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Super Efficient Windows for the Passive Project

by Chad Ludeman on September 11, 2009 · 11 comments

in Building Science,energy modeling,envelope,Passive House,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.

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

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.

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.

Serious Window Technology Diagram

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.

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{ 1 trackback }

Serious Windows - are they all the're cracked up to be? | The Boston Home Team Blog
October 9, 2009 at 12:29 am

{ 10 comments… read them below or add one }

1 Peter Troast September 11, 2009 at 6:46 pm

Have you guys ever seen the 1125 series (R-11) in the flesh? Engineers on a deep retrofit we’re working on say they’ve never come out of the factory.

2 RitaF September 12, 2009 at 5:11 pm

Looks fantastic what’s the cost comparison of the Jeld-Wen windows you used vs the serious windows in the same size?

3 Akat Kobay September 19, 2009 at 1:47 am

what about 1125 series? Can you inform me please ?

4 Peter Troast September 19, 2009 at 11:09 am


Building Science Corp–the architects/engineers on a deep energy retrofit I’m working on here in Maine have been reluctant to spec the 1125 series–Serious’ highest R-value window–because they’ve never actually seen one. Since my prior comment, I’ve been in touch with a Serious dealer here in Maine ( and they’ve assured me these are, in fact, in production. Now awaiting the quote (gulp).


5 Lillian Maurer September 22, 2009 at 8:28 pm

I’m thinking of using Serious Windows in an upcoming project, but having never seen them in person, I’m curious how visible the film layers are. Does the glazing look differently from traditional triple glazing? Thanks for any info from those of you who have used these windows.

6 Kristopher Brill September 24, 2009 at 2:03 pm

To All,
The apearance of the suspended film is very much like the triple glazed.
The difference is that with Serious your are not adding the extra weight of the third pane which extends the life of the hardware. The 1125 is not a myth and it does exist. The cost of the 1125 is almost 2 times the cost of the 925 series and it is my opinion that you get the biggest bang for your dollar with the 925 series. If you calculated $50.00 per square foot on the 925 series you would not be far off the mark. I am availible to help anyone with questions and will work on projects throughout New England.

7 Akat Kobay October 1, 2009 at 4:52 pm

Kristopher thank you for details, and thank you Peter.

8 Peter Troast October 3, 2009 at 6:14 pm


Very thorough post on Passivhaus windows on Martin Holladay’s Green Building Advisor blog yesterday:


9 October 21, 2009 at 2:30 am

Here’s the test for a window company that most of them fail:

If you have unshaded windows on the south side of the building and you’re in a cold place like Philadelphia, you should spec high (.50 or higher) Solar Heat Gain Coefficient (SHGC) low e glass on the south elevation.

So my question is, did PHCC recommend it, and was Serious able to provide this preferred glass? Some window guys call it “hard coat low e”.

10 Gennaro Brooks-Church November 22, 2009 at 8:32 pm

Has anyone looked into just installing two lower cost windows instead of one expensive one?

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