Visible transmittance (VT) is a window rating that shows how much visible daylight passes through the glass, on a scale from 0 to 1. A higher VT means more natural light gets in. There is no single best number, because the right rating depends on how bright you want the room and how VT works with the window's other ratings.

At The Window Depot, VT is one of the numbers we help DIY homeowners and contractors read on the NFRC label when they compare vinyl replacement windows at our stores in Tucson, Phoenix, Chandler, Surprise, El Paso, Albuquerque and National City. Once you know what it measures, choosing between two windows that look identical gets a lot easier.

What Does Visible Transmittance Mean on a Window?

Visible transmittance measures the share of visible light that passes through the whole window compared with an open, unobstructed opening of the same size. It rates the complete product, so coatings, tint, the number of panes and the frame all affect the number.

A window with a VT of 0.60 lets in more daylight than one rated 0.40. That doesn't make the 0.60 window better overall. It just means it lets in more light.

You'll find VT on the NFRC label, listed alongside U-factor and Solar Heat Gain Coefficient (SHGC). NFRC uses standardized test procedures, so ratings from different manufacturers can be compared fairly.

What Is a Good VT Rating for Windows?

There is no universal ideal. ENERGY STAR notes that VT values for windows generally range from about 0.20 to 0.80. Where you should land depends on the room and what you want the window to do.

When a higher VT makes sense

• The room doesn't get much direct sun and feels dim
• You want to rely less on lights during the day
• A bright interior is a big part of your design goals

When a lower VT makes sense

• The room takes intense direct sun, like many west-facing rooms in the desert Southwest
• Glare on screens or floors is a problem
• You prefer a softer, less bright space

Is a Higher VT Rating Better?

A higher VT is better only if daylight is your top priority. It isn't a measure of overall window quality. A window can let in plenty of light and still let in too much heat, which is why VT should always be read next to SHGC.

VT vs. SHGC: What Is the Difference?

VT measures visible light. SHGC measures how much of the sun's heat comes through the glass, and a lower SHGC means less heat.

Light and heat aren't the same thing, so a well-designed window can let in useful daylight while blocking a good share of solar heat. For homes in Tucson, Phoenix, El Paso and other hot, sunny markets, the goal is usually a low SHGC paired with a reasonably high VT. Our guide to energy-efficient windows for hot climates covers the SHGC side in more detail.

How Is VT Different From U-Factor?

Each rating on the NFRC label answers a different question about the window.

Reading all three together tells you what a window is actually built to do.

Does Low-E Glass Change a Window's VT?

Yes, it can. Low-E coatings are designed mainly to control heat, but different coatings and glass combinations let through different amounts of visible light. Two Low-E windows can carry noticeably different VT ratings.

That's why it pays to compare the NFRC ratings for the complete window rather than assuming every Low-E product performs the same.

Does a Higher VT Make a Window More Energy Efficient?

Not on its own. VT describes light, not insulation or heat control. ENERGY STAR bases its window requirements on U-factor and, where it applies, SHGC. VT appears on the label, but it isn't a qualification requirement.

More daylight can still be a real benefit. Just don't use VT alone to judge a window's efficiency.

How to Choose the Right VT for Your Windows

Start with how each room is used. Living areas that feel dark may benefit from higher VT glass. Rooms that bake in the afternoon sun may call for more attention to how VT and SHGC balance out.

Your climate matters too, since ENERGY STAR sets different requirements by climate zone. Our post on choosing the right windows for your climate walks through how that plays out.

If you're a contractor or remodeler ordering for a client, pull the NFRC ratings for the exact glass package you're specifying. The same window line can carry different VT numbers depending on the glass, and that includes the Milgard and SI Windows products we carry.

The same rating also shows up on skylights, where it tells you how much daylight a skylight will bring into the room below.

Frequently Asked Questions

What does VT mean on a window label?

VT stands for visible transmittance. It shows how much visible light passes through the window on a scale from 0 to 1, with higher numbers letting in more light.

Where can I find a window's VT rating?

Look at the NFRC label on the window. You can also search product-specific ratings in NFRC's Certified Products Directory.

Does tinted glass lower VT?

Generally, yes. Tints and some coatings reduce the amount of visible light that reaches the room, which lowers the VT rating.

Do skylights have a VT rating?

Yes. NFRC-certified skylights carry VT on their labels just like windows, which helps you compare how much daylight each one lets in.

Can The Window Depot help me compare VT ratings?

Yes. Our team at any of our locations in Arizona, Texas, New Mexico and California can help you read NFRC labels and compare glass options on the windows we have in stock.

Compare Window Ratings at The Window Depot

The right VT is the one that fits your room, your climate and the rest of the window's ratings. Stop by The Window Depot in Tucson, Phoenix, Chandler, Surprise, El Paso, Albuquerque or National City to compare NFRC labels side by side on our in-stock windows, or browse vinyl windows online first. Find your closest store with our store locator.