Window Decision

We have chosen our windows. If you remember earlier posts about our criteria for windows (April Update and Window Shopping), you’ll know that we were looking for low U-values (more insulation), high SHGC values (more heat from the sun), and low dollar values (more money in our wallets). We found 2 out of 3.

AccurateDorwin (based in Winnipeg, Canada) is going to make us windows with two different types of glazing. Our south facing casement windows will have a 0.24 U-value and 0.47 SHGC. All other windows will have a 0.20 U-value and a 0.44 SHGC.  The fixed window units are the most efficient with a 0.18 U-value. All values are whole window values.

Serious came in second. They make a casement with a 0.17 U-value with 0.33 SHGC (whole window values). Much better U-value but a fairly low SHGC value. And you have to ship them all the way from California and they are not cheap. Plus they use a different technology that can add reflections inside the window, limiting it visibility.

In contrast, the Marvin Integrity All Ultrex windows (our fall-back comparison) are difficult to compare because they only publish center of glass U-values which are always better then the whole window values. Their stated center of glass U-value is 0.25 with a SHGC value of 0.41. Remember, the whole window average is going to be less efficient than center of glass.

We have about 170 sf of glazing area, thats roughly 11% of our conditioned floor area. We added sun shades on the south elevation to block the sun in the mid to late summer.

I’m not sure what the energy guys found, but by my rough calculations, the south facing windows will contribute roughly half of our heating needs in the winter. That includes a bunch of estimates for solar intensity (average sunny days and cloudy days throughout the winter months) and solar heat gain factors (BTU/h-ft2) for our latitude (from the ASHRAE Handbook of Fundamentals).

We’re paying roughly $5k more for windows than if we used our comparison windows, but I’m betting that the extra efficiency will pay for itself. I just don’t want to think about the length of that payback period.

6 Responses to “Window Decision”


  1. 1 Jim Merrithew 7-January-2011 at 4:25 pm

    In an earlier post, you mentioned Thermotech windows, but, in this installment, they were not in the running. Is there a reason for the change? Thanks – Jim

    • 2 Larry 7-January-2011 at 6:37 pm

      To be honest, I don’t think Thermotech wanted my business. I tried several times to get a quote from them. By the time they responded (not even with a quote) I already had quotes from 3 other window manufacturers.

      Accurate had their own issues. But at least I found a rep in Massachusetts that made all the difference. And their customer service after the fact was excellent.

  2. 3 Jim Merrithew 10-January-2011 at 1:44 pm

    I was considering installing Thermotech windows when I build. My understanding is that their windows perform well with a high SHGC and a low U factor for the whole window. Thermotech’s offices are located within an hour of my home, so that would work logistically.
    Customer satisfaction and experience is one of the best and cheapest forms of advertising. Thanks for your input and feedback.

    • 4 Larry 10-January-2011 at 8:23 pm

      I still think Thermotech is one of the best window manufacturers in North America. Many of the award winning passivhaus and net zero houses have them. If you have the money and a good sales rep, then go for it, especially if you live down the street from them. I would recommend dropping in for a visit on a Friday after lunch to sample the quality of workmanship. ;-)

      • 5 W Smith 18-November-2013 at 3:42 pm

        I feel the pain on the windows … same story, just move the names around. Inline didn’t want our business, Thermotech and Serious were too pricey – Dorwin had manufacturing issues, ended up with Fibertech. If yours are anything like ours (they should be), you should love them – we do. Marvin just seemed cheap.


  1. 1 Windows have arrived! « Up Hill House Trackback on 21-November-2010 at 3:14 pm

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