Shopping for replacement windows means decoding an energy label. Learn what U-factor, SHGC, low-E coatings and gas fills actually do, and how to pick the right combination for your climate.
Where to find the numbers
Most replacement windows sold in the U.S. carry a label from the National Fenestration Rating Council (NFRC). It rates the entire window, glass and frame together, under standardized conditions, so you can compare products from different manufacturers fairly. Many windows also carry an ENERGY STAR label indicating they meet efficiency criteria for specific climate zones.
Ratings for the whole unit matter more than 'center of glass' numbers you may see in brochures. The frame and edges are where a lot of heat escapes, so always compare whole-window figures.
U-factor: how well the window holds heat in
U-factor measures how quickly heat passes through the window. Lower is better. Older single-pane windows can be above 1.0, while a good modern double-pane window is typically around 0.30 or lower, and triple-pane units can go lower still.
In cold climates, U-factor is the most important number on the label because it drives winter heating costs and how comfortable you feel sitting near the glass.
SHGC: how much solar heat gets through
The Solar Heat Gain Coefficient is a number between 0 and 1 describing how much of the sun's heat passes through the window. A lower SHGC blocks more heat; a higher SHGC lets more in.
In hot, sunny regions, a low SHGC keeps rooms cooler and reduces air-conditioning loads. In heating-dominated climates, a moderate SHGC on south-facing windows can actually help, letting in free winter warmth. Some homeowners choose different glass packages for different sides of the house for exactly this reason.
Visible transmittance and air leakage
Visible Transmittance (VT) tells you how much daylight comes through, from 0 to 1. Higher means brighter rooms. Some heat-blocking coatings slightly reduce VT, so if natural light matters to you, compare this number alongside SHGC.
Air Leakage (AL), when listed, measures drafts through the window assembly. Lower is better, and casement and awning windows usually score well because they clamp shut against their weatherstripping. Remember that a great rating means little if the window isn't installed and sealed properly.
Low-E coatings: the invisible upgrade
Low-emissivity (low-E) coatings are microscopically thin metallic layers applied to the glass. They reflect long-wave infrared heat while letting visible light through. In winter they bounce your home's heat back inside; in summer they reflect outdoor heat away.
Coatings come in different formulations. Some are tuned for cold climates to allow more passive solar gain, others for hot climates to block it, and multi-layer coatings balance both. Low-E also cuts ultraviolet light, which helps protect floors, furniture and artwork from fading.
Gas fills: argon and krypton
Double- and triple-pane windows are sealed units with a space between the panes. Filling that space with an inert gas instead of air slows heat transfer. Argon is the most common: it's affordable, non-toxic, and noticeably improves U-factor. Krypton insulates better in thinner gaps and is often used in triple-pane units, but it costs more.
Gas fills dissipate very slowly over many years. Quality sealed units with good spacers keep the gas in longer, which is one reason the insulated glass warranty is worth reading.
- Air fill: baseline, least expensive
- Argon: the best value for most homes
- Krypton: top performance, usually in triple-pane
Putting it together for your home
For most homes in mixed or cold climates, a double-pane window with a quality low-E coating, argon fill, warm-edge spacers and a well-insulated frame hits the sweet spot of comfort and value. Triple-pane earns its keep in very cold regions, on noisy streets, or in rooms with large expanses of glass.
During your free estimate, we'll walk through the labels on the windows we recommend and explain why a particular glass package suits each side of your home.
Have questions about your home? We're happy to help.
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