SHGC measures how much solar radiation passes through a window. It ranges from 0 to 1. A value of 0.55 means 55% of solar heat enters the room. Lower SHGC is better for hot climates (reduces cooling costs); higher SHGC can be beneficial in cold climates for passive solar heating. The U.S. ENERGY STAR program recommends SHGC ≤ 0.25 for Southern zones and ≤ 0.40 for Northern zones.

In the Northern Hemisphere:
• West-facing windows receive the strongest afternoon sun, often causing the most summer overheating.
• South-facing windows receive the most winter sun — ideal for passive solar design.
• East-facing windows get strong morning sun.
• North-facing windows receive the least direct sunlight (mostly diffuse).
Use exterior shading on west and east windows to dramatically reduce summer heat gain.

Low‑E (low emissivity) coatings are microscopically thin metallic layers that reflect infrared radiation while allowing visible light to pass. A spectrally selective Low‑E coating can block up to 70% of solar heat (SHGC ≈ 0.27) while maintaining clear visibility. This makes them ideal for hot climates where daylight is desired without the associated heat.

Exterior shading (awnings, roller shutters, exterior blinds) blocks solar radiation before it enters the glass, reducing heat gain by 75–85%. Interior shading (blinds, curtains) only blocks heat after it has already entered the room, reducing effective heat gain by only 35–40%. For maximum energy efficiency, always prioritize exterior shading solutions — they are 3–5 times more effective.

According to ENERGY STAR, replacing single-pane windows with ENERGY STAR certified models can save 12–33% on annual energy bills, depending on climate. For a typical U.S. home, this translates to $125–$465 per year. In hot climates like Phoenix or Miami, upgrading to Low‑E glass with SHGC ≤ 0.30 can reduce cooling costs by 15–25% annually.

Yes, solar heat gain is directly proportional to window area. A window twice as large admits twice the solar heat. This is why large west-facing windows can dramatically increase summer cooling loads. When designing or renovating, consider limiting glass area on east and west façades, or use high-performance glazing with low SHGC values for large windows.