The Pax Futura multifamily passive housing project is designed and built to achieve a near zero energy rating and aims to achieve passivhaus certification. Use of high thermal efficiency construction methods, solar thermal water heaters and a heat recovery ventilation system (HRV) help approach passive house for multi-unit dwelling. HVAC system required an engineering validation to demonstrate ample airflow to satisfy the code requirement.
To achieve the building performance desired, the building’s mechanical engineers installed joined exhaust ventilation for both kitchen and bathrooms. The HRV system was designed with minimal air circulation and exhaust specs did not meet current City of Seattle ventilation requirements. To achieve economic performance, one HRV was used for each six units and the exhaust of 27 cfm was well below the coded 45cfm. These numbers came from ASHRAE’s prescriptive pathways, and the the architects submitted a code modification to the City of Seattle Department of Construction and Inspections with alternate code modification, the current code requirements, the justification and a written analysis describing a reviewed building practice. Seeing this data and modification proposal, the Mechanical Reviewer on the project approved the building’s HRV-based ventilation system. Legislative code updates for high performance building are slowly adopted, and the ASHRAE recommended HRV performance metrics were recognized as an emerging best practice helping to earn the modification approval.
ASHRAE standards suggest air flowing through the kitchen into the bathroom, giving both spaces an exhaust of 30 cfm. NK submitted a Code Modification or Alternate Request form to the City of Seattle Department of Construction citing minimum exhaust rates for private kitchen and toilet rooms by ASHRAE and citing SBC 104.4- code waivers for spirit of the code. The city of Seattle approved the code modification after determining adequate ventilation would be provided by this innovative system.
With passive houses the airflow in cubic feet per minute (CFM) required is substantially less than conventional housing, but the designed CFM for the Pax Futura building was still below Seattle’s minimum code standards for CFM. Obtaining a code modification was required from the City of Seattle Department of Construction and Inspections citing ASHRAE standards. Attached to the form was a description of alternate code modification, the current code requirements, the justification and a written analysis from an engineering firm, Staengl Engineering.
The HRV installed in the pax futura follows Chapter 4 of the mechanical codes. Specific ventilation and exhaust rates are required depending on the building type, room type, and room size. The Pax Futura building is constructed sealed thermal envelope to achieve maximum thermal efficiency. The added HRV system can hinder proper ventilation which can lead to poor air quality in the home. It is important to follow Chapter 4 to provide the necessary air movement and air quality for the health and happiness of the building’s inhabitants. ASHRAE has created the minimum exhaust rates chart as a guide for high performance building to achieve maximum energy recovery while maintaining a well-ventilated dwelling space.
This project aspired to create nearly net-zero passive apartments and to give future residents of Columbia city access to sustainable and economical housing options. Having an airtight building reduces energy costs and waste associated with heating and air conditioning. The solar thermal water system was an initiative by the owner and a catalyst to the HRV innovation and many of the other energy-efficient systems in Pax Futura. Along with this vision for passive hot water, the owner initially wanted to avoid air conditioning, a Pacific Northwest rarity. However, to achieve the building envelope performance desired, the HRV units were selected as an energy-saving solution for the building. Pax Futura is built near transit lines and in a walkable neighborhood, features the owner selected in citing the project despite premium land values in the hot market. The decision to build small square footage units was made to increase affordability for young urban professionals in an up and coming neighborhood.
This project was conceived through a passion of sustainable housing. Architects, engineers, and clients worked closely together to determine how to design and build passive apartments that satisfy the demographics and lifestyles of the potential residents. The design of this project took ten months to be reviewed. The engineers began by doing an initial energy analysis on the project to work out the performance targets and how they will be achieved. This project in particular was interesting to the engineers because they were working in a different climate than what they were used to, Seattle’s climate doesn’t require a lot of home dehumidification and cooling is not an issue. One challenge of incorporating the HRV was that they are expensive, and the economical single HRV system per six apartments made duct work more challenging.
The HRV and solar water heater systems that were installed in this building were costly and the further inspections and permitting added to the costs. These costs were were offset by the very low energy requirements, approximately 90% lower than a comparable building, for heating of both air and water through the controlled thermal envelope, HRV and solar water heaters. This will allow for the builder, Cascade Built, to recover those costs by charging competitive rent while not having to pay or pay very little in utilities.
Proving the apartments meet airflow requirements for healthy indoor living engineering assessment that cites another building best practice can help in earning compliance. In this case, the application of ASHRAE standards was appropriate as their standards are tuned towards high performance buildings that the Seattle Municipal Code has been slow to adopt.