Windows for Hot Climates: SHGC, Low-E and Frames

In a hot climate, the window that suits the house is the one that keeps the sun's heat out while still letting daylight in. The rating that measures this is the solar heat gain coefficient (SHGC), and lower means less solar heat. ENERGY STAR's two southern zones, South-Central and Southern, cap SHGC at 0.23, against 0.40 in North-Central ¹. A window at or below that cap, with a whole-window U-factor inside the zone's limit and low air leakage, is the kind to shortlist.

No single product or brand suits every hot-climate house. What separates good choices is a solar-control (spectrally selective) low-E coating that blocks heat without darkening the room too much, glass chosen for the sides of the house that get the most sun, and a frame that holds up to heat. This article is the warm-weather counterpart to windows for cold climates.

The ENERGY STAR southern-zone limits

ENERGY STAR sets window limits by climate zone. The two zones that cover the hot parts of the country look like this ¹:

All zones share an air leakage limit of 0.3 cubic feet per minute per square foot ¹. ENERGY STAR's criteria page links a search for finding which zone your address is in ².

Two things stand out. In the south, SHGC has a ceiling; in the Northern zone it has a floor, and the table's equivalent-performance options there allow U-factors of 0.23 to 0.24 only with an SHGC of 0.35 or higher, and 0.25 to 0.26 only with 0.40 or higher ¹. And the U-factor limits are looser in the south than in the north, but they are still limits: heat also moves through glass and frame from the hot side to the cool side, which is the U-factor's job to measure. DOE's consumer guide suggests a low U-factor on every side of a house in a warmer climate, not only a low SHGC ³. What SHGC measures, and how it is tested, is covered in solar heat gain coefficient explained; this article stays with how to use it in the heat.

Why SHGC leads in a hot climate

Sun through the glass becomes heat in the room, and in a hot climate the air conditioner then has to remove it. The Efficient Windows Collaborative, NFRC's consumer site, says a low SHGC can reduce cooling costs in hot climates and keep rooms from overheating in the afternoon sun ⁵.

One practical point: ask for whole-window numbers. DOE's Energy Saver Guide advises using whole-unit U-factors and SHGCs rather than center-of-glass figures, because the whole-unit numbers more accurately reflect the entire product ⁴. A glass maker's data sheet describes the glass alone; the NFRC label on the window is the figure to compare.

Keeping daylight while cutting heat

The obvious way to cut solar heat is darker glass, but NFRC's glossary notes that any tinting of glass reduces both visual and radiant transmittance ⁶: the room loses daylight along with heat. Spectrally selective glazing, which NFRC defines as transparent to some wavelengths of the solar spectrum and reflective to others, can be achieved with low-E coatings or high-performance tints ⁶. The low-E side is explained in low-E glass explained, and the SHGC article compares two real coatings.

For comparing quotes, one ratio helps. NFRC's glossary defines the light-to-solar-gain ratio (LSG) as visible transmittance (VT) divided by SHGC, a measure of how well glazing provides light without excessive solar heat gain ⁶. If a quote does not list LSG, divide the window's VT by its SHGC; VT is a mandatory NFRC label rating ⁵. When two windows have the same SHGC, the one with the higher VT, and so the higher LSG, lets in more daylight for the same solar heat.

Shade and orientation

Not every window in a house faces the same sun. DOE's consumer guide suggests choosing glass by the direction each window faces, with a low SHGC on south, east and west windows in a warmer climate and lots of shade on the south side ³; the SHGC article walks through that table. So it is reasonable to ask whether a quote can use a lower-SHGC glass on the west wall than on the north.

The glass is not the only tool. For warm weather, DOE's Energy Saver Guide suggests ⁴:

• awnings on south- and west-facing windows to create shade;
• closing draperies on windows that get direct sunlight on summer days, and medium-colored draperies with white plastic backings to reduce heat gain;
• sun-control or other reflective films on east- and west-facing windows to reduce solar heat gain.

Those options suit windows you are keeping. The article on window tint and film covers the film route, including what it can mean for a window warranty.

Frames in the heat

The frame matters less in a hot climate than the glass, but it still counts. The Efficient Windows Collaborative says the sash and frame make up 10% to 30% of a window unit's area, so the frame significantly influences the whole window's performance ⁷.

• Aluminum. The Efficient Windows Collaborative names its high thermal conductance as its biggest disadvantage. A thermal break, a less conductive material joining the inner and outer parts of the frame, has cut aluminum frame U-factors from roughly 2.0 to about 1.0 Btu/hr-sq. ft-°F ⁸. The same page says that in hot climates, where solar gain often matters more than heat conducted through the frame, improving the frame's insulation can be much less important than using a higher-performance glazing system ⁸.
• Vinyl. Standard vinyl performs about as well as wood thermally, and the collaborative says recent advances have improved its dimensional stability and resistance to degradation from sunlight and temperature extremes ⁹.
• Wood and composites. The collaborative gives wood frames U-factors of 0.30 to 0.50 Btu/hr-sq. ft-°F, and wood-polymer composites are described as very stable, with structural and thermal properties as good as or better than wood ⁹.

So in a hot climate, a thermally broken aluminum frame with excellent glass may perform well, while a well-insulated frame with ordinary glass may not. Material trade-offs are compared in window frame materials compared and aluminum vs vinyl windows. Coastal homes in hurricane areas may also need impact-rated products; see hurricane impact window cost.

How to compare hot-climate quotes

Ask each company for the NFRC label values of the exact window quoted, and set them side by side:

1. Whole-window SHGC, 0.23 or lower in the two southern zones ¹.
2. Whole-window U-factor, inside your zone's limit ¹.
3. VT, if daylight matters to you ⁵.
4. Air leakage, 0.3 or lower ¹.
5. Whether the glass changes on east- and west-facing windows ³.
6. Frame material, and whether an aluminum frame has a thermal break ⁸.

The full label is explained in energy-efficient windows explained, and state pages such as Texas, Florida and Arizona cover the installer checks where you live. Is the installer licensed where your state licenses this work? That question, and what a written quote should list, are in how to compare window quotes. When you are ready, you can compare quotes from window installers.

About these figures

• ¹ Official record: ENERGY STAR certification criteria table for residential windows (October 2023 file): Southern zone U-factor 0.32 and SHGC 0.23, South-Central U-factor 0.28 and SHGC 0.23, North-Central SHGC 0.40, Northern U-factor 0.22 with SHGC 0.17 or higher (prescriptive) and the equivalent-performance options of 0.23 to 0.26, and the 0.3 air leakage limit, checked October 27, 2026.
• ² Official record: ENERGY STAR key product criteria page and its climate zone search, checked October 27, 2026.
• ³ Official record: DOE consumer guide on a low U-factor and low SHGC by orientation in warmer climates and on choosing windows for different sides of the house, checked October 27, 2026.
• ⁴ Official record: DOE Energy Saver Guide (2022) on whole-unit ratings and its warm-weather tips on awnings, draperies and sun-control films, checked October 27, 2026.
• ⁵ Official record: Efficient Windows Collaborative ratings page on low SHGC in hot climates and visible transmittance as a mandatory label rating, checked October 27, 2026.
• ⁶ Official record: Efficient Windows Collaborative glossary definitions of tinted glass, spectrally selective glazing (low-E coatings or high-performance tints) and the light-to-solar-gain ratio as VT divided by SHGC, checked October 27, 2026.
• ⁷ Official record: Efficient Windows Collaborative frames page stating that sash and frame make up 10% to 30% of a window unit's area, checked October 27, 2026.
• ⁸ Official record: Efficient Windows Collaborative metal frames page on aluminum's conductance, thermal breaks lowering frame U-factors from roughly 2.0 to about 1.0, and frame insulation mattering less than glazing in hot climates, checked October 27, 2026.
• ⁹ Official record: Efficient Windows Collaborative non-metal frames page on vinyl stability, wood frame U-factors of 0.30 to 0.50 and composites, checked October 27, 2026.