What is a U-factor Value?

In this guide, we’ll tell you everything you need to know about the U-Factor value of your replacement windows, as well as the NRFC sticker.

Allow us to first explain the U-Factor then.

Simply put, the U-factor is the most important insulation rating for your windows.

It represents how much heat passes through your windows.

Ideally, you want your replacement windows to have the lowest U-Factor rating possible, so you won’t spend as much on heating bills.

However, one more thing to keep in mind with this value is the climate you live in.


How is a U-Factor determined?

Your windows’ U-Factor rating is expressed as Btu/h·ft²·°F. The Btu stands for British thermal units, the ft. is feet, and the °F is degrees Fahrenheit. On the lower end, replacement windows can have a U-Factor of 0.25. The highest U-Factor rating is 1.25.

The U-Factor rating includes all parts of the window, such as the spacers, the frame, and the glazing. The glazing can be an important part of the U-Factor, enough so that sometimes you can get a rating for the center-of-glass-U-Factor.

This is how well the glazing insulates before your frame is added. The center-of-glass-U-Factor is rarely as high as the overall U-Factor.


How Do You Know What a Good U-Factor Is? NFRC Window Sticker

In the prior section, we mentioned that the U-Factor value is between 0.25 and 1.25 Btu/h·ft²·°F. We also said that windows with a lower U-Factor rating were more adept at insulation.

What if you checked the NFRC sticker and saw you had a higher U-Factor? Does that mean your windows are totally useless?

Not at all. The U-Factor for your windows is never randomly chosen.

Energy Star will typically suggest a U-Factor range depending on where you live.

If it’s a hotter climate, then it’s better to have replacement windows with a lower U-Factor. Even if you make your home somewhere cold, lower U-Factor windows are also good. You can also get away with a higher U-Factor rating in these environments.


What Is The Lowest U-Factor Value Possible?

The lowest U-factor value that we have seen is .15. This is for a triple pane window that has krypton & argon gas fills & Low-E glass to maximize the performance.

Each manufacturer has different materials and combinations, so it may be slightly tough to find one that offers all of these options.


Is the U-Factor Different from the R-Value?

You may never have heard of a replacement window’s U-Factor before, but you’re quite familiar with the term R-Value. Is that the same thing as a U-Factor?

Not exactly. The R-Value also deals in insulation and heat flow resistance, that much is true. However, it doesn’t have anything to do with windows. Instead, it’s how well-insulated the roof, floors, and walls are.

Unlike the U-Factor, you want a higher R-Value, as it indicates better insulation.

It’s good to know both the U-Factor and R-Value of your home to get an idea of how insulated your property is.

You can even use the U-Factor to calculate the R-Value to a point. You want to divide one by your U-Factor. If your U-Factor is 0.25 and you do 1/0.25, you get an R-Value of 4.


What Is an NRFC Sticker?

Okay, first, let’s clear this up: what is an NRFC sticker anyway? The NRFC stands for the National Fenestration Rating Council.

They’re a nonprofit that labels and certifies energy-efficient windows and more. These stickers can be placed on skylights, doors, and windows.

To earn an NRFC sticker, the window in question must have been tested. It then passes verification and certification processes by the NRFC. The sticker will give you a series of performance

ratings, categories, one of which is the U-Factor.


What Do All Those Other Abbreviations Mean on the NFRC sticker?

Okay, getting back to your NFRC sticker, you’re going to see other abbreviations beside the U-Factor. What are they and what do they mean? In this section, we’ll elaborate on these categories.

Condensation Resistance

Okay, the first category on your NRFC sticker doesn’t have an abbreviation, admittedly. It’s Condensation Resistance. As the name might tell you, this category determines how well your windows are at preventing a buildup of water, aka condensation.

Condensation can form on the outer or interior surface of your windows as well as within two panes.

It occurs quite often in the winter when the temperature lowers and window glass gets cold as a result.

The rooms in your home retain moisture, which can also increase the rate of condensation.

If you don’t monitor your windows, mold, and mildew can develop around windows that are heavy with condensation. These bacteria prefer warm, moist environments, which they can get in a home with lots of condensation.
If you have replacement windows with Condensation Resistance, they will be assigned a score between 0 and 100. You want a Condensation Resistance number that’s higher, as it’s better at warding off condensation.

Visible Transmittance or VT

Another term you’ll see on your NRFC sticker is Visible Transmittance or VT. This determines how much light your windows will reflect into your home.

If you’re the type who always thinks your home is too bright, then you’ll appreciate windows with a lower VT rating. These will not pass as much light inside.

It’s almost like having tinted windows or wearing sunglasses. Your house is nowhere near as bright as it usually is.

VT windows are rated at 0.20 on the lower end and 0.80 on the higher end. Windows that are 0.20 and up are going to be much brighter and allow for a lot more light.

Air Leakage or AL

Next, there’s the Air Leakage or AL rating. As you can imagine, this is a category that determines how much air your windows will leak. AL focuses on air exiting the window’s joints specifically.

Per minute, AL calculates how much air goes through a small portion of your window, just a square foot. The air is calculated in cubic feet.

This is one of those categories where you want a lower rating. If your AL is 0.3 cf·m/ft² or lower, you’re good. Once it starts going over 0.3 cf·m/ft², your windows are probably letting a lot of air in.

This is problematic. We alluded to this issue in the intro, but when your windows allow in cold air, you have to crank the heater up. In the summertime, as hot air leaks in, you’re blasting your AC. This leads to your energy bills skyrocketing.
If you’re buying new replacement windows, they are mandated to be at least 0.3 cf·m/ft². Energy Star windows must meet certain certifications to achieve that AL rating.

Solar Heat Gain Coefficient or SHGC

Finally, there’s the Solar Heat Gain Coefficient or SHGC. This one is a little more complicated. The SHGC is used to determine the window’s efficiency at keeping out natural heat from the sun. It does this by tracking solar energy in small portions.

You want a lower SHGC, as this means your windows will pass solar heat through the house in smaller quantities.

The SHGC may be as little as 0 and as high as 1. Most commonly, windows will be rated at 0.25 through 0.80. If you’re buying replacement windows with Energy Star certifications, there will be strict criteria on SHGC.



When it comes to buying replacement windows, you want these to be as energy-efficient as possible.

You also want them to adequately insulate your home. If so, then it’s time to find your NRFC sticker and look at the U-Factor.

The U-Factor tells you how adequate your windows are at insulation. It’s expressed in Btu/h·ft²·°F and the range is 0.25 to 1.25.

The higher your U-Factor rating, the better, as that means your windows are well-insulated.

It’s better to know the U-Factor ahead of replacement window installation since you can ensure you get the most energy-efficient windows.

You should also familiarize yourself with the Condensation Resistance, Visible Transmittance or VT, Air Leakage or AL, and Solar Heat Gain Coefficient or SHGC ratings of your windows.

This way, you can be certain your new windows aren’t just pretty, but efficient, too.

What inspired you to replace your windows? Perhaps it was because they were old and drafty.

Perhaps, you got beyond tired of always having to crank up the heat or AC and pay those high energy bills every month. Maybe the windows were outdated and you wanted to freshen them up with new ones. When you got your replacement windows, did you bother to look at the NRFC sticker?

If you did, then you would have seen the U-Factor rating of your replacement windows. Maybe this is something you just glossed over, however, it does have significance.