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U-Factor: How Well Does A Window Insulate?

What is U-Factor?

U-Factor and Solar Heat Gain Coefficient (SHGC) are the two most important factors in choosing energy efficient windows. The U-Factor determines how well your windows keep warm air inside. The Solar Heat Gain Coefficient is a measure of how much outside heat is blocked from coming into your home.

If you want to cut energy costs, make keeping the heated or cooled air inside your home a priority. Replacement windows significantly reduce electric and gas bills because they stop unwanted heat exchange through windows and also stop drafts. Alternatively, if you live in a cold climate, they may allow more of the sun’s heat into your home. A full window replacement project can cut utility bills by up to 50%. We recommend using a professional window replacement installer to ensures that you achieve optimal performance from your windows.

Why Is It Called U-Factor?

It seems a bit daunting to a home buyer but we will make this all simple to understand in this article. The entire window assembly’s ability to insulate is called U-Factor. The whole window refers to the frame materials and glazing. Less frequently, the U-Factor may also be referred to as the U-Value. Now we are getting somewhere.

The U-Value or U-Factor measures how much heat a window, including the frame, glass, and insulating materials, allows out of the house, which is also known as its thermal transmittance.

Windows are peculiar compared to other home products. You will usually hear about the R-value when discussing insulation values of materials, which is a measure of thermal resistance, or its insulation ability. There are a variety of insulation values that use catchy first-letter naming conventions.

The Best U-Factor For Your Climate Zone

The U.S. Environmental Protection Agency and the Department of Energy’s Energy STAR program breaks down the variations by different climate zones. Energy STAR has four zones, designated as:

The divisions assure that homeowners can make solid window buying decisions appropriate for their climate. Let’s talk extremes to understand more about U-factor. In states like New York, which are in the Northern climate zone, the sun’s light and the sun’s heat are welcome guests to a household in the cold and darker months of the long, drawn out winter.

Now compare New York to Southwest Arizona, which is in the Southern climate zone, where it could easily hit 110 degrees Fahrenheit often. If you buy windows made for the Northern climate zone and your house is in Arizona, your energy bills will sky rocket. The sky is so big that the sun blazes indoors, bringing in even more of its heat. Air conditioning would have a hard time keeping up.

Energy STAR is driven by the independent testing of window products by the National Fenestration Rating Council, or NFRC. The two most important factors for NFRC and Energy STAR are U-Factor and Solar Heat Gain Coefficient (SHGC). For that reason, the two numbers are often paired up and reported on the labels of most windows.

Recent Updates: Energy STAR Version 6.0

As of January 1st 2015, all but the Northern climate zone were required to increase their energy efficiency in order to stay Energy STAR eligible. Windows for the Northern climate zone were required to meet new standards that are much more stringent. Due to the stricter guidelines, the windows marketed to Energy STAR's Northern climate zone went into effect on January 1, 2016.

Manufacturers had to determine if they could retrofit their Energy-STAR qualified windows to stay on the list of energy efficient products. Manufacturers could add glazing, coatings to the glass, increase insulation of the framing material, such as adding thermal breaks to stamp out conduction of air across the frame. The manufacturers also had to determine whether it was financially feasible to provide products at a cost that most consumers would be willing to pay.

Initially the U.S. Department of Energy (DOE) and the Environmental Protection Agency (EPA), that run the Energy STAR program wanted much stricter guidelines for Energy-STAR-eligible products.

It’s estimated that 50% of products would have fallen out of Energy STAR’s program if the original stricter guidelines had been put into place. Industry-wide concerns about offering cost-effective windows in the Northern division hampered the time frame. The EPA relaxed its original demands to meet the present-day manufacturing and materials limitations. The loosening of these new requirements resulted in windows that are still very energy efficient but also 105 to 50% less expensive than they would have been had manufacturers such as Pella and Andersen had to meet the original Energy Star requirements.

Climate ZoneU-factorSHGC
Northern≤ 0.27Any
North Central≤ 0.30≤ 0.40
South Central≤ 0.30≤ 0.25
Southern≤ 0.40≤ 0.25

The two most important factors in choosing energy efficient windows are U-Factor and Solar Heat Gain Coefficient, or SHGC. The U-Factor determines how well your window keeps warm air inside. SHGC is a measure of how much heat is blocked from the sun.

How Do Manufacturers Decrease The 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.

An animated demonstration image of how low u-factor replacement windows keeps heat inside a home.

The low-emissivity coatings, depending upon how they are used, will either bounce heat back into or back out of the house. Thermal breaks are pieces of plastic that prevent conduction of air across metal frames, which makes for a more beneficial U-Factor.

Double-paned windows may have a U-Factor at or below 0.30. Triple glazed windows may have a U-Factor as low as 0.15. The U-Factor will approach the very low 0.15 point if there is a gas, such as Argon or Krypton, between the layers of glass (glazing). Gases are heavier than air and therefore provide insulation resistance to air. That means the gases effectively block the air exchange between outside and inside.

An animated demonstration image of how low u-factor replacement windows keeps heat inside a home.

Manufacturing and an attention to detail in the construction of a window makes a great deal of difference in the U-Factor as well. For instance, there are spacers between the glazing and the frame material. When well executed with high quality materials and craftsmanship, the spacers create a lower U-Factor, which means the window is a energy efficient barrier to exchange of air between the outside and interior of the home.

The Quick Guide: Selecting the Correct U-Factor For Your Climate Zone:

When shopping for replacement windows, look for the Energy STAR label. Review the NFRC label to learn how energy efficient the product will be in your home. Look at the table above to determine the U-Factor and corresponding Solar Heat Gain Coefficient that will work most effectively where you live.

The U-Factor is important regardless of the climate. Though, it is most important for homes in places where there is a great deal of reliance on heating the home. This includes mostly the Northern climate. It is also important in places where it gets cold, such as the North Central and South Central. Though, the U-Factor is important in places that rely predominantly on air conditioning, because if it is unfavorable the home will lock in too much heat, which means the air conditioning would have to run continuously to maintain cooler temperatures.

Northern Climate Zone U-Factor Options

For instance, homeowners in the Northern Climate Zone would use the following correlation between insulation properties of the window, called U-Factor, and the amount of the sun’s heat that is allowed in through the windows, called the Solar Heat Gain Coefficient, or SHGC.

The recommendation for the Northern region to meet the standards for Energy STAR is less than or equal to 0.27. By comparison a skylight in this region is only expected to have a U-Factor of 0.55. It is a lot higher than for windows, because skylights take in more direct heat and light than a window. Skylights are in direct contact with the downpouring of the sun at its height, which means it is more challenging to keep heat inside.

Low U-Factor Windows Should Be Used in Cold Climates with Mild Summers

Always read the Energy STAR Climate Zone Map and follow it. The Northern climate does not include households that require great reliance on a mixture of heating and cooling. The energy efficiency in such places focuses on keeping heat inside. Homeowners in Maine could safely go with the lowest U-Factor they can afford, while also going with the maximum solar heat gain.

North Central And South Central Climate Zone U-Factor Options

The Solar Heat Gain coefficient can range significantly from just 0.32 to 0.42 for the Northern Climate Zone. It depends on if you are living in a place that is plagued with a cold winter, such as in Idaho and a hot and humid summer felt on the coast of Rhode Island. While the extra sunlight and solar radiation is appreciated in the winter, it could make a house uncomfortable in the grips of a sweltering summer temperatures.

The North Central and South Central Climate Zones have the difficult balancing act of having to rely upon heating and cooling units. It means a portion of the year allowing the sun’s heat into the home is beneficial, and another part of the year is unwelcome. North Central Climate Zone relies more on heat, and the South Central Climate Zones, such as in Alabama, has a longer summer, and may rely more on air conditioning.

Both of these "mixed" regions need to look for a U-Factor of 0.30 or less. If you want a warmer home, go with a lower U-Factor.

If you prefer a cooler home, combine a higher U-Factor with a lower Solar Heat Gain Coefficient. Use the table up above to find out the minimum standards you need to purchase in your Energy STAR Climate Zone to receive tax credits and achieve maximum energy efficiency.

The Energy STAR 6.0 updates went into effect for all regions but the Northern on January 1, 2015. The other Climate Zone changes went into effect on time because the changes were not drastic for manufacturers to meet the expectations set for by Energy STAR 6.0.

Southern Climate Zone U-Factor Options

High U-Factor Windows Should Be Used in Hot Climates with Hot and Humid Summers

It’s good to focus on getting replacement windows with a Solar Heat Gain Coefficient that is less than 0.25. This blocks more of the sun’s heat. Also, focus on buying windows with a U-Factor that is as high as possible, such as 0.40, to keep cooler indoors.

U-Factor in Skylights

Skylights increase natural light and solar heat allowed into the home. The size of the skylight impacts both temperature and light inside the home. For these reasons, the rule of thumb is minimize the skylight size to only five percent of the floor space. If there is little other natural light from windows, then 15 percent is a good ratio.

Roof DirectionLightSun's HeatTime of DayFeatures

South (Winter)DesirableDesirable
Shades or Glazing
South (Summer)MaxHot
Shades or Glazing

Skylights are a popular option to add more daylight and ventilation. Recent developments include sun tracking, reflectors to allow in light while keeping out excess daylight heat gain or nighttime heat loss.