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.
|Location: Greenwood, Seattle
Star Level: 5-Star
Checklist: 2021 Single Family/Townhome New Construction
Verifier: Evergreen Certified
|Site and Water
|Health and Indoor Air Quality
|Equity and Social Justice
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.
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.
Exterior cork insulation
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.
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.
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.
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
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.