Energy Star is a voluntary program established by the U.S. Environmental Protection Agency (EPA) to help individuals and businesses save money and protect the environment through energy efficiency. Energy Star certified windows and doors are designed to meet certain criteria to ensure they are energy-efficient and contribute to reducing energy consumption in buildings. Here are some key requirements for Energy Star windows and doors:

1. U-Factor: U-factor measures the rate of heat transfer through a material. For windows and doors, a lower U-factor indicates better insulation and reduced heat loss. Energy Star requirements vary depending on climate zones, with lower U-factor requirements for colder climates to maximize energy efficiency.
2. Solar Heat Gain Coefficient (SHGC): SHGC measures the fraction of solar radiation admitted through a window or door. Energy Star sets maximum SHGC values to control the amount of heat gain in warmer climates and minimize the need for air conditioning.
3. Visible Transmittance (VT): VT measures how much visible light is transmitted through a window or door. Energy Star requires a minimum VT to ensure adequate daylighting while still meeting energy efficiency standards.
4. Air Leakage (AL): Air leakage measures the amount of air that passes through cracks in a window or door assembly. Energy Star sets maximum allowable air leakage rates to ensure windows and doors are properly sealed to prevent drafts and improve energy efficiency.
5. Condensation Resistance: Energy Star certified windows and doors must meet minimum requirements for condensation resistance to reduce the potential for moisture buildup and mold growth.
6. Certification: Products must be independently tested and certified by an EPA-recognized certification body to ensure they meet Energy Star requirements.
7. Labeling: Energy Star certified windows and doors are labeled with the Energy Star logo and information about their energy performance, including U-factor, SHGC, VT, and air leakage ratings.

It's important to note that requirements may vary depending on factors such as window or door type, glazing options, and geographic location. Additionally, Energy Star periodically updates its requirements to reflect advancements in technology and changes in building codes and standards.

Choosing Energy Star certified windows and doors can help homeowners and businesses reduce energy costs, improve indoor comfort, and minimize environmental impact by reducing greenhouse gas emissions associated with energy consumption.