Visible Transmittance in Efficient Windows and Light Entering Your Home.

Visible transmittance (VT) is a measure of how much visible light passes through a window. It is an important factor to consider when choosing windows, as it can affect the amount of natural light that enters your home.

The sun emits different kinds of light, and some of them are bad for us. Good sunglasses come with UV protection to block harmful ultraviolet rays. Good windows – efficient windows – do something similar. They give you the best of the sun and protect you from the worst.

Sun shiningLet’s start with the good stuff: solar rays allow natural light into a home, enhancing its natural beauty, providing you and your family with vitamin D, and lowering your reliance on anything requiring a light bulb. In cold climates, the sun also offers you some free heating. A window optimized for cold climates helps lower your utility bill.

The bad stuff includes overexposure and the harm of solar rays over time, such as fading your furniture, floors, carpets, art, and another valuable home décor. In hot climates, allowing solar heat into your home makes your air conditioner work harder, wasting energy and raising your utility bills.

Efficient windows give you the best of the sun while protecting you from the worst by using reflective low-e (emissivity) coatings. Low-e coatings are transparent metallic layers, thinner than human hair, that are typically applied to windows in the manufacturing process but can also be added later. They filter out infrared light – the kind you don’t want. And they also reduce damage from UV light by 75%, block up to 70% of heat from passing through a window, and create $1.3 billion in annual residential savings on utility bills, according to this awesome fact sheet from the Lawrence Berkeley National Laboratory. This technology can be customized to your climate and needs by controlling the amount of light and heat that can pass through your windows.

Our parent organization, the National Fenestration Ratings Council, makes it easy for you to find windows that offer these technologies. It rates windows on three main energy performance factors, including Visible Transmittance. The NFRC label you see on new windows contains a number between 0 and 1 for Visible Transmittance, with a higher number meaning that more beneficial natural light will pass through them. You can also use our Window Selection Tool to find windows with the right level of Visible Transmittance for your home and climate.