What is low-E glass?

What this means for your home project

1. Record the climate zone on the ENERGY STAR label beside the NFRC label on a candidate window before ordering. (source)
2. Compare visible transmittance (VT) values; favor a higher value when more daylight is your priority. Source: ENERGY STAR / U.S. Environmental Protection Agency, read 2026-11-05.
3. Request a written bid separating labor from materials so the replies show those two amounts individually. Source: ENERGY STAR / U.S. Environmental Protection Agency, read 2026-11-05.
4. Request the manufacturer’s installation instructions and have the proposed work follow them to protect any instruction-dependent warranty. Source: ENERGY STAR / U.S. Environmental Protection Agency, read 2026-11-05.

What is the difference between low-E types?

Use NFRC’s guidance (read 2026-11-05) to distinguish the coating’s insulation role from its effect on sunlight entering your rooms. “Different types of low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain.” (source) “Windows with moderate-solar-gain low-E typically have a SHGC of around 0.25-0.35, but higher or lower values are available with high-solar-gain or low-solar-gain low-E glass.” (source) “Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy transmitted and tells you how well the product blocks heat caused by sunlight.” Source: ENERGY STAR / U.S. Environmental Protection Agency, read 2026-11-05. “The lower the SHGC, the less solar heat the window transmits.” Source: ENERGY STAR / U.S. Environmental Protection Agency, read 2026-11-05.

Do low-E windows make a house darker?

Use ENERGY STAR’s daylight rating (read 2026-11-05) to compare proposed glazing; its storm-window guidance also identifies a darkening concern. “Visible Transmittance (VT) measures the amount of light the window lets through.” (source) “VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80.” (source) “The higher the VT, the more light you see.” (source) “If your current windows already have tinted glass, adding a low-e storm window may reduce the visible light transmitted through the window, making them even darker.” (source)

Can I add low-E to existing windows?

ENERGY STAR’s guidance (read 2026-11-05) explains the storm-window option, when existing-window condition limits it, and when heat trapping changes the decision. “Storm windows are installed as attachments to existing primary windows.” (source) “Storm windows work best when installed over working primary windows that are in good condition.” (source) “If your current windows are rotted, not operating properly, or damaged beyond reasonable repair, adding low-e storm windows may not be appropriate.” (source) “If you already have double-pane or low-e coated windows and you live in a hot, sunny climate where the temperature frequently goes above 115 degrees Fahrenheit, you should consider awnings, solar screens or full replacement with ENERGY STAR certified low-SHGC windows as an alternative to storm windows.” (source)

Do your current windows have metal frames?

Before adding low-E storm glass, use ENERGY STAR’s installation guidance (read 2026-11-05) to identify the separator needed at metal frames. “It is very important to use thermal breaks (small non-metal separators) when installing ENERGY STAR certified storm windows over metal-framed primary windows.” (source)

What SHGC should I choose?

Use ENERGY STAR specification 7.0 (read 2026-11-05) and NFRC’s guidance to request ratings suited to your climate and window exposure. “Following is the Version 7.0 ENERGY STAR product specification for Windows, Doors, and Skylights.” (source) “Windows, doors and skylights must meet U-Factor and, where applicable, Solar Heat Gain Coefficient (SHGC) requirements based on climate zone.” (source) “U-Factor measures the rate of heat transfer and tells you how well the window insulates.” (source) “The lower the U-factor, the better the window insulates.” (source) “SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80.” (source) “What level of solar gain is most desirable depends on the climate, orientation, shading conditions, house design and personal preferences.” (source)

What affects the cost of Low-E glass?

For a low-E window replacement, ENERGY STAR’s buying guidance (read 2026-11-05) helps separate the product package from installation in written bids. “When interviewing contractors, ask them to break down the price quote by labor and materials.” (source) “ENERGY STAR certified windows, doors and skylights may cost more than non-certified products, but the labor involved should be comparable for both.” (source)

Sources

• Frequently Asked Questions (FAQ) — National Fenestration Rating Council; read 2026-11-05.
• Independently Tested and Certified Energy Performance — ENERGY STAR / U.S. Environmental Protection Agency; read 2026-11-05.
• Storm Windows: Buying Guidance — ENERGY STAR / U.S. Environmental Protection Agency; read 2026-11-05.
• Residential Windows, Doors, and Skylights: Version 7.0 Specification — ENERGY STAR / U.S. Environmental Protection Agency; published 2022; read 2026-11-05.
• Residential Windows, Doors, & Skylights: Buying Guidance — ENERGY STAR / U.S. Environmental Protection Agency; read 2026-11-05.