW; 11,000kWh/yr; 49.95 KBtu/sf/yr

Percent of building's total energy use provided by solar/wind: 90%

TOOLS UTILIZED

Modeling software: After construction, the Weidt Group developed an energy study comparing the cabin with a code based building.

Models constructed: Models were tested at the Daylighting Lab at the Architecture Department, University of Minnesota.

MOTIVATION FOR INSTALLATION

The motivation for Medura Woods was to renovate her cabin for use throughout the year without the materials waste, energy consumption, and site destruction such projects can typically cause. In essence, she wanted to preserve the site while developing a net zero energy building that could drive the market forward.

LESSONS LEARNED

As Nettleton worked through the process of connecting daylighting and passive solar heating with zero energy design, a meeting point of strategies conflicted with available technologies.

Appliances with the low energy efficiency loads needed for incorporating renewable energy systems at that time were not quite at the off-the-shelf stage and had to be sought out. In order to utilize renewable energy systems effectively, the energy loads needed to be reduced significantly. Each new solution brought forward more challenges. Reducing the load profile was accomplished with the simple design decisions first: orientation, ventilation, placement of windows and overhangs. The more complex strategies such as super-insulation, ground-source heating, radiant floors, wind and solar energy required technical knowledge from engineers.

Making the connection with the energy of the site meant placing the 3kW Jacobs Longcase wind turbine and solar panels out in the open, away from the shade and turbulence of the trees. Placing the 8.2kW panel array on the garage roof enabled Nettleton to keep the curved roof and clerestory division of the space. The balance of systems was placed in the garage to keep the technical systems at a minimum within the house. One of the largest challenges of solar technology is merging design to include large arrays on the roof that get full sun. Including solar panels can compromise design elements and conflict with natural shading provided by deciduous trees.

In the end, Nettleton and the design team moved forward with a project that has helped inform other zero energy design projects. Slowly over time, more are being developed in the cold climate upper-midwest.

OTHER INFORMATION

Other Sustainable Features:

  • Space and material use reduced by design
  • Waste reduction
  • Certified lumber
  • Sustainable materials selection
  • Materials reuse/recycling of 95% of the old cabin material

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