Low-E Glass: What It Is, How It Works and Is It Worth It?

Reviewed by Mario Garcia, General Manager at Windows Hawaii

If you’ve shopped for new windows, you’ve seen “Low-E glass” on every quote. It’s one of the most useful upgrades you can buy, especially in a sunny place like Hawaii, but most people never get a clear explanation of what it is. Here’s how Low-E glass works, what the numbers on the label mean and how to choose the right type.

What Is Low-E Glass?

Low-E stands for low emissivity. A Low-E coating is a microscopically thin, nearly invisible layer of metal or metallic oxide applied to the glass. It lets visible light through but reflects much of the infrared energy that carries heat. According to the U.S. Department of Energy, Low-E windows can reduce energy loss by as much as 30% to 50%, and they typically cost about 10% to 15% more than windows with plain glass.

In a hot climate, that means less of the sun’s heat gets into your rooms. In a cold climate, it means less of your heat escapes. Most Low-E coatings also block a large share of ultraviolet light, which is what fades floors, furniture and artwork.

Types of Low-E Coatings

For Hawaii homes, solar-control Low-E is almost always the right choice. Our Ali’i Extreme windows offer SunMatrix, a four-layer Low-E coating designed to reflect solar heat.

How to Read the Window Label

Every rated window has an NFRC label with a few key numbers. Here’s what they mean:

• U-factor: How well the window keeps heat from passing through. Lower is better.
• Solar Heat Gain Coefficient (SHGC): How much of the sun’s heat gets through. Lower means less heat. In hot climates, this is the most important number.
• Visible Transmittance (VT): How much light gets through. Higher means brighter rooms.

For reference, ENERGY STAR version 7.0 requires windows in the Southern climate zone to have a U-factor of 0.32 or lower and an SHGC of 0.23 or lower.

Does Low-E Glass Make Windows Darker?

Modern Low-E coatings are designed to be nearly invisible. Some solar-control coatings have a very slight tint, and the more heat a coating blocks, the more noticeable it can be. Spectrally selective coatings are made to block heat while keeping visible light high. If you’re worried about the look, ask to see a sample before you order.

Low-E Glass vs. Window Film

Window film can be added to existing glass and costs less upfront, but it doesn’t match the performance or durability of a factory coating sealed inside an insulated glass unit. Film can also bubble or peel in strong sun and may affect some window warranties. If your windows are old, leaky or foggy between the panes, new windows with Low-E glass will do far more than film.

Is Low-E Glass Worth It in Hawaii?

Yes. Hawaii has some of the highest electricity prices in the country, and windows are one of the biggest sources of heat gain in a home. Low-E glass keeps rooms that face the afternoon sun noticeably cooler, cuts down on fading and helps the AC work less. Combined with argon gas and a good seal, it’s the biggest energy upgrade in a replacement window. It can also help reduce interior window condensation by keeping the inside pane closer to room temperature.

Low-E Glass FAQs

Can you add Low-E coating to existing windows?

Not the factory kind. Low-E coatings are applied during manufacturing. The only way to add Low-E to an existing window is film or replacing the glass unit.

Does Low-E glass block UV?

Most Low-E coatings block a large share of UV light, which helps protect floors, furniture and fabrics from fading. The exact amount depends on the coating.

Does Low-E glass affect plants?

Indoor plants usually do fine because visible light still gets through. Plants that need a lot of direct sun may grow a little slower behind heavy solar-control glass.

How much does Low-E add to the cost of a window?

The Department of Energy puts the premium at about 10% to 15%. See typical Oahu pricing in our Hawaii window replacement cost guide.

Want cooler rooms? Call 808-671-0808 or request a free estimate on Oahu.