Modern Window Technology

Look for National Fenestration Council Ratings:

• U-factor:  Conduction losses and far infrared (nonsolar) radiation
• Solar Heat Gain Coefficient (SHGC): Transmissivity to the entire spectrum of sunlight (both visible and near infrared).
• Visible Transmittance (VT): Transmissivity to visible solar spectrum only. 

All windows:

• Low-E (low emissivity) double-glazed windows. U-value < .37 

South Windows:

•  "High Transmission" low-E: Large SHGC

Nonsouth Windows:

• "Selective Transmission" low-E: High VT, Low SHGC.