Comparing window performance is easy, thanks to labels from the National Fenestration Rating Council (NFRC)
. Computer simulations create the key numbers for each window1. Energy Star
The shaded portions of the accompanying map show where the window meets regional Energy Star performance criteria. This one passes muster nationwide. 2. U-Factor
Measures how well a window stops heat flow, on a scale of 0.10 to 1.20. The smaller the number, the better the performance in both cold and hot climates.3. Solar Heat Gain Coefficient (SHGC)
Measures how well glazing blocks the sun's heat, on a scale of 0 to 1. The lower the number, the less heat gets in.
Go for 0.40 or lower in warmer climates. 4. Visible Transmittance (VT):
Tells how much light passes through; 0 is opaque, 1 is transparent. Glazing with a VT of 0.60 or more looks clear. A lower VT limits glare but can slightly darken rooms.
Some labels also rate Air Leakage (look for 0.30 or lower) and Condensation Resistance (the higher the number, the better).TOH Tip: Score a Tax Credit
Federal tax credits can cut the cost of new energy-efficient windows by 30 percent, up to $1,500 per household. Windows installed in 2009 and 2010 qualify if they have an NFRC-certified U-factor and an SHGC of 0.30 or less. The credit applies only to window costs, not installation. Save the "qualification statement" from the manufacturer and your receipts. A word of caution: Not all Energy Star windows are shoe-ins for the credit.