Habitat Northside Net Zero Home

Minneapolis, Minnesota
Anticipated Completion Date: December 2013

The students began with initial designs based on Habitat for Humanity prototypes, developed over the first 6 weeks of the semester.

A number of open community meetings were held with the Hawthorne Neighborhood Housing Committee to gather and integrate feedback from Hawthorne neighborhood residents, and the designs were presented to construction supervisors in the Twin Cities affiliate of Habitat for Humanity over the course of the semester to ensure that they met Habitat for Humanity specifications and that they could be built by Habitat for Humanity volunteers.

The students completed a preliminary set of construction documents at the end of May 2012. The document set was given to Habitat for Humanity and over the summer and fall of 2012 the construction drawings were thoroughly analyzed and validated to ensure the integrity of structure, construction sequence, and energy balance.

Ground was broken in May of 2013, and is completion is expected in December of 2013, with the family moving into the house in January 2014.

  SITE

Neighborhood residents and leaders, community agencies, and the City of Minneapolis developed a shared vision for the EcoVillage. The area is part of a "cluster" approach developed by the City of Minneapolis and the Family Housing Fund for neighborhood groups to identify development sites with the potential for making a positive and stabilizing impact in neighborhoods deeply affected by foreclosures. The Hawthorne EcoVillage community will include both new and existing homes with opportunities for mixed-income living. Overall cost for the development of the four block area is $32 million and funders included Home Depot Foundation, Twin Cities Community Land Bank, the Neighborhood Revitalization Program (NRP), Wells Fargo Foundation and the City of Minneapolis.

The house was built on a 0.11 acre urban infill lot purchased from the city of Minneapolis for $4,400, with a density of 8.1 units/acre, qualifying as a moderate density development per LEED for Homes. A density of at least 6 units/acre is required to qualify as a compact development for the Green Communities Criteria. The site fulfills LEED's mandatory site selection measure that restricts building within the 100 year flood plain, in areas that threaten habitat of endangered species, within 100 feet of a water body, on prior public parkland or an area with prime agricultural soils. The site is adjacent to existing utilities and connected to the pedestrian grid. Doors at the front and back of the house and side of the garage connect to pathways and sidewalks and promote walkability to the rest of the neighborhood. The NSNZ home is well connected to the rest of the community, with over 14 basic community resources within ½ mile away, a community park within a few blocks and a newly revitalized commercial corridor within walking distance (Green Communities 2.1c) Within ¼ mile, bus stops service over 60 transit rides per weekday (LEEDH SS5.3, Green Communities 2.7). There is a strong participatory and educational component to the EcoVillage, with a public web site that contains information on green features, LEED educational signage, an on-site nursery and demonstration garden serving the neighborhood.

Design
Students made extensive use of the Passive House Planning Package (PHPP) energy-modeling software. The PHPP was developed by the German Passive House Institute to perform energy use calculations on single zone structures, with particular accuracy pertaining to issues of thermal bridging, airtightness, and solar gain. By manipulating variables within the PHPP, the students were able to balance the design factors influence energy consumption. Initial airtightness was assumed to be 1.5ACH50, or what could be assumed reasonably achievable using unskilled volunteer labor. Insulation levels were optimized depending on insulation material, location of insulation installation, and ease of installation.

The site has a North/South orientation, 46' x 116', which posed a challenge to solar design. Ideally, structures optimized for solar design are oriented with at least a 1.5:1 ratio of E/W facade: N/S facade. A one-story home to the east and a two-story home to the west also shaded the site.

The resulting narrow solar window on the site affected window size and placement on each facade and the direction of the ridgeline of both the house and garage roofs. The garage is seen as integral to the energy balance of this home, in that the entire PV array is placed on the garage roof to avoid shading from trees on the boulevard to the south that would shade much of the property during the winter months when the sun is low and most needed.

The potential for shading led to a deliberate height increase to maximize the potential for solar gain on the roof, and placement of the garage as far north on the lot as was permissible. Careful attention was paid to the North slope of the house roof so as not to shade the garage, while still providing plenty of comfortable living space within the home.

To help ensure that the energy performance goals were met, the unique construction of the building envelope had to be carefully described to all involved parties. The students developed step-by-step diagrams to illustrate various assembly sequences. Additionally, the students constructed full-scale mock-ups of important details at the window openings that showed the proper assembly sequence and layering of waterproofing and insulation materials.

The house is designed to be easily wheelchair visitable, including access ramps to main doors and zero-step entries.

  ENERGY

The house is superinsulated, with above-grade walls at approximately R-48, below grade walls at approximately R-39, the roof at approximately R-61, slab edge at R-20, and sub-slab at approximately R-30. This was achievable in part due to a unique relationship between Habitat for Humanity and a major extruded foam manufacturer who donated $8,000 worth of extruded polystyrene panels. It should be noted that the thickness of foam used in this project (8" of XPS on the exterior walls) would typically be cost prohibitive. The students' designs capitalized on two unique aspects not usually present in high-performance housing design: Availability of inexpensive rigid foam insulation, and availability volunteer labor.

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Heating and cooling is achieved with two high-efficiency mini-split wall-mounted heat pump units (Mitsubishi MUZ-FEI2NA; Cooling 12,000Btu/hr, SEER 17; Heating 8,300-13,600 Btu/h, HSPF 8.2). One delivery unit is installed on each floor. The system includes programmable thermostats to optimize efficiency. Heating and cooling is distributed via the ventilation system, described in greater detail in the IAQ section.

Six inches of rigid insulation (R-24) were installed under the slab, with four inches of XPS on the sides of the footing. Original plans called for two inches of high-compressive strength foam underneath the footing, but this was not installed. Rim joists are sealed and insulated with six inches of polyurethane spray foam. All wood-to-wood connections on exterior walls were sealed with caulk during assembly, and a visual inspection was completed for the Energy Star program's Thermal Bypass checklist to ensure that air and thermal barriers throughout the home are continuous and properly aligned, with insulation installed correctly to minimize gaps and voids. Blower door results for airtightness are forthcoming.

Above-grade walls were constructed with conventional 2x6 platform stud framing. The stud cavities are filled with R-19 fiberglass insulation, with two additional layers of 4" extruded polystyrene (R-4.9/in.) applied to the exterior. Continuous box beams fastened at each rim joist support the foam and exterior siding.

Domestic hot water is heated with three flat plate solar panels (Solar Skies SS-40) that heat water in a hot water storage tank (Solar Skies 915-018). An integrated "drainback" system eliminates heat loss whenever the sun is not shining, and protects the system against freezing in extremely cold temperatures. An estimated 80% of the DHW heating will be met by the solar system. Total cost of the solar DHW system is $6576.00, which includes the panels, pumps, controllers, and storage tanks. Cost does not include installation labor, which was donated.

The house is designed to have net-zero annual electrical consumption, with solar electricity for the home generated by a 5.7kW system composed of thirty 190w panels (manufacturer: TenK) mounted on the garage roof. The panels run to a Sunergy ELV inverter inside the garage, and then to a bidirectional meter that will tally power used and created by the house. Total cost of the PV system is approximately $19,000 (price includes panels, inverter, shutoffs, and mounting racks. It does not include installation labor, which will be donated, or state and federal rebates for solar system installation). Students in the Solar and Renewable Energy Program at Century College specified the layout and components of both the photovoltaic and solar DHW systems, and aided in installation.

All windows in the house are triple-paned with moderate solar heat gain (Richlin 500 series; tri-pane with krypton fill, foam-filled vinyl frames, U-.13, SHGC .22). The windows are mounted on the outside face of the 16" thick walls to provide the best opportunity for solar gain.

All appliances in the house will be Energy Star certified. Refrigerator: Whirlpool, W8TXEWFYQ, 18 cu.ft, 343 kWh; Range: Whirlpool, WFC130S0AB; Clothes washer: TBD; Clothes dryer: TBD. To minimize lighting energy demand, approximately 80% of installed lighting will be LED, with the remaining 20% as CFL.

Total Energy Consumption
The NSNZ home will be Energy Star certified, and HERS rating information will be posted when it becomes available. Subsequent annual utility consumption will be tracked by Habitat for Humanity of the Twin Cities.

  POTABLE WATER

"Very high efficiency" water conserving fixtures are installed at the NSNZ home per LEED for Homes guidelines (WE 3.2). The maximum flow rates specified are 1.28 gallons per flush (gpf) for toilets, 1.5 gallons per minute (gpm) for lavatory faucets, and 1.5 gpm for showerheads. The home also met an advanced Green Communities measure that also required a maximum flow rate of 2 gpm the kitchen faucet (Green Communities 4.1c).

  MATERIALS

A framing order waste factor limit and framing efficiencies were specified to reduce the amount of wood used for construction. Habitat for Humanity of the Twin Cities prefabricates the walls for all of their residential projects using volunteer labor in a warehouse that minimizes construction waste and protects construction materials from on-site weather and damage. All wall panels are preplanned using right-size headers and advanced framing techniques. Lumber used for framing was finger-jointed stock, which allows use of smaller pieces of lumber in full-length stud applications. Framing efficiencies were also met through pre-engineered open-web trusses.

Interior floor finish surfaces are all durable surfaces that require little maintenance and do not abrade.

Exterior paneling is composed of a mixture of cementitious panels and cementitious boards mounted in a rainscreen-type assembly for durability and minimal maintenance.

  WASTE

A local waste handing company (Atomic) was retained to handle all construction waste. They are able to divert 60% of construction waste from the landfill using a single-stream sorting facility.

  INDOOR AIR QUALITY AND OCCUPANT HEALTH

An effective whole house ventilation system is a necessity for a tightly constructed home in order to prevent moisture and contaminant accumulation. The NSNZ Home incorporated an air-to-air heat recovery ventilator (Zehnder Comfoair 350; >90% efficiency) to provide outdoor air and a balanced ventilation system. Heat recovery reduces heat energy loss by transferring heat between fresh air and exhaust air. Fresh air circulation within the home is assured by using transfer grills and room transfer fans to allow air to flow between rooms even if doors are closed. Using manifold ducts connected to the HRV, stale air is exhausted from the kitchen and bathrooms, and fresh air is delivered to the bedrooms and living room areas.

The HVAC system was properly sized using ACCA Manual J to increase occupant comfort and optimize energy performance. This measure was an item in all four building checklists: LEED for Homes, Green Communities, Energy Star for Homes with the Indoor Air Package, and Health House. All closed rooms, with the exception of the kitchen, laundry room, baths or closets, were provided with a transfer grille or jump duct to minimize pressure differentials, and all ventilation system components were sealed during construction to prevent contamination with construction dust or debris.

MERV 7/8 air filters are installed in the Heat Recovery Ventilator to reduce particulate matter from the air supply system while maintaining adequate pressure and air flow through the air handlers. All interior paints are low-VOC (Valspar Medallion), and all interior finish sealants are low- or no-VOC (Manus). A durable laminate flooring (Shaw) is installed throughout the home except in moisture prone areas such as entryways and bathrooms, where moisture-resistant vinyl flooring was used. Shoe removal and storage spaces near the entry also prevent the tracking of contaminants throughout the NSNZ home. The stair treads will be durable solid oak. Tub and shower enclosures have smooth, durable and cleanable surfaces, with non-paper faced backer boards at the walls and hard surfaces at the floor.

A passive radon mitigation system was installed at the NSNZ home, where a collection system at the perimeter of the footings connects to a 4" diameter PVC pipe that is discharged above the roof. The system will become active if necessary.

Exterior moisture protection at the foundation, walls, penetrations and roof prevent water from entering the home, which can cause mold growth and the deterioration of building materials. A pathway was developed for water to exit the exterior wall assembly, flashing was used at intersections and penetrations, and a drip edge, kick-out flashing at eaves and clearance between wall cladding and the roof all work towards divert water from the building. Below the slab, a vapor retarder over a capillary break keeps water from seeping up or ponding beneath the slab. The foundation walls were enclosed in rigid insulation to ensure that interior surface temperatures stay above the dew point, preventing condensation and damage to building materials. The site was graded to drain rainwater away from the building and plants were separated from the foundation by at least two feet.

  FOOD

There are two small lots in the Ecovillage development that have been identified as potential community garden areas. Although this land use is currently limited in the development due to municipal zoning restrictions, the project team is exploring methods of integrating urban agriculture into the future of the neighborhood.