Replacement Window GuideIn a hot climate the priority is keeping solar heat out, which makes Solar Heat Gain Coefficient the rating to watch. The NFRC states that a lower SHGC number means more heat blocked, and lists the ENERGY STAR SHGC threshold at 0.25 for southern zones. This guide explains how to pick a hot-climate window, with every figure tied to its source.
Why SHGC leads in the heat
The NFRC hot-climate guidance describes SHGC as one of three primary energy-efficiency measures on the NFRC label, telling you exactly what to expect from a window in a number between 0 and 1, where a lower number means more heat blocked.
In a cooling-dominated climate, solar heat coming through the glass is heat your air conditioner has to remove. That is why SHGC, not U-factor, is the first number to check on a hot-climate window label.
The ENERGY STAR SHGC thresholds
The NFRC hot-climate guidance lists the ENERGY STAR SHGC thresholds by zone: the Southern and South-Central zones require an SHGC at or below 0.25, the North-Central zone at 0.40, and the Northern zone at 0.32.
A hotter zone demands a lower SHGC. For a southern home, targeting 0.25 or below is how you match the certification bar built for that climate.
How low-e blocks solar heat
Coatings are what let a window hit a low SHGC without going dark. The NFRC hot-climate guidance states that in hot climates technologies such as low-e coatings prevent hot, stuffy air.
ENERGY STAR quantifies the stakes, noting that a standard double-pane window allows approximately 75 percent of the sun's heat into a home, while certified options substantially reduce that heat gain. A hot-climate low-e coating is tuned to reject that solar heat.
Fading and comfort
The same coatings that block heat also protect interiors. ENERGY STAR reports that the coatings on certified windows reduce fading by up to 75 percent from ultraviolet light exposure.
In a sunny hot climate, that reduction protects flooring, furniture, and fabrics from sun damage while the low SHGC keeps rooms cooler. Comfort and interior protection move together when the glass is specified for heat rejection.
Certification and savings
ENERGY STAR requires that windows meet U-Factor and, where applicable, SHGC requirements based on climate zone, and directs buyers to determine their climate zone first.
ENERGY STAR reports that replacing old single-pane windows with certified windows lowers household energy bills by an average of up to 13 percent nationwide. In hot climates, where cooling dominates the bill, a properly specified low SHGC window is central to reaching that saving.