U-Factor Value On Replacement Windows Explained (2023)

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? U-Factor Value On Replacement Windows Explained (1)

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.

(Video) What is U-Factor for Windows?

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.

(Video) What is a Window's U-Factor Value?

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?

U-Factor Value On Replacement Windows Explained (2)

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.

(Video) Window Ratings: U factor and SHGC

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.

(Video) Energy Star map with different performance zones for U factors on windows and doors.

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.


Is U-factor .32 good? ›

U-Factors in Different Climates

In areas where air-conditioning needs are minimal, windows that allow for solar heat gain during the day (a solar heat-gain coefficient of 0.40 or higher) can be considered energy-efficient with a U-factor as high as 0.32.

Is .30 a good U-factor for windows? ›

In a cold climate a good U-factor for a window is between 0.17 and 0.39. (That's between R-6 and R-2.5). Lower is better with U-factor--the opposite of R-value, when higher is better. The low end of that range is only achievable with higher-quality triple-glazed windows--windows with three layers of glass.

What is a good U-value for windows? ›

Generally, the lower the U-Value of a window the better. Low U-Values indicate that a window will perform well in maintaining the indoor temperature of a customer's home. Recently building regulations have changed, meaning that new windows must have a u-Value below 1.6 W/m2k.

How much of a difference does U-factor make windows? ›

The lower the U-factor, the better the window is at preventing heat loss. The NRFC range is between 0.20-1.20. For climates where cold weather is a factor, installing window units with a U-factor between 0.20 and 0.40 is a major benefit when it comes to improving the energy efficiency of your home.

Is 0.29 a good U factor? ›

U-Factor values generally range from 0.25 to 1.25 and are measured in Btu/h·ft²·°F. Ideally, Florida homeowners should aim to have a U-factor under 0.30 paired with a solar heat gain coefficient (SHGC) under 0.30.

What is an acceptable U-value? ›

Under current requirements, an external wall U-Value cannot be higher than 0.30. When the regulations change, this limit is being reduced to 0.26. So, as long as the wall construction achieves 0.26 or lower, the wall will meet the heat loss requirements of the new regulations.

What is the U-factor of Pella windows? ›

The NFRC website states U-factor ratings for fenestration products generally fall between 0.15 and 1.20.

Does glass thickness affect U-value? ›

Thicker glass has much better U value than thin glass, laminated glass U value is significantly lower than that of the same thickness glass. For clear float glass, the increase of glass thickness does not increase the insulation performance.

What is the U-factor on Renewal by Andersen windows? ›

It has a U-factor of 0.24, promising the best heat insulation among our glass packages. In addition, it blocks the highest percentage of UV light at 95%.

Is it better to have a low or high U-value? ›

U-values can tell you how well an insulated glass unit (IGU) will hold in heated or cooled air. The lower the number, the better the insulating performance. U-values generally range from 0.1 (very little heat loss) to 1.0 (high heat loss).

Is the higher the U-value the better? ›

A U-value value shows, in units of W/m²K, the ability of an element to transmit heat from a warm space to a cold space in a building, and vice versa. The lower the U-value, the better insulated the building element.

Is 0.18 U-value good? ›

The best insulating materials have a U-Value of close to zero; the closer to zero the better. Under LABC guidelines, the retrofitting of insulation to existing buildings requires the following U-Value targets: Wall – 0.3 W/m2k. Roof – 0.18 W/m2k.

Is 1.2 A good U-value for windows? ›

', according to Part L1A of current Building Regulations a domestic window installation should achieve a U-value no higher than 1.4 W/m²K*. Typical u values for windows that are double glazed record a U-value of between 1.2 W/m²K and 3.7 W/m²K, while triple glazed windows are usually rated below 1 W/m²K.

What are the most energy efficient replacement windows? ›

The most efficient window will have a fiberglass frame — which is the most insulative frame material — triple-pane glass, a Low-E coating and window tinting. Other energy-efficient options include vinyl and composite window frames and double-pane glass.

How much difference does U factor make? ›

The general rule is that one-hundredth of a point in U factor equals a 3% change in performance.

Is 0.28 a good U-value? ›

The lower the U-value, the more thermally efficient the wall construction is, so a U value of 0.28 is much better than one of 1.6 .

Is 0.3 A good U-value? ›

Building Regulations (opens in new tab) require a wall of no worse than 0.3, roof 0.16, and ground floor 0.22. All of these U values are written as W/m2K. That is, Watts (of heat) passing through every square metre (m2) for each 1°C temperature difference (between the inside and outside of the building).

What is the U-value of Low E glass? ›

This type of glass is called low-emissivity, or low-e, glass. A double-pane window with a low-e coating has a U-value of about 0.36, which translates to 35 percent less heat gain or loss than conventional double-pane windows.

What is the U value of double glazed windows? ›

The current industry standard double glazed unit is made up of one 4mm pane of “Low E” (Low Emissivity) coated glass*, one pane of 4mm plain annealed glass and a 20mm cavity of argon gas (make up = 4-20-4). This will provide a centre pane u-value of 1.1 W/m²K.

What is U factor on Andersen window? ›

Andersen® products are tested to determine the U-Factor. The U-Factor is a commonly used measurement (ranging from 0-1) of the rate which heat flows through a window or patio door due to differences between indoor and outdoor air temperatures. The lower the U-Factor, the better the insulating value.

What is the U factor for Jeld Wen windows? ›

Jeld-Wen vinyl window U-Factor rankings

The best Jeld-Wen vinyl windows can have a U-Factor as low as 0.18 for triple-pane windows, and double-pane high-performing windows often have a U-Factor of 0.30 or less.

What is the U factor of Marvin windows? ›

The Marvin Ultimate double-hung window has a U-factor ranging from 0.20-0.32. The Marvin Essential double-hung window has a U-factor ranging from 0.25-0.48.

Is Thicker glass better for windows? ›


Thick glass windows with double-strength are far more durable and less likely to break, Thick glass also enhances the structural stability of the window. Most importantly, thicker double-strength glass provides improved sound control.

How thick should double pane windows be? ›

Glass Thickness

The overall measurement, from outside surface to outside surface, is ½-inch — the typical width for double-glazed windows. Triple-glazed windows are generally 1 inch thick and made of three sheets of glass. They are also much more energy-efficient than single or double glazed windows.

What is a good thickness for glass? ›

¼ inch thickness: This is the ideal glass size for tabletops and glass protective covers for tables such as coffee tables and patio tables. While the ½ inch thick glass feels sturdier, you can also feel safe with the ¼ inch thickness.

Is there a difference between Renewal by Andersen and Andersen windows? ›

Andersen Windows manufactures windows and sells them through a network of authorized dealers, whereas Renewal by Andersen is made up of franchises that sell and install one line of home replacement windows that are manufactured by Andersen.

What is the average U-value of a window? ›

To comply with building regulations, windows (like every other element of your property) must meet a certain minimum U-value. In the case of a window, it's 1.6 W/m2K. Double-glazed windows, filled with argon, are typically 1.4 W/m2K, while thicker triple-glazed windows can go as low as 0.7 W/m2K.

How do I increase my window U-value? ›

In order to achieve a further improvement of the U-values of the window frame, a larger insulating zone could be utilized, for example, by wider distances between the aluminum frame shells of the window profile.

What is a good U factor? ›

While the U-Factor can take any value, in general for windows it ranges from 0.20 to 1.20. The lower the U-Factor, the better the window insulates.

Is 1.1 A good U-value? ›

In 2018, Part L building regulations for standard replacement doors and windows specified that a product should have a U-Value no higher than 1.6 for windows, and 1.8 for doors.

What two things does a lower U-value indicate? ›

A unique term for each component

The u-value is measured in W/(m2K). In short: A lower u-value results in less heat energy lost through windows. Lower u-values represent better thermal insulation.

What is the difference between U-value and U factor? ›

U-value (also known as U-factor) is a measure of heat transfer (heat gain or loss through glass), while R-value is a measure of heat resistance. U-value is not a material rating; it is a calculation of the conduction properties of various materials used in the construction of windows, doors and skylights.

Why is the U-value important? ›

A U-Value tells you how thermally efficient a product happens to be, so you need to know what a good U-Value actually is and how you should factor in the U-Value of a home improvement solution before buying one. This will help you identify the level of insulation offered in seconds and make a sound investment.

What does a high U factor mean? ›

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.

What is the U-value of SuperQuilt? ›

SuperQuilt Insulation have a range of methods to achieve a 0.18 U-value. Dependent on the access to your roof would determine the best solution for your project.

Is 1.8 an adequate U-value for a new window? ›

What is the minimum U-Value and Window Energy Rating for Windows? All new or replacement aluminium, PVCu or timber windows require a minimum C Window Energy Rating or a U-Value of 1.6Wm2K or less.

Are C rated windows any good? ›

A 'C' rated window is the WER that is required to meet part L of the building regulations. A 'C' Rated window is still very energy efficient and could save you up to as much as £425 on your house hold bills (according to GGF). A window energy rating 'C' would be far more energy efficient than an 'E' rated window.

Which type of window has the highest heat loss? ›

According to www.efficientwindows.org, single-glazed windows with clear glass allow “the highest transfer of energy (i.e., heat loss or heat gain depending on local climate conditions) while permitting the highest daylight transmission.” No wonder such windows are practically extinct in residential architecture.

Is it better to replace all windows at once? ›

Many professionals agree that if homeowners can afford it, they are better served by replacing all their windows in one order. The installation team can complete the full job in a couple of days. When you do all of the windows at once there is less paperwork, less time, and less stress.

What are the hardest windows to break into? ›

Polycarbonate Unbreakable Glass

A polycarbonate window is much more difficult to break through than a standard glass window. In fact, according to some estimates a polycarbonate window is over 200 times more resistant to impact!

When talking about a windows U-value or U factor which is true? ›

The u-value of windows describe the window's ability to keep heat inside your home. If a window has a low u-factor, it's more energy efficient compared to one with a higher u-factor.

What is the perfect factor of 32? ›

All factors of 32 are 1, 2, 4, 8, 16, and 32.

Is a high U factor good? ›

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.

Is 0.3 A good U-Value? ›

Building Regulations (opens in new tab) require a wall of no worse than 0.3, roof 0.16, and ground floor 0.22. All of these U values are written as W/m2K. That is, Watts (of heat) passing through every square metre (m2) for each 1°C temperature difference (between the inside and outside of the building).

Do you want U-value high or low? ›

U-values measure how effective a material is an insulator. The lower the U-value is, the better the material is as a heat insulator.

Why is 20 not a square number? ›

A square number has a whole number root that produces the square number when multiplied by itself. The number 20 doesn't have a whole number that can be multiplied by itself to equal 20.

Why 33 is not a prime number? ›

No, 33 is not a prime number. The number 33 is divisible by 1, 3, 11, 33. For a number to be classified as a prime number, it should have exactly two factors. Since 33 has more than two factors, i.e. 1, 3, 11, 33, it is not a prime number.

Can you get 6 factors of 32? ›

Answer: The factors of 32 are 1, 2, 4, 8, 16, 32 and of 64 are 1, 2, 4, 8, 16, 32, 64.

How do I know if my windows are good quality? ›

Ask to see cutouts (windows & doors cut in half), look for think walls and multiple chambers in windows. Check the weather-stripping. You want neoprene and multiple layers/applications of it in each window. Stay away from brush type as its inferior in every way.

How do I lower my U-factor? ›

To decrease the U-Factor (a lower U-Factor indicates the windows keep more of the heat inside) , manufacturers have several options. They may add low-emissivity coatings to the window glazing (layers of glass), use multiple layers of glass (glazing), insert gases between the layers of glass, or add thermal breaks.

When talking about a windows U-value or U-factor which is true? ›

The u-value of windows describe the window's ability to keep heat inside your home. If a window has a low u-factor, it's more energy efficient compared to one with a higher u-factor.

How do you achieve 0.15 U-value? ›

How to achieve 0.15 u value in a pitched roof with TLX Silver
  1. Fit a breathable membrane over the rafters.
  2. Install 115mm of PIR board between 175mm deep rafter at 400mm centres.
  3. Fix a batten to create an air gap between the PIR board and TLX Silver.
5 Sept 2022


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