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What Is Visible Transmittance In House Windows?

Last updated on 05/11/2022

The windows in a home serve a few purposes. They obviously allow for ventilation and fresh air where it's necessary. In some cases, they allow a homeowner to take advantage of a beautiful view. But, one of their main purposes is to allow light into the home. And while it might seem that all windows of similar sizes should allow the same amount of light in, this isn't the case. In fact, the amount of light can differ so much that the NFRC (National Fenestration Rating Council) devotes a whole section to it on their labels called "visible transmittance.";

But what is visible transmittance and why does it matter? Keep reading to learn more about this window rating.

What is Visible Transmittance?

Most folks understand that natural light can put them in a better mood, reduce fatigue, and improve mental and physical health among many other benefits. But what some homeowners don't know is that the natural light that makes it into their home through the windows can have similar effects-they don't even have to go outside.

But, that depends on their windows' visible transmittance rating.

In very simple terms, visible transmittance is a term that describes the amount of light that a window allows into a room. Third-party examining bodies perform tests and calculations to determine how much light is able to pass through the window.

They then assign that window a visible transmittance score, which shoppers can find on the NFRC label.

How Visible Transmittance is Calculated

One might think that the only parts of the window that matters when testing and calculating visible transmittance are the window panes. However, visible transmittance is a whole-window rating, meaning there is more to the equation than shiny, transparent glass.

Independent testing bodies use the NFRC's criteria when calculating Visible Transmittance and other ratings, such as Air Leakage, U-Factor, and Solar Heat Gain Coefficient. This criterion requires taking the entire window's construction, size, and materials into account when determining visible transmittance.

For example, the sash, frame, and any grids, which are not at all transparent, are considered when calculating visible transmittance. These materials block the natural light completely, which brings the window's visible transmittance rating down.

And, not all window panes are created equal. For example, some window panes feature low-E coatings designed to block light from entering the home. Other windows might also feature three layers of glass, further reducing the amount of light that makes it through.

With all of these factors considered, the testing body assigns the window a visible transmittance score between 0 and 1. Lower scores, such as .3 or .4 indicate that the windows let in less light than higher scores, such as .6 or .7. Replacement windows with wide sashes, grids, three panes, and low-E coatings will have lower visible transmittance ratings than new construction windows with thinner frames and fewer light obstructions.

Where You Can Find a Window's Visible Transmittance Rating

Most shoppers know to look for the Energy Star symbol, but the NFRC takes the rating a step further. Rather than just certifying a product, the NFRC supplies the shopper with that window's rating in an easily-understood sticker label. Every window must come with this sticker attached or it is considered to be uncertified.

The NFRC certifies and labels products with:

If a window fails just one of the test ranges above, the NFRC will not certify it. But, in the case of visible transmittance, all windows fall within the 0 to 1 range. Zero would have the equivalent visible transmittance of a brickwall. A score of 1 would mean it lets 100% of light through the opening. It's really an all-encompassing range.

Why Visible Transmittance Matters

On the surface, it would appear that visible transmittance only affects the light within the space, but that's not always the case. This rating can directly impact energy efficiency and the lifespan of home furnishings.

Energy Efficiency

The amount of light a window lets in can directly impact energy efficiency, and these effects can be both good and bad. High visible transmittance scores typically come with higher Solar Heat Gain Coefficient ratings. This means that, because the window allows so much light into the space, the light brings solar heat with it.

In colder climates where homes require heating eight or nine months out of the year, higher visible transmittance can be a benefit. The higher transmittance and accompanying Solar Heat Gain Coefficient can help warm a home during the day, reducing the home's reliance on heat.

In warmer climates, the opposite is true. In these areas, homes are more reliant on air conditioning than heat, which means they need a lower Solar Heat Gain Coefficient. Along with this lower coefficient typically comes lower visible transmittance.

The Impact on Furnishings

Lots of natural light is a good thing for mental health and physical well-being, but it can also take a toll on a home's furnishings.

It's not uncommon to walk into homes with high visible transmittance windows and find fading on hardwood floors. This fading is caused by the sun and is usually found on the floor under a window, just a few feet from the wall. Depending on which direction the wall is facing, these fade marks can even be in the shape of an arc, caused by the sun's path as it travels through the sky.

The same can be true for couches and curtains. High visible transmittance ratings might make the room feel bright and cheery, but they can also cause the fabric on couches, pillows, rugs, curtains, and other objects in direct sunlight to fade prematurely.

In these cases, a lower visible transmittance rating could preserve the floor or textiles, though the room would feel darker.

There are Exceptions

It isn't always the case that a high visible transmittance rating means a higher Solar Heat Gain Coefficient. Today's windows and technology have come so far that it's easier than ever to get brighter windows without sacrificing heat gain.

Low-E coatings on modern, high-performance windows can resist heat gain while still letting in relatively high amounts of light. Special coatings, films, and gasses help filter out the solar gain while also allowing high visible transmittance.

Also, there can be exceptions or special circumstances within a single home. For example, it's not uncommon for homeowners in the northern hemisphere to install windows with lower visible transmittance ratings on the south side of the home. On the northern side, they'll often use higher visible transmittance windows to let as much light in as possible. The windows on the east and west sides typically fall somewhere in the middle.

Visible Transmittance Will Affect Your Home: You Decide How

Visible transmittance, whether it be low or high, will have an impact on a home. All windows allow some light in, but how much light and how it affects the rest of the home are important to understand. If planned correctly, a homeowner can really bolster their home's look and efficiency with high-quality, well-planned windows.

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