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.