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Visible Transmittance (VT): How Well Do Your Windows Manage Natural Light?

| March 22, 2022
Visible Transmittance


It’s a whole lot easier to be eco-conscious around the home when there’s a chance to see the benefits in your wallet. Outfitting your home with energy-efficient windows is the perfect example of this: it’s an effective way of reducing your home’s carbon footprint and saving on utility bills at the same time. But just how energy-efficient are your windows?

The energy performance ratings found on your glass units tell you how well they work to let in natural light, keep out unwanted heat and prevent air from leaking. One of these important metrics is called Visible Transmittance. In this blog, we’ll cover what visible transmittance is, why it’s important and what design elements can help improve it.

What is Visible Transmittance (VT)?

Visible Transmittance (VT) is a unit of measurement that describes how well glass window and door units can transmit light.

This measurement is important: it refers to the extent to which your units work to let natural sunlight into the home, reducing the need for artificial light sources. Plus, access to more natural light can liven up your living space and positively influence your mood!

Here’s How Visible Transmittance is Measured

A window’s VT is calculated based on the percentage of light that passes through a glass unit. “Visible” refers to the fraction of the spectrum of sunlight that’s visible to the human eye. Visible transmittance is expressed as a number between 0 and 1, with a higher number indicating more natural light transmittance.

What is a Good Visible Light Transmittance Rating?

There is no one specific VT rating to look for. The general recommendation is that homeowners seek out higher numbers closer to 1. However, a VT rating on its own doesn’t tell the full story when it comes to energy efficiency.

Visible Transmittance And Other Energy Efficiency Indicators

Visible transmittance is one of four key metrics used by the National Fenestration Rating Council (NFRC) to rate the energy efficiency of glass windows, doors and skylights. As an independent third party, the NFRC is able to provide objective testing and certifications for organizations such as ENERGY STAR®. These metrics allow them to hold windows to a consistent standard of performance which, in turn, helps homeowners make informed decisions about their windows.

The four key metrics used by the NFRC are:

    1. Visible Transmittance
    2. U-Factor: The rate at which heat escapes through a window.
    3. Solar Heat Gain Coefficient: How well a window can resist heat gain.
    4. Air Leakage: The amount of airflow into and out of a building via the windows.

Learn More About Window Energy Efficiency For Your Home or Cottage Here.

Visible Transmittance

Is High Visual Transmittance Good for Energy Efficiency?

A high VT rating alone isn’t enough to inform you on how energy-efficient your windows are. It’s crucial to look at this rating in conjunction with other performance metrics, namely the solar heat gain coefficient. Together, these two metrics will tell you just how well your windows balance letting in natural light and heat from the sun.

For example, a high VT rating coupled with a high SHGC would indicate that your windows let in a lot of light and do not resist the heat gain that naturally comes with it. This may be considered “good” or “bad” depending on the climate where you live:

  • Homes in Northern Canada exposed to long winters and minimal warm weather would want a higher SHGC to take advantage of more solar heat gain.
  • Homes exposed to sunshine year-round in the southwestern parts of the United States would want a lower SHGC.
  • Conversely, homes in Southern Ontario want an optimal balance to manage the hot summers and cold winters.

As you can see, balance and customization are important parts of energy efficiency.

Tip: Light-to-solar gain (LSG) is a more specific energy rating that measures this relationship between VT and SHGC. The higher the number, the more light is transmitted without adding excessive amounts of heat.

3 Window Design Elements That Will Influence Visible Transmittance

Now that you know the ins and outs of visible transmittance, here are some design features that will influence how much natural light streams through your windows:

Low-E Coatings

Low-E coatings are transparent, ultra-thin coatings added to the glazings of a window unit. They help to control or regulate the passage of solar radiation. The goal of low-E coatings is to limit the passage of certain wavelengths such as ultraviolet and infrared light while still permitting visible, natural light. This means protecting your home from the types of light that can fade fabrics and cause unwanted heat gain.

Even though low-E coatings do add a slight tint to windows, the primary point of this technology is to control and reflect unwanted radiation without noticeably impacting the flow of light.

low-E coatings diagram

The Magic™ Difference: Most window manufacturers purchase their glass units directly from suppliers. Magic is unique in the sense that we do not purchase our glass units. From glass types and glazings to gas fills and coatings, we formulate our own glass packages and customize them to suit your specific preferences, climate and home’s construction. Therefore, we can provide customized low-E coatings that will deliver optimized natural light transmittance.

Learn More About Low-E Coatings Here.

Retractable Screens

When window manufacturers report and promote the energy efficiency of their window units, they are reporting measurements taken without an insect screen in place. Why is this a problem? Because fixed insect screens can cut your natural light transmittance by 50% or more. This means that these promoted metrics are false.

Conversely, you’ll be able to fully capitalize on your window’s VT year-round with retractable screens that remain completely out of sight when they’re not needed.

Visible Transmittance

The Magic™ Difference: Our fully retractable screens built with patented, Slide ‘N’ Hide® technology give you complete control. Plus, with the addition of our dynamic shade and blind system, you can also see improvements to your windows’ solar heat gain resistance. The use of all three retractable screens from Magic can improve a window’s overall efficiency by 40% or more.

Learn More About Retractable Screens Here.

Slimmer Frames

High-quality frames are key to whole-home energy efficiency. They keep the elements out and keep your heating and cooling in. But what you may realize is that they can affect your home’s natural light transmittance, too. An important design feature to look out for when attempting to fully capitalize on your property’s natural sunlight is the size of the frame. Put simply, a window frame that maximizes the glass surface will also help to improve visual transmittance.

Visible Transmittance

The Magic™ Difference: Our frames have a slim, stylish profile that places visual emphasis on the window’s glass and the outdoors beyond. The profile of our frames is a full three inches thinner than our competition. This ultra-thin design reduces the frame’s size and maximizes the glass surface. This can help create a more contemporary look that fully capitalizes on natural sunlight. Plus, homeowners can also choose to customize the look of these high-performance frames.

Learn More About Our Hybrid Fusion Frames Here.

Learn More About Installing High-Performance Windows

Looking for more information on visible transmittance, energy-efficient windows or general information about replacing your windows? Reach out today to book a free, no-hassle consultation with one of our representatives!



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