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What is the U-Factor?

It is an insulation value to help compare the efficiency of windows.

Window U-Factor Rating

If you have ever been shopping for new windows, you might have noticed that it is common for stickers to be placed on the windows that state what the “U-Factor” rating is. The U-factor is a number that is extremely important to take into account when deciding on what windows to purchase and you should definitely pay close attention to what this number is for each window you are looking at.
The U-Factor represents the rate of heat loss for a window or how much heat passes through your windows. In other words, the U-Factor is the most crucial rating in terms of insulation for a window. Usually, the U-Factor ranges anywhere from 0.15 to 1.25 and you want to make sure you pay close attention to what a windows U-Factor is when you are out shopping for new windows. The lower the U-Factor, the greater resistance a window has to letting heat escape, and therefore, providing better insulation. Whereas, on the other hand, the higher the U-Factor, the less insulation the window will provide resulting in more heat escaping. Typically, you want your windows to have the lowest possible U-Factor because that will save you more money on the heating and cooling bills in the long run.

What U-Factor Numbers Mean

Typically, high-performance double pane windows must have a U-Factor of 0.27 or lower. However, there are some special cases in which the U-Factors go slightly outside of the average range of 0.15-1.25. In fact, there are some triple pane windows that can reach a U-Factor as low as 0.15.  For comparison purposes, the triple pane Intelligent Windows™ that we offer at have a U-Factor that ranges from 0.22-0.16, dependent on the size and style of window.

What is the U-factor rating actually measuring?  The actual U-Factor rating on a window is expressed as Btu/h·ft²·°F. The Btu stands for British thermal units, the ft. is feet, and the °F is degrees Fahrenheit. Essentially it is a measurement of heat loss over time.

The important thing to keep in mind is that the lower the number, the better.  There is no such thing as a window that is too efficient.  As mentioned above, the U-Factor ratings typically range between 0.15 and 1.25 Btu/h·ft²·°F, where windows with a lower U-Factor rating were better insulated. However, this value is never randomly chosen. In fact, Energy Star will require a minimum U-Factor rating depending on where you live and the typical weather conditions in that area. For example, in the northernmost areas of the United States, for a window to achieve an Energy Star rating, it must have a minimum U-Factor rating of 0.27.

The U-Factor has a couple different elements that contribute to the overall U-factor. The glazing, the frame, and the spacer are all parts of the window that are included in the U-Factor as a whole. The spacers are what separate the glazing panels of the window and can actually reduce the U-Factor near the edge of the glazing. Sometimes, it is possible to get a rating for just the glazing because it is such an important factor. This is called the center-of-glass U-Factor. Even though the center-of-glass U-Factor is less commonly rated, the point of getting it is to find out how effective the insulation of the glazing is by itself, without the frame or spacers. Although, most of the time, the U-Factor for the center of the glass is not as high as the overall U-Factor for the entire window.


Energy Star Ratings for Windows Map


How Do I Find the U-Factor?

Every window has a U-Factor rating, and it has that number stated on something called an NRFC sticker. The NRFC stands for the National Fenestration Rating Council. They are the entity that actually certifies and labels energy efficient windows, doors, and skylights. In order to earn the NRFC sticker, the specific window has to be tested. Once it passes verification and certification processes, the NRFC will give it a sticker that will state a number of different ratings and categories, including the U-Factor. The ratings that come from the testing are based on how well the windows perform in terms of energy efficiency. Therefore, when you see a window that has a NRFC sticker on it, you know that the window has been properly tested and rated by professionals based on it’s abilities to insulate.

NFRC Window Label


Is U-Factor Important?

You want to make sure that when you purchase replacement windows, you keep the U-factor in mind.  Though windows are replaced in homes for a number of reasons, comfort and efficiency are often high on that list of reasons.  The U-factor given for the windows you are considering are basically a way for you to compare windows so that you know how well they will keep the outside weather outside.

All you need to do is make sure that you work to get the windows with the lowest U-factor possible, while also offering the other window features you want in your replacement windows.

The Window Hound’s Take

Efficiency and comfort are extremely important when it comes to replacement windows for your home.  Windows are not nearly as efficient as a wall, so you want the most efficient windows possible.  Windows were meant to be seen through, not felt.  My suggestion is to keep the U-factor in mind when selecting a window for your home.  Aim for a U-factor lower than 0.25 and less than $1000 per window, including installation, and you can be proud of your new wndows.


The Window Hound

The Window Hound is helping you find the best replacement windows for your home.  Reading through pages of data, researching replacement window companies and bringing you the best information possible so that you can be a better window shopper.  The Window Hound writes for and lets us know which windows are the best for our clients.

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Low-E Coated Glass

Most quality replacement windows have some form of Low-E coating on their glass.  The quality of that coating varies, but in order be anywhere near efficient, it is a must have.

Better SHGC Rating

The amount a solar heating allowed through a window can be adjusted at the time of manufacturing of the window by using different Low-E coatings. 

In some parts of the country, you welcome the sun warming your living space.  In warmer climates, you want the sun’s warm rays to stay outside.


To most people, a window is a window.  when you go to sell your home and you inform would-be buyers of the Low-E coated windows in your home, you have now shown them that you gave your home the best care.

Let the Light In

When glass is coated with inferior Low-E coatings, it can block or blur what you see out the window.  Our Low-E coatings are visibly transparent, allowing you to see out, while preventing damaging rays in.


Our windows have three panes of glass, giving you a better window.  That also gives us more glass area to put our efficent Low-E coatings on, giving you a superior window than possible with a double pane window.

Our Promise

When we created Easy Order Windows we knew we would need to offer the the best windows possible for our customers.  We would need to offer the best because we would have a hard time explaining how good a mediocre window is.  That is why we only offer superior Low-E coated, triple pane windows, at a double pane window price.

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