Sizing and placing windows using the Passive House Planning Package

The availability of not just good, but great, windows and doors has posed one of the greatest challenges for the growing Passive House (PH) movement in North America. As designers, builders, and owners, we take such care to provide a continuous layer of insulation and an extremely high level of air-tightness in the envelope that we almost donʼt want to poke any holes in it. At the very least, we want to control our holes and fill them with windows and doors that are worthy of our painstakingly created thermal envelope.

Ultra-performing windows have at least three layers of insulating glass or film, a thermally-broken frame, and several locking points to achieve the best in air-tightness. U.S. manufacturers have been slow to get on board, so PH and near-PH projects look to Europe for better performance. (Canada also has some pretty good options). The price for these imports can be high – partly due to quality, partly due to shipping; and, shipping from such distances goes against the ‘green’ wisdom of sourcing products locally. Thankfully, a properly chosen and well-placed window or door can redeem itself by collecting the sunʼs free heat – which is integral to PH strategies.

Our search for windows and doors led us to Zola. John and Nancy found the Thermoclad units to be the best price among other similar windows and doors.

For assistance in sizing and placing the windows, we utilized the Passive House Planning Package (or PHPP) software, which offers the ability to compare the solar gains from a window with its associated heat losses. This feature enabled us to adjust the size of windows to an optimal size depending on its orientation, gains, and losses. A bigger window will loose more heat, but may be worth it if the window is facing towards the southern sun. On the other hand, windows on the north should be kept quite small because they constitute all heat loss, and no gain. PHPP also models shading elements, which are essential for controlling solar gain in the summer months and for encouraging it in the winter months.

The biggest challenge regarding these PH-caliber windows and doors (other than their availability and price) is figuring out how best the units will sit in the wall without compromising the thermal envelope and air barrier and still reliably shedding water. For all of their advantages, Zola windows lack some of the installation innovations that North American-made residential windows commonly have. Instead, they are designed more like windows intended for masonry construction. For example, they donʼt come with nailing fins or a drainage sill – you get a sash in a frame, and that’s it. To make the installation work, we considered a lot of different construction details. In the end we went with a detail that combines sealant, backer rod, and spray foam with a made-in-place pan system and Tyvek flashing.

Despite the challenges, putting time and energy (and money) into high quality windows is worth it when you are building a house like this one. It just doesnʼt make sense to invest in the envelope, but then skimp on the windows. And, I think John and Nancy will enjoy the performance, not to mention the amazing view, for years to come.



Lanesboro, Minnesota
Climate Zone 6 (cold/moist)
Latitude: 43° 44' 18'' N
Longitude: 91° 54' 48'' W

House Size

Net Treated Floor Area: 1,514 SF
Gross Square Footage (House only): 2,210 SF

Building Envelope

Roof: R-99
Wall: R-61
Ground: R-53

Windows & Doors

Glazing: U-0.10 BTU / hour / sq. ft.
Solar Heat Gain Coefficient (SHGC): 0.48”
Frame: U-0.19 BTU / hour / sq. ft.

Modeled Performance

Specific Primary Energy Demand (Source Energy Demand): 12.1 kBTU / sq. ft. / year

Specific Space Heat Demand: 7.0 kBTU/sq. ft. / year

Peak Heating Load: 7,047 BTU / hour

Space Cooling Demand: 0.44 kBTU / sq. ft. / year

Peak Cooling Load: 3,625 BTU / hour

Pressure Test Goal: Whole House Air Changes Per Hour (ACH) = 0.4 ACH 50


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