Holzraum System is a design and consulting partnership combining decades of experience in architectural, structural and heavy timber design, high performance and Passive House construction, CADCAM (computer aided manufacturing) for wooden structures and software development for streamlining design and manufacturing processes. Formed as a Passive House consulting and building envelope design to manufacturing firm, Holzraum System was based on our commitment to connecting builders and architects with passive house and off-site manufacturing in order to achieve unmatched comfort, superb air quality, extreme building resiliency, and among the lowest embodied  + operational carbon footprints in the building envelope industry.

Ilka Cassidy (co-founder) is passionate about Passive House envelope design and building science, focusing on the benefits of natural building materials. She grew up in Germany and received her Diploma of Engineering/ Architecture degree from the RWTH Aachen. Ilka connected to the US during her year at the University of Washington in Seattle where she won a Scholarship to study within the graduate program. Eventually she moved to Philadelphia where she co-founded C2 Architecture in 2005. She has worked on multiple projects as a Certified Passive House Consultant and is an active member of Green Building United in Philadelphia.


Steve Hessler (co-founder) is a heavy timber structural designer, project manager and PHIUS+ certified passive house consultant with over 20 years of experience designing, building and managing structural-thermal envelope offsite fabrication projects as well as on-site high performance construction projects.  Since 2008 Steve has been combining 3D modeling, integrated project delivery and building science principals to design and manage off-the-grid, net-zero, and passive house projects. Steve received his bachelors in Communications Strategies from the Pennsylvania State University in 1990 before completing a five year apprenticeship program as journeyman and lead restoration carpenter.

David Hessler (co-founder) is a biologist, computer programmer, and entrepreneur who has worked in biopharm development, and co-founded three startups: a biotechnology company and two scientific software development companies. He has served as a senior software director and product portfolio manager for BIOVIA Dassault Systemes, and served on the project management team for construction of 30 custom homes in Ecovillage @ Ithaca, NY. David received a Biology degree from the Pennsylvania State University, and did graduate work in biochemistry at the University of Rochester.

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