LBC requires net-zero water, meaning the water that falls on the defined LBC project site in a year determines the project’s allowable water use per year[1]. Composting toilets are a commonly used strategy to achieve this level of water use reduction. The composting process allows water to be saved from use as a carriage medium. The project plans on using foam flush composting toilets by Clivus Multrum with a composter beneath which requires continuous ventilation. Exposure to constant air flow is necessary to allow the mixture of toilet waste and bulking material to convert to usable compost and liquid fertilizer. The continuously operating composter fan creates negative pressure at the toilet fixtures for odor control. The challenge is how to deal with this specialized system while meeting the heating goals set by PHIUS – direct exhaust would result in an insurmountable energy penalty. Two approaches are being investigated:

Keeping the composter ventilation separate from the main ERV:
This would involve a secondary ERV unit to serve the composting toilets only.

Integrating the composting toilets’ exhaust into the overall ventilation system:
This requires the use of an ERV that has virtually no cross flow leakage. This will prevent the exhaust from entering the supply air.


Conclusion to the Passive House Series of Posts:
To create a regenerative and resilient world where humans live in alignment and contribute to the natural systems we are part of, the best strategies and thinking need to be combined, synthesized and deployed. Combining Passive House and Living Building Challenge forced the project team to think and rethink goals, processes and outcomes in a way that maximized benefit and created abundance. For example using Passive House standards to achieve a net positive energy building that meets the Living Building Challenge the design team is creating a building that uses less energy than allowed by LBC and at the same time eliminates the need to use the fossil fuel allowed by Passive House. Finding components and materials that can achieve the rigorous thermal and airtightness requirements of Passive House as well as satisfy the toxicity reduction goals of LBC makes for a better building and a healthier planet. Combining the deep and thoughtful work embodied in Passive House and Living Building Challenge is a wonderful struggle that is leading the team to be innovative, creative and regenerative.

[1] Living Building Challenge 3.0 Water Petal Handbook

Written by Christina Aßmann and Nicole Schuster, Ashley McGraw Architects