Every rated window carries an NFRC label. U-factor measures heat loss: lower is better, especially in cold climates. Solar heat gain coefficient (SHGC) measures how much of the sun's heat gets through: low SHGC suits hot climates, while higher SHGC can help in cold climates. ENERGY STAR sets limits for four US climate zones, so check that a window qualifies for your zone, not just in general.
The NFRC label is the most useful thing on a window. It lets you compare brands on the same tested numbers.
The NFRC label
| Rating | Range | What it means |
|---|---|---|
| U-factor | About 0.15 to 1.2 | Heat loss; lower is better |
| SHGC | 0 to 1 | Solar heat let through; lower blocks more |
| Visible transmittance | 0 to 1 | Daylight let through; higher is brighter |
| Air leakage (optional) | Lower is better | Drafts through the frame |
| Condensation resistance (optional) | 1 to 100 | Higher resists condensation |
ENERGY STAR climate zones
ENERGY STAR divides the US into Northern, North-Central, South-Central and Southern zones, each with U-factor and SHGC limits. Cold zones focus on low U-factor; hot zones focus on low SHGC. Look up your zone and the current criteria before buying.
Frames and glass
- Vinyl: affordable, low maintenance.
- Fiberglass: stable, strong, paintable.
- Wood and clad wood: traditional look, more upkeep.
- Aluminum: slim and strong; needs a thermal break in cold climates.
- Double versus triple pane: triple lowers U-factor further.
- Gas fills and low-E coatings: improve performance without extra panes.
Installation
Even the best window leaks if poorly installed. Ask whether the installer uses full-frame or insert installation, and how they flash and seal. Coastal homes may need impact or design-pressure ratings.
Pair windows with insulation and air sealing for the biggest efficiency gain, and see hiring a contractor.
Browse windows and doors.
Common questions
What is a good U-factor for windows?
Lower is better. Compare against the current ENERGY STAR limit for your climate zone, since requirements differ between northern and southern regions.
What does SHGC mean?
Solar heat gain coefficient, from 0 to 1. A lower number blocks more of the sun's heat.
Are triple-pane windows worth it?
In cold climates they lower U-factor and reduce drafts and noise. In mild climates double-pane low-E windows usually pay back sooner.
What is low-E glass?
Glass with a thin metallic coating that reflects heat, used to lower U-factor and SHGC.