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Project Description:
Eliakims Way is a neighborhood of eight single-family affordable homes on Martha’s Vineyard in the town of West Tisbury. A local nonprofit affordable housing developer, The Island Housing Trust, made a collaborative land purchase with the Martha’s Vineyard Land Bank to develop eight units of housing for qualified low and moderate income residents that were chosen by lottery.
South Mountain was hired to do the planning, architecture, and building. Habitat for Humanity was responsible for building one of the houses using SMC design and assistance. The houses are clustered around a central pedestrian green with parking at the perimeter. All houses have unobstructed southern orientations. Four of the houses are two-bedroom units of 1,251 sf and the other four are three-bedroom units of 1,447 sf, all with full basements. The main living area and upstairs bedrooms and bath are identical in the two house types; the third bedroom is a north extension of the two-bedroom plan.
Designed and built to be net zero possible, they are all-electric homes with 5.04 kW Sunpower solar electric (PV) arrays. They were designed to be fossil fuel-free and “zero energy possible” (this depends of course on the occupants). They were occupied on June 1st 2010 and after one year, two households achieved zero annual net energy – using less energy than the PV array had generated. Two other households were very close: within ~1,100 kWh of reaching net zero. We have carefully monitored energy usage since occupancy among the residents.
The houses are superinsulated and have unobstructed southern orientations. Basements are within the thermal envelope, with R-20 walls and sub-slab insulation. Above-grade walls are R-31, roofs are R-50, (note, these are effective R values for the entire assembly) windows are triple glazed Thermotech with two layers low-e and argon fill, (south facing windows are coated with Energy Advantage low-e with a SHGC of 0.62, all others are 0.48) and the blower door results range from 117 to 184 CFM50 for the seven SMC homes and 236 CFM50 for the Habitat home. This is with no mechanical openings taped off and with the HRV running.
Heating and cooling is provided by a Daikin single zone minisplit heat pump (RXS24 DVJU) with a wall cassette in the main living area. Supplemental heating is provided by ceiling-mounted Enerjoy electric radiant ceiling panels. The houses are designed such that the single point source of heat – the heat pump – should be able to provide all the required heating, as long as the doors to the bedrooms are left open to allow heating by natural convection. The radiant panels allow heating in the event of a doors-closed operation, or to provide supplemental heating in extreme cold conditions.
Ventilation is provided by a constantly-operating Fantech 704 heat recovery ventilator. This unit draws about 30-35W and exhausts 25 CFM from each bathroom and supplies 15 CFM to each bedroom (in the case of the two bedroom unit, 15 CFM is also supplied to the living area.)
Domestic hot water (DHW) is supplied by a 50 gallon Marathon electric water heater. This tank is polybutylene lined and is insulated with two-and-a-half inches of closed cell foam.
All houses received LEED Platinum ratings - the highest designation of the US Green Building Council and have permanent affordability restrictions. They were designed and built for the long term to serve the island community.
Homeowner Bonnie Tilton, a high school guidance counselor commented on her first year living without a utility bill. “When I first saw the usage monitors, I thought, ‘What do all these numbers mean?’ but I’m figuring it out. This is a beautiful house that literally heats itself and I give South Mountain a lot of credit, they have built not just houses but a neighborhood community here. We all get together every once in a while. I love this neighborhood.”
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All of the homes are occupied year round with the exception of occasional vacation travel.
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Energy Highlights:
The super insulated envelope of these all-electric homes is paired with 5.04 kW Sunpower solar electric (PV) arrays to match the modeled loads. In addition to the standard utility electric meter, each home has sub-metering of major components. They are:
- Minisplit heat pump
- Electric radiant panels
- DHW
- PV inverter output
- Water to the DHW tank
These sub-meters allow measurement of energy used for heating, cooling, DHW, and plug loads/lighting/ventilation, as well as the energy generated by the PV system.
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We installed separate meters to monitor production and various loads in the buildings.
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CHP System Description:
n/a
Subslab assembly:
4" concrete slab over Tu-Tuff ground moisture barrier with taped seams over (2) layers 2" Type IX EPS rigid insulation over compacted soil
Slab edge assembly:
(2) layers 1 1/2" Polyiso. rigid insulation taped to subslab taped to moisture barrier
Foundation wall assembly:
(2) layers 1 1/2" Polyiso. rigid insulation with staggered seams over 8" concrete foundation wall
Above grade wall assembly:
White Cedar shingles over drainage plane over 5/8" sheathing with taped seams over 9 1/4" double stud wall at 24"o.c. with dense packed cellulose insulation over 1/2" drywall
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Process:
The Island Housing Trust worked with the Martha’s Vineyard Land Bank to create a 3 acre area of development amongst 18 acres of conservation lands in West Tisbury. A special permit was granted for affordable housing that allows density development beyond the standard 3-acre zoning. Board of Health restrictions limited the property to twenty bedrooms.
South Mountain was hired to do the planning, architecture, and building of the resultant eight single-family affordable homes. Habitat for Humanity partnered on one of the houses. The cluster of homes access a central common creating a pedestrian friendly environment.
The homes were designed to serve low to moderate income families earning between 60% and 120% of area median income.
Design for Adaptability:
Fully insulated basements are readily adaptable to owner build-out. A portion of the foundation wall has a reduced concrete thickness to facilitate adding a code compliant egress window.
The timber frame porches are fabricated from potentially demountable components.
Finances Description:
The project aspirations and budget required that we bring subcontractors in at their best prices, cut our typical overhead and profit by more than half and seek financial contributions for the renewable energy systems.
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Published References:
Journal of Light Construction January 2011 http://www.southmountain.com/press-and-media
Energy Strategies:
Very tight construction assemblies, appropriate levels of insulation and a modest footprint allowed us to keep mechanical systems simple with short runs for plumbing and ventilation. The design of the heat pump heating system, HRV, efficient lighting, low-flow plumbing and passive solar strategies brought the anticipated loads down to a level that could be compensated for with the use of a modest PV system.
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Other Purchased Fuels Description:
no
Storage Strategies & System Details:
Grid tied system.
Summary of enclosure strategy/description:
Basements are within the thermal envelope, with R-20 walls and sub-slab insulation. Above-grade walls are R-31, roofs are R-50, (note, these are effective R values for the entire assembly) windows are triple glazed Thermotech with two layers low-e and argon fill, (south facing windows are coated with Energy Advantage low-e with a SHGC of 0.62, all others are 0.48)
Roof Assembly:
asphalt roofing over drainage plane over 5/8" sheathing with taped seams over 14" I-Joists @24" o.c. with dense packed cellulose insulation over 1/2" drywall
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Window Description:
Thermotech fiberglass clear argon-filled, triple glazed, double low-E coated, dual seal insulating glass, simulated divided light, with warm edge spacers.
o South-facing windows to be high heat gain LOE triple glazed glass
Door Description:
Masonite fiberglass outswing BELS-106-010-1 LOW-E