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Case Study: Stack House Apartments surpasses industry standards for sustainable innovations and energy efficiency

Stack House Apartments surpassed industry standards – both locally and nationally – through many sustainable innovations and energy efficiency measures. This full city-block project in Seattle’s evolving South Lake Union neighborhood combines two seven-story, mixed-use residential buildings with the restoration and adaptive reuse of the adjacent Supply Laundry Building, a historic landmark.

This project achieved LEED Platinum certification and met the Seattle 2030 District high-performance building challenge. In addition, the Supply Laundry Building participated in Preservation Green Lab’s pilot program for outcome-based energy modeling and permit approval.

Summary of Innovations

Stack House Apartments incorporates numerous innovative design elements including:
1. Central heat pump water heating system with Reverse Cycle Chillers (RCC)
2. Compartmentalization of central circulation areas
3. Ductless heat pumps for 40% of the units
4. 8,000 square foot green roof
5.  Rainwater catchment and re-use
6. Biofiltration swales (bioswales) to greatly improve the quality of stormwater runoff from the Capitol Hill neighborhood
7. Historic landmarked Supply Laundry building participated in pilot outcome-based energy code

  • Innovation 1 - Central heat pump water heating system using a Reverse Cycle Chiller (RCC)

    The central heat pump water heating system uses RCCs to reduce electricity costs for the building. By using heat already created from underground parking, RCCs supply 100% of the energy needed to heat the domestic water for the building.
    – The RCCs reduce domestic hot water use by a factor of three.
    – Provides a simple six-year payback.
    – Allows significant downsizing of the garage exhaust fans.

  • Innovation 2 - Compartmentalization of central circulation areas

    Compartmentalizing the central circulation areas significantly reduces envelope heat loss. It eliminates the leakiest portions of corridors from heat space and the natural ventilation cut the heating energy by 80% in corridors. This helps create a transition zone between the interior and the exterior. The elevator stack is isolated from corridors to eliminate the stack effect.

  • Innovation 3 - Ductless heat pumps for 40% of units

    The south and west facing units, which comprise 40% of the total apartments, have the highest seasonal cooling loads.

    By providing ductless heat pumps for these units, coupled with improved glazing, the heating energy loads are reduced by a factor of three.

  • Innovation 4 - 8,000 Square Foot Green Roof

    Stack House’s expansive green roof reduces urban heat island effect and filters rainwater. A portion of the green roof is also dedicated to urban agriculture, providing an opportunity for residents to grow their own vegetables.

  • Innovation 5 - Rainwater catchment and reuse

    Onsite cisterns hold approximately 43,000 gallons of water, saving about 450 potable gallons of water per day through the rainwater catchment system.

    The cisterns reduce demand on Seattle’s water supply and slow the discharge of stormwater to the city’s sewer system.

  • Innovation 6 - Stormwater treatment pilot for bio-filtration swales

    Large bioswales on the east and west perimeters of the site, filled with native sedges and rushes, remove 92% of contaminants from 190,000,000 gallons of untreated stormwater annually. This is equal to about 580 acres of untreated surface runoff. The bioswales were installed in partnership between the developer and Seattle Public Utilities.

    The bioswales significantly reduce contaminants discharged to Lake Union and provide additional greenery and habitat on the public right-of-way.

  • Standard energy saving measures

    Stack House also utilizes energy saving measures that are standard in sustainable buildings. Daylight and motion sensors cut electricity costs by 50% in corridors. The units and amenity spaces are equipped with high-performance lighting and low-flow plumbing fixtures. Energy efficient windows throughout and external sun shades help to reduce the heat gain, while the slab edge insulation reduces heat loss.

Major Learnings

Stack House Apartments leverages cutting-edge energy savings features to achieve significant energy savings: The entire building has a measured Energy Use Intensity (EUI) of 22 kBtu/sf-yr, translating to 51% in energy savings.

Stack House’s building design and features have also resulted in the following measured savings:
– 60% less energy than baseline commercial buildings
– 50% less energy than baseline residential buildings
– 40% less water than baseline commercial buildings
– 60% less water than baseline residential buildings

Overall, the Stack House Apartments and Supply Laundry building are paving the way for innovative design and sustainable practices. The project team saw the opportunity to leverage existing and new construction in a creative, sustainable, and community-friendly manner that conserves natural resources and supports the city’s greater environmental- and historic-preservation objectives. The project team was successful in achieving these results while also exceeding national industry standards for energy savings.