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Originally Posted by Wikipedia
Properly designed triple glazed IGUs with low emissivity coatings on surfaces #2 and #4 and filled with argon gas in the cavities result in IG units with R-values as high as R-5. Certain vacuum insulated glass units (VIG) or multi-chambered IG units using coated plastic films result in R-values as high as R-12.5
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And since you can't afford VIG glass, it's R5, versus about R20 for a 2x4 wall with proper sheating, drywall, &c.
You'd lose about 60W/mē through conductive losses 24/7 (barring curtains), but the sun puts out 1000W/mē (perpendicular area, before losses to low-emissivity glass) on a sunny day. With intelligent curtain management, I'm pretty sure even we R-3 double-glazers benefit from the windows.
Also, I never run lights when the sun is up, because I painted the interior of the house the second-lightest shade of each hue available at the Depot.