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Solar Heat Gain Coefficient (SHGC)

An In-Depth Look at Choosing the Most Energy Efficient Replacement Windows

In this article, Window Joe guides you to the factors to consider then determining the best SHGC for your window replacement project. Be sure to read the article on u-factor as well, as it is closely related to energy efficiency in windows.  

Steps to Choosing the Best SHGC

  1. Determine your climate zone
  2. Find out how much sun each side of your home gets
  3. Review how much shade each side gets
  4. Count the number of windows in each room
  5. Review window frame material options
  6. Choose glass coatings and glazing
  7. Refer to the table with recommended SHGC and U Factor combinations

What is a Good SHGC for Energy Efficiency?

The answer to this question depends on where you live.

If your climate zone experiences many cold days, the sun's warmth, also referred to as the sun's radiation, is welcome because it allows the heat from the sun into your home. Conversely, if your are trying to keep your home cool in a hot climate, adding the intensity of the sun’s warmth could create a suffocating situation, particularly if the energy is trapped inside by an incorrect u factor

When too much heat is let in by an inappropriate SHGC and too much heat is trapped in by an incorrect u factor, you have what is commonly known as the greenhouse effect. This is a particular concern in hot, tropical climates such as southern Florida, Georgia, Texas and other states in that experience extreme heat.

The challenge in selecting the appropriate SHGC is balancing out the ideal amount of incoming solar radiation in cold months (to heat your home from the sun's warmth and to lower heating bills), versus keeping solar heat out in hotter months of the year in order to save on air conditioning costs. 

If you live in a climate where it is hot and sunny most of the year, then you may want to block more of the sun’s heat with an SHGC closer to 1.  If you live in a hotter climate zone, an SHGC closer to 0 to block the heat is more appropriate. You can maybe start to imagine how you might have different window needs if you are in Texas or Florida as compared with Rhode Island or Massachusetts.

Calculating the SHGC for Your Home

While you may like having free heat in the winter, it might be too much of a good thing in the summer months if you live in a climate zone with extreme seasons. Beyond climate region, there are some other factors in place that impact how SHGC will affect your life. 

The other two factors related to Solar Heat Gain Coefficient when choosing replacement windows are:

  1. The direction the windows face in relation to the sun
  2. The amount of shade your home gets each day

Window Joe went to look at a home recently that required different solar heat gain coefficients based on the two factors above.  The home is a three story house, with a basement comprising the bottom floor. The basement windows look out only on the east and south sides of the home where they barely see the light of day. Yet, the living room and the sun room, get a little bit more but welcome heat, while the bedrooms on the top floor facing west get a wallop in the afternoon.

In this case:

  1. The basement windows got a higher SHGC to allow in warmth
  2. The living room got a middle of the road SHGC
  3. The sunroom got a higher SHGC because of the sheer number of windows it has
  4. The westward facing bedroom windows got an SHGC closer to 1 to prevent it from heating up in the afternoon hours

While it isn't absolutely necessary to be so exacting when choosing new replacement windows, every little bit of energy efficiency helps when planning your window replacement project.

Additionally, the homeowner may choose to augment solar heat coming in the windows with exterior awnings or interior window blinds or curtains.  

Overall, it is best to take your time, build your knowledge, and scrutinize each portion of the process and product when choosing new replacement windows.  The more you learn about energy efficiency in windows, the more you will be able to evaluate a quality window replacement installer.

The Best SHGC for Each Climate Zone

The reason Energy STAR, from the U.S. Department of Energy, designates four climate zones is to help make it easier for homeowners to know which new house windows will work in their climate. The Energy STAR program provides window buying guidance on the most energy efficient windows for buyers based upon their regions: 

Some areas of the country are plagued with a hot, dripping summer as well as a long, cold, we winter. For some locations, the heat is more pronounced and longer in duration than the short months of frigid temperatures with little snow. Window Joe says to always keep it simple and follow NFRC and Energy STAR ratings that are suggested for your specific county.

Energy STAR and National Fenestration Rating Council 

The National Fenestration Rating Council(NFRC) is an independent organization that tests windows and doors for the Energy STAR program. NFRC is so well-trusted that its testing results inform Energy STAR ratings. 

All you need to do is look for the ratings that indicate a good fit for your climate. Fortunately, retailers will tailor their stock to homes in your area. 

Search for Your U Factor and SHGC

Use our Energy Star climate zone tool to look up the specific combinations of u factor and solar heat gain coefficient for the county you live in. Simply choose your state and then your county below and we will display the combinations of u factor and SHGC as recommended by Energy STAR and the National Fenestration Rating Council.

Energy STAR Northern Climate Zone

The Northern Energy STAR Climate Zone recently underwent changes it its criteria of what constitutes a window that insulates well.  This is covered in the U-Factor article, but it is worth mentioning because it also impacted the solar heat gain coefficient for the Northern Zone.

If you started shopping prior to January 1, 2016, then you might notice some of the windows you were considering buying no longer carry the Energy STAR sticker. Most manufacturers worked to retrofit products to meet the new and more stringent standards.

Though the original proposed changes to U-Factor and SHGC to Energy STAR would have cut the market share by 50% due to vastly increased costs of manufacturing that would have been passed on to the consumer. Fortunately, Energy STAR pulled back a bit, which means windows could remain a bit more affordable.

To remain as Energy Star products, manufacturers had to either make changes to their effected products or lose the rating altogether. The manufacturers may have improved the materials that made up the frames or the coatings to the glass to meet demands. Depending on how many changes a manufacturer had to make, it could have greatly impacted the retail price of your window replacement installation.  

The other climate zones underwent more stringent Energy Star qualifications that took effect on January 1, 2015.

Factors for Energy STAR Ratings in the Northern Climate Zone

  1. Keeping homes warmer
  2. Blocking out cold temperatures in winter months
  3. Allowing in more of the sun's thermal radiation (heat)

U Factor & SHGC Combinations for the Northern Climate Zone

Energy STAR North Central Climate Zone

The North Central Climate Zone has more moderate seasons as such, the Energy Star Factors are as follows:

Factors for Energy STAR Ratings in the North Central Climate Zone

  1. Keeping homes comfortable in more moderate weather
  2. Finding balance in letting heat in and keeping cold air out

U Factor & SHGC Combinations for the North Central Climate Zone

Energy STAR South Central Climate Zone

The South Central Climate Zone has warmer summers and milder winters. As such, the Energy Star Factors are as follows:Factors for Energy STAR Ratings in the South Central Climate Zone

  1. Keeping more heat out in the summer to lower air conditioning costs
  2. Letting some heat in during winter months

U Factor & SHGC Combinations for the South Central Climate Zone

Energy STAR South Southern Climate Zone

The Southern Climate Zone has hot summers and mild winters. As such, the Energy Star Factors are as follows:

Factors for Energy STAR Ratings in the Southern Climate Zone

  1. Keeping heat out in the spring and summer to lower air conditioning costs
  2. Lowering energy bills by keeping heat out most of the year

U Factor & SHGC Combinations for the Southern Climate Zone

SHGC for the Glass or Window Frame or Both

When windows are rated for heat transference, the information may be for:

  1. The whole window; meaning the window frame and glazing (glass) or:
  2. Center of the glass; meaning it only includes the SHGC for the the glass

In general, the values for the whole window will appear lower than for the center of the glass readings. Remember, the solar heat gain coefficient ranges between 0 and 1, the lower the number the less heat (radiation) is let in, the higher the SHGC, the more heat is let in. Explained differently, the lower the SHGC ratio, the better the new window is at blocking the sun’s heat from coming inside your home, the higher the SHGC ratio, the more heat is let in to your home.

Remember when choosing the coefficient for each window:

If you live in a warm climate, choose windows with an SHGC closer to 0 

In colder climates choose a window with an SHGC closer to 1

When reading an Energy Star or NFRC tag, they report the SHGC on the WHOLE window, including the frame, not just the center point of the glass

SHGC for Replacement Window Frames

Choosing a window frame material that does a fine job of insulating against thermal (heat) transference is one way to reduce the solar heat gain coefficient. 

Traditional metal windows allow heat to transfer along the metal into the home, so if you live in a hot climate region, metal isn't a great choice for your replacement window frame. Metal is an excellent conductor not only of electricity but also of hot and cold air. 

On the flip side that means that cold air would normally go from outdoors to indoors with metal framed windows. 

Fortunately, these days, thermal breaks which are small pieces of plastic, are built into the window frame, to “break” the conductivity, and therefore do a better job of insulating by disrupting the heat transfer through the frame material. 

It is a safe bet to buy any material, so long as the overall replacement window is more energy efficient.

Basically, any window frame material is going to insulate well whether it is wood, vinyl, aluminum clad wood, or fiberglass.  

Vinyl replacement windows, those with a vinyl window frame, are a great economic choice and also provide great energy efficiency for your home, lowering bills.  Vinyl windows are generally made of one continuous piece.  The lack of joints in vinyl windows increases their energy efficiency and reduces their solar heat gain coefficient because there are not any joints for heat to travel through.

Remember, always rely upon the NFRC and Energy STAR ratings to tell you exactly how effective the replacement windows you might buy will work in your home and climate. 

Window Replacement Glazing

There are a couple of ways that glass can maintain the indoor temperatures to keep you comfortable indoors. 

It comes down to three ways windows are improved with the use of glass technology, coatings and gases: 

  1. Coatings bounce heat and light out or inside the home to increase or decrease light and heat that enters the house
  2. Glazing comes in single or multiple layers to keep mother nature outdoors
  3. Gases are added between the layers of glazing to further increase insulation, the gases are heavier than air, making it less likely air will penetrate the glazing.

Review in-depth information on energy efficient glass for your window replacements

In summary, there are many things to consider when choosing the best window replacement options.  Be sure to educate yourself and to work with a professional installer for your project.  The most important factors are budget and energy efficiency.  Other factors like style, finish, hardware, and function can also play a big part in the curb appeal and aesthetics of your home.  Remember, Window Joe and the staff at WindowReplacement.net are always here to answer any questions.