Super Efficient Windows for the Passive Project

In addition to insulation levels and air sealing details, the windows and external window shading in the Passive Project is upgraded from the 100K House Project.

There is a lot more to windows than insulation levels, but in order to keep things simple we will focus on this aspect and cover energy efficient windows in more details in a future post. For windows, U-values are used that equate to the more recognized R-values used to define wall and roof insulation. The lower the U-value, the higher the R-value. In our region of the US, a U-value of 0.32 (R-3.1) is required to reach Energy Star status. Many are surprised at how low insulation values for energy efficient windows are compared to even the lowest R-19 code built wall.

In the 100K project, we used the bare minimum for Energy Star in a Jeld-Wen builder grade window. For the Passiv Project, we upgraded to the only US window manufacturer capable of meeting the specs required by the Passiv Haus standard, Serious Windows. Below is the comparison.

Soil Testing and Next Steps

Energy Star: U 0.32 (R-3.1)

100K House: U 0.32 (R-3.1) Jeld-Wen Builder Grade Wood Clad Casement

Passive Project: U 0.17 (R-5.9) Serious Windows 925 Series

Soil Testing and Next Steps

To further increase the overall window insulation in the Passive Project, we increased the amount of glazing % taken up by picture windows. The 925 serious of Serious Windows figures for picture windows increases to U 0.14 (R-7.1). Over 2/3′s of the glazing in the Passive House is comprised of these picture windows.

See the diagram below for a comparison of some of the leading window manufacturers in the US. Notice that Serious offers windows that achieve R values all the way up to R-11.2.

Window Article and Thoughts on Fiberglass Windows

So how does Serious achieve these higher efficiency figures? Below is a diagram of how the Serious Windows are constructed. They start with their frames by using fiberglass which is a better insulator than any other window frame material commonly seen such as wood, aluminum and even vinyl. They then fill these frames completely with spray foam. There are a bunch of small energy saving features inside the windows, but the biggest difference with Serious is that they offer triple and even quadruple glazing. They save cost and weight by using suspended films rather than full sheets of glass for the layers inside the inner and outer most glass panes.


There is a lot more to cover on the subject of uber-efficient windows, but we’ll cut off this discussion for here. For those interested in cost difference, we spent just over double the amount on the Passive Project windows as we did on the 100K windows.