U-Factor & SHGC

Notes

• Code minimum and ENERGY STAR are different numbers, and salespeople blur them. In the North-Central zone the code lets you install U-0.3; ENERGY STAR wants U-0.25. A window described as "meets code" can be well short of the level that qualifies for rebates and tax credits — and in the Northern zone the gap between those two is the difference between double and triple glazing.
• Lower U-factor is better; it is the inverse of R-value. U-0.28 is about R-3.6. That is worth holding on to when comparing a window against the R-19 wall it sits in: even an excellent window is a hole in the insulation, which is why window area matters as much as window quality.
• SHGC matters more than U-factor here. In a cooling-dominated climate the sun coming through the glass is a larger load than the heat conducting through it. ENERGY STAR caps SHGC at ≤ 0.40 in this zone for exactly that reason, and a window with a superb U-factor and an unrestricted SHGC will make a south or west room worse.
• The numbers must be NFRC-certified. The label on the window carries U-factor, SHGC, visible transmittance and air leakage, tested to a common standard for the whole window — frame included. A manufacturer quoting "centre of glass" performance is quoting the best part of the window and omitting the frame, which is where most of the loss happens.
• Installation can undo the rating. A U-0.28 window fitted without air sealing around the frame, or shimmed out of square so the sash does not compress its seals, performs like a far worse window. Air leakage around windows is usually a larger loss than conduction through them.
• Replacing working windows rarely pays back on energy alone. The saving is real but small against the cost, and simple payback commonly runs past twenty years. The defensible reasons are rot, failed seals, single glazing, comfort next to the glass, and noise — not the utility bill.