The Impacts Of A WUFI Passive Model On Window And Door Design & Selection

In a Passive House, windows are one of the most complex puzzles to solve. While they’re critical for natural daylighting, you could reasonably also think of them as thermal holes in a carefully insulated wall. We don’t want to castigate them, of course. Windows are essential sources of daylight, which create that unique sense of connection to the outdoors that we are so excited about in our beautiful surroundings. Striking the perfect balance between these competing functions is where science meets art, and the primary tool for that science is the WUFI Passive energy model.

Achieving Phius certification requires creating a detailed digital twin of the entire building in WUFI (which stands for Wärme- und Feuchtigkeitsinstationär, or Heat and Moisture Transiency). This model simulates how the home will perform, hour-by-hour, for an entire year. And it is ruthless.

Our desire for huge, view-capturing windows on the north side of the house immediately ran into the realities of the model. North-facing glass loses a lot of heat and gets very little direct sun, putting a huge dent in the energy balance. 

The model forced a series of tough, data-driven decisions. We had to work with our architect, Scott Witt, and Certified Passive House Consultant Cameron Caja, to carefully size the windows and ensure we were focusing our upgraded performance specifications in the right places. We needed to meticulously calculate the impact of every single door and window on our overall energy targets. It was a fascinating, iterative process that proved that you can't just pick a window out of a catalog; you have to design it as part of a holistic, high-performance system.