At 115°F, most of the heat a window lets in is sunlight, not hot air, and dual-pane low-E glass blocks most of that sunlight while still letting you see out. One of our LoE³-340+ windows lets in about 14% of the sun's heat that hits it, against roughly 80% for an old clear single-pane window: about 195 watts through a 3-by-5-foot window in full sun, instead of more than 1,100.
Two ways heat gets through a window
A window passes heat two ways, and each has its own rating on the NFRC label:
- Sunlight (solar heat gain coefficient, SHGC): the share of the sun's heat that gets through. 0.14 means 14%. Lower is better in Phoenix.
- Hot air against the glass (U-factor): how fast heat soaks through when it is hotter outside than in. Lower is better.
In a Phoenix summer the first one matters far more, because the sun is strong and on the glass for hours.
The numbers on a 115°F afternoon
Take a 3-by-5-foot window (15 square feet) on a west wall in full afternoon sun. Full sun delivers about 1,000 watts per square meter (the "one sun" standard used by the National Renewable Energy Laboratory), so about 1,390 watts reach the glass.
| Window | SHGC | Sun heat let in | Heat from 115°F air (78°F inside) |
|---|---|---|---|
| Old clear single pane, aluminum frame | about 0.80 | about 1,110 W | about 195 W |
| Energy Shield, LoE³-366 glass (single hung) | 0.22 | about 305 W | about 46 W |
| Energy Shield, LoE³-340+ glass (single hung) | 0.14 | about 195 W | about 39 W |
The single-pane figures are typical values from the Efficient Windows Collaborative; ours are NFRC-certified whole-window ratings. Heat from the air uses each window's U-factor (about 1.2 for single pane, 0.28 and 0.24 for ours) across the 37°F gap between 115°F outside and 78°F inside.
Two things stand out:
- The sun is the big number. Even with 115°F air, sunlight brings in five to six times more heat than the air does through the old window.
- Low-E cuts that by about 80%. The 340+ window lets in about 900 fewer watts in full sun. That is close to a small space heater you are no longer running against your air conditioner, per window, for every hour that wall is in the sun.
What "low-E" and "dual pane" each do
- Low-E is a microscopically thin metal coating on the glass. Cardinal's LoE³-340+ has three silver layers that reflect the sun's infrared heat while letting most visible light through. That is why the SHGC drops to 0.14 without the glass looking dark.
- Dual pane with argon puts a sealed gap of argon gas between two panes of double-strength 3 mm glass. That slows the heat that soaks through from hot air, which is the U-factor.
- The inner pane on our 340+ package carries a second coating (LoE-i89) on the room side. It brings the center-of-glass U-factor down to 0.20 and keeps the inside pane closer to room temperature, so the glass does not radiate heat at you on a hot afternoon.
Cardinal's own figures for the 340+ / i89 pair, glass only: SHGC 0.17, U 0.20, visible light 38%, and 2% of UV through, which is why furniture and floors fade much more slowly.
How that compares to Phoenix code
Across the 2012 to 2024 editions of the International Energy Conservation Code, the residential prescriptive path caps window SHGC at 0.25 for the Phoenix area's climate zone (2B). Every current Energy Shield window and sliding glass door is at or under 0.25:
- With LoE³-340+ glass, every window is 0.14 to 0.16.
- With LoE³-366 glass, windows run 0.22 to 0.25.
See every product and both glass packages on our solar heat gain page, or download the full performance ratings dataset.
340+ or 366: which should you pick?
- LoE³-340+ blocks the most heat (SHGC 0.14 to 0.16) and lets in less light (visible transmittance about 0.31 to 0.34). Best for west and south walls and rooms that bake in the afternoon.
- LoE³-366 lets in more daylight (about 0.52 to 0.58) and still meets code. Good for north walls and rooms where you want brightness.
Many Phoenix homes mix them: 340+ on the sunny sides, 366 where light matters more. In 2024 Phoenix ran 113 days in a row at 100°F or hotter (National Weather Service), so for most west-facing rooms the 340+ package pays off fastest.
See it on real homes
Recent installs: replacement windows in Mesa and patio doors in Phoenix. Installed prices for every window are on our pricing page, and every window carries our lifetime warranty.
Want numbers for your own windows? Send photos and sizes for a virtual quote, book a free estimate, or call (623) 231-2451.
Questions
Does low-E glass make windows darker? Somewhat. Our LoE³-340+ windows let through 31% to 34% of visible light, and LoE³-366 windows 52% to 58%. The coating reflects heat more than light, so 340+ cuts the sun's heat by more than it cuts brightness.
Is dual-pane low-E glass worth it in Phoenix? Yes, for sunny walls. In full sun a 3-by-5-foot LoE³-340+ window lets in about 195 watts of solar heat, against about 1,110 watts for an old clear single-pane window.
What SHGC do I need in Phoenix? Phoenix-area code caps window SHGC at 0.25. Every current Energy Shield window meets it, and with LoE³-340+ glass every window is 0.14 to 0.16.
Written by Juan Valles, co-owner, Energy Shield Window & Door Company. Updated November 6, 2026.
