Window U-value, SGHC, and VT
We are building a house in zone 5 near Chicago, IL with 2 x 6 walls, 2″ of exterior insulation, rain screen and looking at Marvin essential line of fiberglass windows. I have received two quotes, one for regular thermopanes and one for an upgrade glass with a lower u value. Here is my question: if I choose the lower u value windows (better, more expensive glass), the SHGC goes down and VT goes down. For example the sliding patio door has a U of .29, SHGC of .30 and VT of .51 for the regular glass. The upgraded glass has a U of .24, SHGC of .19 and VT of .44. The cost difference for this one item alone is about $800 regular glass to upgrade glass. All of the windows with the upgraded glass have a lower U value, lower SGHC and lower VT. I am thinking go with the regular glass, save the money and put it into air sealing, insulation, or better HVAC system. The lower SHGC really concerns me. Plus I am only gaining about .5 r value with the lower U number. Any comments?
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Those windows R values are 3.45 and 4.17. About a 21% improvement for the lower U glass. Will you notice this in practice? Probably not, unless you have a very large area of glass. The lower VT will be noticeable, but it won’t be really severe. You may have a bit of a noticeable tint (color shift) though and not just “darker”. See if you can go see a sample of this glass in person to see if that tint is objectionable to you or not.
I don’t think the lower SHGC is particularly important in the Chicago area — you’re too far north and in a very heating dominated climate.
I personally would see if I could get a less aggressive lowE coating on a triple pane (something thicker than the 7/8” triple panes too, the skinny ones don’t see much benefit compared with good double panes). If the triple panes were really expensive or unavailable, I’d probably go with the less aggressive window option you have and put the savings towards better air sealing or possible better windows (the frames this time, not the glazing).