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ZeroZero House

(Zero Embodied Carbon + Zero Operational Carbon)

Buildings account for 40% of all global greenhouse gas emissions. The Embodied Carbon of new construction (the greenhouse gas emissions released during the mining and manufacturing of building materials and their transportation to and assembly on site) accounts for 80% of these total emissions, while Operational Carbon (the greenhouse gas emissions released during the lifetime of the building, predominantly from space heating and water heating) accounts for the remaining 20%. So while it is important to transition our buildings to 100% renewable energy, the more urgent question is how to reduce the Embodied Carbon of our new buildings. The good news is that many design and material choices can not only lower the Embodied Carbon but also not have an overall impact on construction costs. The ZeroZero House is a case study to show that by using new Embodied Carbon design tools, alongside contractor input on material costs, it has never been easier to achieve good design without having a significant impact on the budget(3).

 

The Embodied Carbon of a modest 1,200 SF home is an average of 23 tons of CO2e(1), the equivalent of driving a gasoline car 62,630 miles, or 2.5 times around the world(2). REMAKE uses the BEAM calculator(4) to quantify the Embodied Carbon of building materials. BEAM sources its data from Environmental Product Declarations (EPD) which are third-party verified (by well-known organizations like ASTM, NSF and UL-Certified) that summarize (using the ISO 14040 standard) the lifecycle assessment of a material. Using this data we were able to reduce this 23 tons of CO2e down to below zero, without raising the overall cost.

 

ZeroZero House is an all-electric, 2-story detached Accessory Dwelling Unit (ADU) that has a self-contained 2-bedroom residence on the 2nd floor and a 2-car garage on the ground floor, which can later be converted into an aging-in-place 2-bedroom residence. Best of all, it has both a negative Embodied Carbon and a roof area capable of housing a PV array to be Zero Operational Carbon.

DESIGN STRATEGIES:

  • Efficient Building Footprint

  • Efficient Organization of Space

  • Future Flexibility (Aging-in-Place Ground Floor Apartment)

  • Tight Thermal Building Envelope for Energy Efficiency

  • Healthy Indoor Air Quality (All-Electric Equipment & Appliances, Induction Cooktop, ERV Mechanical Ventilation, and Non-Toxic Material Selections)

  • Frost-Protected Shallow Foundations to Reduce Excavation Costs for deep foundations and to Reduce the Quantity of Concrete – a main contributor of CO2e in new construction (often accounting for a 1/3rd of the building’s lifecycle carbon footprint).

  • Heat Pump Space Heating/Cooling and Water Heating

  • Minimize the size of the rooftop PV array required to achieve Net Zero

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KEY MATERIAL SELECTIONS: (pricing sourced in Nov 2020)

  • Foundations: Frost-Protected Shallow Foundations (FPSF) to reduce the amount of excavation and concrete

  • Concrete: >50% SCM mix (no cost premium over regular mix)

  • Sub-slab Insulation (R-10): 2” DuPont Styrofoam ST-100 (Material Price: $1.03 /SF [$0.52 /SF per inch thickness]; Compare with 2” Owens Corning Foamular 250 R-10: $1.75 /SF [$0.88 /SF per inch thickness])

  • Below Grade Insulation (FPSF vertical & horizontal insulation, R-10): 2” DuPont Styrofoam ST-100 (Material Price: $1.03/SF [$0.52 /SF per inch thickness]; Compare with 2” Owens Corning Foamular 250 R-10: $1.75 /SF [$0.88 /SF per inch thickness])

  • Exterior Wall Structure: 2x6 Wood + 1/2” plywood sheathing

  • Exterior Wall Insulation (cavity, R-20.4): Cellulose Dense Pack, R-3.7 /in. (Installed Price: $11 to $12.65 /SF [$2.00 to $2.30 /SF per inch thickness]; Compare with 3” Closed-Cell Spray Foam Insulation, R-7 /in. $3.00 to $4.50 /SF [$1.00 to $1.50 /SF per inch thickness]

  • Exterior Wall Insulation (continuous, R-5.4): 2” Wood Fiber Board, R-2.7 /IN.

  • Exterior Wall Cladding: 3/4” MOSO Bamboo X-Treme Outdoor Pre-finished Shiplap Siding

  • Drywall: 5/8” USG EcoSmart Firecode (Material Price: $0.33 /SF; Compare with USG Firecode $0.33 /SF)

  • Windows: Anderson Fibrex PVC-clad Wood, Double-Glazed

  • Floor Structure: TJI Wood Joists

  • Flooring: 1/2” x 3-3/4” Solid Bamboo (Material Price: $2.99 /SF; Compare with 3/4” x 3-1/4” Solid Oak $4.44 /SF)

  • Flooring: Porcelain Tile

  • Ceilings: 1/2” Drywall CertainTeed AirRenew (Material Price: $0.33 /SF; Compare with 1/2” USG UltraLight $0.29 /SF)

  • Roof Structure: 2x12 Wood + 5/8” OSB Roof Decking

  • Roof Insulation, cavity (R-41.6): 11-1/2” Cellulose Dense Pack, R-3.7 /in. (Installed Price: $23 to $26.45 /SF [$2.00 to $2.30 /SF per inch thickness]; Compare with 6” Closed-Cell Spray Foam Insulation, R-7 /in. $6.00 to $9.00 /SF [$1.00 to $1.50 /SF per inch thickness]

  • Roof Insulation, continuous (R-8.1): 3” Wood Fiber Board, R-2.7 /IN.

  • Roofing: Asphalt Shingles Owens Corning, Supreme

Footnotes:

[1] Builders for Climate Action (www.buildersforclimateaction.org) conducted a survey of 550 homes in Canada using their BEAM calculator. For an equivalent 1,200 SF home, the results showed the embodied carbon of the homes ranging from 10 tons to 66 tons CO2e, with the average being 23 tons of CO2e.

[2] 23 tons CO2e is the equivalent of 2,588 gallons of gasoline. The US Environmental Protection Agency (EPA) reports that the CO2 emissions from a car burning a gallon of gasoline is 0.008887 tons CO2 /gal. EPA (“Greenhouse Gas Emissions from a Typical passenger Vehicle, 2014, https://bit.ly/2HAwL5p). The Average fuel economy of a car is 24.2 mi/gal (https://afdc.energy.gov/data/10310). The circumference of the earth is 24,901 mi.

[3] The top contributors of CO2e in new construction are concrete (~33% of the total building’s carbon footprint), insulation (~26%), and cladding (~13%). There are many low-hanging fruit decisions that can have zero-to-little impact on cost, particularly with regard to the types of concrete mix, insulation and cladding. Where possible, we have provided the estimated cost per SF of materials and, for comparison, an equivalent “standard” material cost pf SF.

[4] Created by Builders for Climate Action (www.buildersforclimateaction.org)

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