Tuesday, July 24, 2012

A bit about our windows

Turn function of the window
In a Passive House the windows are like an appliance that performs a specific task efficiently. They are an integral part the passive solar function of the house. The windows capture solar energy into the house and then must help the house retain that energy,  i.e. not let all that good heat go right out the window, a one way valve.  We decided on Intus Eforte windows manufactured in Lithuania, http://intuswindows.com/passive-house-window.html

While the windows are not officially certified by the Passive House organization, their performance metrics are comparable to windows that are.

There are 3 main metrics for analyzing window performance.  U-value is a measure of a window's ability to keep heat from escaping the house, lower is better.  It is the inverse of R-value, a metric commonly used with insulation.  SHGC is Solar Heat Gain Coefficient, a measure of the fraction of heat from the sun that makes it into the house.  VT is visual transmittance, a measure of the amount of light from the sun that makes it into the house. 

The eastern and western windows have U-values of 0.088, an SHGC of 0.4 and a VT of 0.58.  The northern and southern facing windows have U-values of 0.105, an SHGC of 0.5 and a VT of 0.74.  Originally, the SHGC of the southern facing windows was going to be higher, but there was concern about too much solar gain in the spring and fall when the sun is lower in the sky. 

Why did we choose Intus?  There are no American-made windows that meet the performance requirements of our Passive House, so we had to look at European manufacturers.  Wood framed windows were too expensive.  That left us with UPVC.  We looked at Intus, Shuco and Unilux.  Since we have no idea which if any of these companies will still be in business in the US 10 years from now, it ultimately came down to cost.  Intus was 25% less expensive than the second place finisher, Shuco.  We ended up paying about $50/SF of glass, but that includes a large slider and two doors.  I was surprised at how affordable these windows were. 

You can see the "tilt" function in the top left window. The windows are nicely shaded in June.
 We're very happy with the windows.  They seem to be really well made.  The windows are a little different that what we are used to in the US.  They are in-swing windows which are popular in Europe.  They have a really cool tilt-turn function that enables you to tilt the window in just a little for ventilation (see top left window in the picture above) or swing the window all the way open like a door.  They seal up very tightly when closed.  We were able to get some really large panes of glass for an uninterrupted visual frame of the trees and beautiful garden I'm still dreaming about.

Our architect Jamie also designed a beautiful curved overhang like the brim of a hat on the south side of the house to provide just the right shading for the first floor windows for the time of year and position of the sun.  We have been at the house during these hot summer days and have seen the upstairs windows perfectly shaded by the roof overhang and the large slider in the dining room perfectly shaded by the hat brim as well.  I  look forward to feeling the sun shining in brightly in the colder months to warm our New England winter bones.


  1. What has been your experience with the in-swing windows in the summer with drapes? Also, in reading another blog, they were debating between Intus and Bieber and their deciding factor was the screens. They thought Intus' screens seemed like an afterthought. What has been your experience with the screens? And, for less money, I can be persuaded to accept a less than ideal screen if it's still functional and not completely annoying.

  2. The tilt-in works fine in the summer months with the drapes. The windows only tilt in a few inches and our walls are a foot thick, so there's plenty of room to tilt-in the windows. We rarely swing in the windows like a door. Last summer we kept the windows closed and ran the air conditioner much of the time, due to the humidity outside.
    The look good from the outside and work well keeping the bugs out. They're easy to install. The only thing I don't like is that they're mounted with metal clips on one of the 3 window gaskets. The gaskets seem to regain their original shape after the clips come off.
    I also looked at Bieber windows, but the Intus ones were about half the price.