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Targa Homes—Greenwood Reclaimed DADU

With a sharp focus on low-carbon, high performance construction, Targa Homes recently completed this 5-Star DADU in the Greenwood neighborhood of North Seattle. Salvaged and carbon sequestering materials were used at every opportunity, and the airtight all-electric home features an advanced envelope and mechanical systems for superior comfort and indoor air quality.

Greenwood Reclaimed DADU, exterior,

Vital Stats

Section

Points

Location: Greenwood, Seattle
Star Level: 5-Star
Checklist: 2021 Single Family/Townhome New Construction
Verifier: Evergreen Certified
Site and Water 159
Energy Efficiency 150
Health and Indoor Air Quality 107
Material Efficiency 133
Equity and Social Justice 45
  Total Score 702

Freehand Studio Architects engaged with the homeowner to create a multigenerational site for their family with the addition of this backyard cottage. The design features a bedroom and bathroom on each floor, as well as flex space and 2nd story deck. The soaring double height great room provides an airy, light-filled atrium which belies the 1000 SF floor area.

Greenwood Reclaimed DADU, interior
Greenwood Reclaimed DADU, interior

Lowering the Upfront Material Emissions

Using the BEAM tool, Targa set out from the beginning to use low embodied carbon materials wherever possible, and ultimately achieved a 30% reduction in material carbon emissions. The home is framed with reclaimed lumber provided by Sledge Seattle—a company that deconstructs homes that are otherwise slated for demolition. The centuries old Fir that comes out of these homes is dense, dry, and ready to serve another 100+ years. BEAM was utilized to show the team that using blown-in cellulose cavity insulation and cork exterior insulation makes a major impact on lowering the total kg CO2e for the project compared to the typical fiberglass and foam. Other notable decisions include using LP SmartSide lap siding (an engineered wood product) instead of the typical Hardie plank, as well as generally substituting wood based products for foam and plastic wherever feasible.

BEAM comparison
Greenwood Reclaimed DADU, exterior cork insulation

Exterior cork insulation

Greenwood Reclaimed DADU, Dark color of reclaimed fir

Dark color of reclaimed fir

While framing the home, the team was continually stunned by the tight grain of the reclaimed old growth Fir. The team could not let any of it go to waste, so the lead carpenter Matt Kull fabricated a custom end-grain butcher block countertop out of framing lumber. This centrally located focal point provides a visible connection to the invisible bones of the home.

Greenwood Reclaimed DADU, upcycled reclaimed butcherblock

Supplying Fresh Air in an Air-Tight Home

Superior Indoor Air Quality was a priority for architect and builder and the first step towards this is airtight construction. Starting with a fully taped sub-slab vapor barrier, the air barrier then wraps under the bottom plates which are sealed with sill seal foam and Contega HF flexible adhesive caulk. The bottom of wall is also sealed to the concrete stem wall using Siga tape for a belt & suspenders approach at this critical joint. Zip System sheathing with taped seams on the walls and roof form the primary air barrier above grade. The unvented roof is framed using a unique over-roof assembly (also known as “Monopoly Framing”) where rafter tails are applied on top of the structural roof sheathing after the roof sheathing and roof/wall joints are air sealed with tape. Rigid foam is then installed on top of the roof sheathing providing continuous insulation that keeps the sheathing above dewpoint at all temperatures. This high performance assembly avoids spray foam and is a much more durable and forgiving design.

Greenwood Reclaimed DADU, roof under construction

The high glazing ratio can be a detriment to energy efficiency, so the team chose Vetta windows to mitigate this. These European made triple pane tilt/turn windows are made from FSC wood and clad in an aluminum exterior. Although the team found these to have a large carbon footprint, the beauty and fantastic durability were offsetting factors.

Greenwood Reclaimed DADU, interior

The team runs a preliminary blower door test just before insulation to identify and address any problem areas. This home scored a final blower door score of 0.02 CFM/SF shell (0.35 ACH50) placing it well below the PHIUS passive house air tightness standard. With extreme air tightness comes a necessity for ventilation and this home uses a Zehnder Energy Recovery Ventilator (ERV) running continuously to provide fresh filtered air and exhaust stale air. Low-emitting interior finishes are the final piece in a healthy IAQ home.

High Performance Systems and Fixtures

Heating and Cooling are primarily from two ductless minisplits, with backup bedroom heat from Ducoterra radiant panels on the ceiling. Hot water is made by a SanCO2 split system heat pump water heater. This unit uses CO2 refrigerant (much friendlier to the atmosphere) and runs at an astounding efficiency to quietly provide hot water year-round without electric resistance. A heat pump clothes dryer is also an efficiency upgrade and doesn’t require a penetration through the wall for exhaust. An induction cooktop and LED lighting round out the specs for the all-electric home.

Greenwood Reclaimed DADU, exterior roof solar panels

To provide energy usage feedback to the occupants Targa installed a Leviton Smart LoadCenter electrical panel which reports usage for each individual circuit in the home to a smartphone app. Paired with the homes 3.6kW Solar PV array, occupants will be able to live as close to net zero as possible.

Low flow plumbing fixtures were chosen throughout including ultra-efficient Niagara 0.8 gallon per flush toilets. The team also chose to infiltrate rainwater on site using a drywell which wasn’t required by the city. Fireclay tile, Valinge engineered wood flooring, and Paperstone countertops were chosen as sustainable, functional, and attractive finishes.

Built Green highlights:

Site and Water

  • Infill site with a 26” DBH Maple tree preserved
  • New stormwater runoff managed by a drywell and light-colored permeable pavers
  • Modeled 61% reduction of total potable water use
  • No turf grass

Energy Efficiency

  • Achieved a total of 9.0 energy efficiency WSEC credits
  • HERS Index of 6.0
  • Energy Star and DOE Zero Energy Ready Home Certified
  • Air tightness of 0.35 ACH50
  • CO2 heat pump water heater
  • Ductless heat pump and radiant panel heating system
  • All Electric home
  • Smart electrical panel
  • Level II EV charging
  • 3.6kW Solar PV array

Health and Indoor Air Quality

  • Rigid insulation with taped seams
  • Rainscreen under siding
  • Greenfiber cellulose and seam tape have Health Product Declarations
  • EPA Indoor AirPLUS Certified
  • Low VOC and CARB II compliant interior paints, finishes and millwork
  • No carpet
  • Skylight has U-value of 0.14
  • Whole home ERV

Materials Efficiency

  • Open and stacked floorplan
  • Carbon-storing and locally-produced materials
  • Significant amount of framing was reclaimed framing lumber
  • Rigid exterior cork insulation
  • Thermally-modified and certified wood siding and decking
  • No vinyl flooring or siding
  • Countertops made from recycled paper or cut wood waste from framing
  • 90% source-separated recycling rate for all new construction waste
  • 30% reduction in embodied carbon over a conventional modeled baseline

Equity and Social Justice

  • Multigenerational accessory dwelling unit
  • Stepless entries and exterior gathering space
  • Accessible bathroom on 1st floor
  • Lever-style door handles
  • Low maintenance outdoor space
  • ¼ mile from transit stop
  • Two W/MBE firms hired on project
  • Builder provides job training opportunities to underserved community members

Photo credit: Chuan Photography

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