If you’re in the market for new windows, you’ve likely seen terms like U-value, U-factor, and SHGC on the specs sheet.
Do you pick the units with the highest rating? Or is lower better?
Read on to learn all about the performance indicators and find the best ratings for the climate in Illinois.
What Is U-Factor in Windows?
U-factor describes the rate at which your windows transmit non-solar heat flow. In other words, it’s the amount of heat that can escape from inside your home through the window.
Note that the U-factor is sometimes called the U-value.
Numerical Value
For windows, you’ll typically see the rating ranging between 0.2 and 0.1.
Higher values indicate that your home loses lots of heat. So, the lower the number, the higher the insulation level. In fact, the term “superwindow” is typically given to windows with U-factor ratings below 0.15.
Side Note: U-factor is expressed in units of Btu/hr-sf-°F. There’s a way to convert from imperial to metric units, but you likely won’t need to since the rating on the NFRC label is provided in the US/imperial system.
NFRC Label
In some cases, the U-factor rating is used to refer to the glass and its glazing alone.
However, the rating given by the National Fenestration Rating Council (NFRC) is more comprehensive. It expresses the overall performance of the window assembly. The council takes into account the glazing, frame, and spacer material as well.
Just note that the centers of energy-efficient windows may have a lower U-factor rating than the unit’s overall rating.
You’ll find the U-factor rating in the NFRC label’s “Energy Performance Ratings” section.
What Is SHGC in Windows?
SHGC stands for solar heat gain coefficient, and it represents the percentage of radiation that can enter your home through a glazing assembly.
This heat could be gained in one of two ways. The first is a simple transmission directly through the window. The other is with the heat being absorbed by the window materials and then released into the home.
For a while, the shading coefficient (SC) was the standard indicator of shading ability. It was measured relative to the ability of a ⅛-inch clear, double-strength glass pane to transmit solar heat inwards.
However, the SHGC has replaced the SC and is now the modern standard. If, for some reason, you want to calculate the SC, all you need to do is multiply the SHGC by 1.15.
Numerical Value
SHGC is dimensionless, so you don’t have to worry about the units here.
The rating range starts at 0 and goes up to 1. Most windows range between 0.2 and 0.9, with lower values indicating less heat gain (thanks to higher resistance).
NFRC Label
The SHGC value is available on the NFRC label, right next to the U-factor rating.
And much like the U-factor, the SHGC can represent the glass alone or the entire window assembly. That being said, when people use SHGC, they’re mostly referring to the unit as a whole, including the glass and the frame.
The older indicator (the SC), however, was based only on the glass part of the assembly.
U-Factor vs. SHGC
Let’s check the two indicators in more detail.
Climate Considerations
We’ve explained what U-factor and SHGC mean for a window unit. But the short version is this:
- U-factor is an indicator of the window’s ability to let indoor heat escape outwards.
- SHGC is an indicator of the window’s ability to let the sun’s heat inside.
With this in mind, you can see how climate and heating/cooling demands should influence people’s assembly choices.
In warm areas where cooling is the priority, low-SHGC windows would be particularly helpful. This way, you’re blocking a bigger fraction of the solar radiation. But windows with higher SHGC and low U-factors work well in cold climates.
The Department of Energy’s Energy Star Program uses four zones for the different climates in the US.
We’ll take a closer look at where Illinois falls and what this classification means in terms of energy-efficiency recommendations in a minute. But for now, you can take a quick look at the general heating/cooling demands for each zone.
| Climate Zone | Heating/Cooling Patterns |
| Northern | Heating-dominated |
| North-Central | Heating and cooling |
| South-Central | Heating and cooling |
| Southern | Cooling-dominated |
Cost Comparison
It’s hard to compare how U-factor and SHGC affect a unit’s price.
However, it’s fair to say that the more insulating a window is, the more it’s going to cost. Triple-pane windows, for one, can cost 10–15% more than the double-paned counterparts.
While energy-efficient windows do pay for themselves over time through energy bill savings, you still don’t want to pay for more insulation than you really need. You’ll find tips on finding the ideal rating for your county later in the article.
Project Type
Window installers have to consider all aspects of the project before recommending a U-factor and an SHGC rating. Everything from size to orientation matters.
Let’s say someone lives in an area with a cold climate. The ideal choice for their south-facing and east-facing windows won’t be the same at all.
For the south-facing windows, the goal is to pick the highest SHGC possible. East-facing ones, however, need low SHGC.
The household’s energy plan matters as well. Windows that let plenty of the sun’s energy in (high SHGC) can be better for passive solar home designs, for example.
Materials, Durability, & Lifespan
Any alteration or upgrade that affects the window’s resistance to heat transmission will affect its U-value and SHGC.
For instance, a single-glazing, uncoated window may have a relatively high SHGC of around 0.7. Adding a second glazing will boost the resistance and lower the transmission value to around 0.6.
Manufacturers incorporate low-emission coatings and gas fillings to improve the insulation ability even more.
| Material | Argon Gas Fill | Frame | U-Factor | SHGC |
| Double glazing, clear glass | No | Aluminum (with thermal break) | 0.63 | 0.62 |
| Double glazing, low-E coating (low solar gain) | Yes | Aluminium (with thermal break) | 0.47 | 0.33 |
| Double glazing, clear glass | No | Vinyl/wood | 0.49 | 0.56 |
| Double glazing, low-E coating (low solar gain) | Yes | Vinyl/wood | 0.34 | 0.3 |
| Triple glazing, low-E coating | Yes | Insulated vinyl | 0.18 | 0.4 |
According to InterNACHI, windows with aluminum frames, in general, are expected to last 15–20 years.
Vinyl and fiberglass alternatives tend to be more durable, with a lifespan of around 20–40 years. Meanwhile, wood frames can last 30+ years.
Side Note: KobyCo offers a one-year labor warranty that covers installation errors.
The Illinois Climate Challenge
In Illinois, the climate is typically continental, which translates to frequent but short temperature fluctuations.
During early summer, the solar energy is 3–4 times greater than that of early winter. This, along with the state’s inland location, creates warm summers and cold winters.
The average winter high in the north of the state is usually in the 30s, with lows dropping to the 10s. In the south, however, the winter highs are in the mid-40s, with the lows in the 20s.
The summer temperature is often between the 60s and 80s across the state. But it’s worth noting that the south gets 40+ days above 90°F, compared to a mere 10 days for the north.
Naturally, the north sees harsher winters, with about 140 days at 32°F or lower, compared to 80 such days in the south.
Why Window Ratings Matter for Illinois Homes
There are a lot of ways your home can gain/lose heat, and your window unit is a major one. After all, windows alone account for 25–30% of all heating/cooling energy use in residential properties.
Cold air can leak through the window. Plus, heat radiation can be transferred through the glass, multi-layer glazing, or even the frame itself.
Finding the right window rating will help you insulate your home against winter heat and minimize summer heat gain.
Choosing the Right Ratings for Illinois
The state combines two zones on the Energy Star climate zone map: Northern and North-Central.
As you can see, there are areas in Illinois with a heating-dominated climate, while others feature a mixed climate.
For reference, Springfield pretty much falls on the line between the two climate zones. Any city south of Springfield falls in the North-Central classification.
With that in mind, we can take a closer look at the U-factor and SHGC recommendations.
U-Factor Rating Recommendations
Homeowners in parts of the state that fall into the Northern Zone should aim for a U-factor of 0.22 or lower.
For areas that fall in the North-Central zone, the recommended U-factor window rating is 0.25 or lower.
SHGC Rating Recommendations
If your home falls in the North-Central zone, you should aim for an SHGC that doesn’t exceed 0.4. Otherwise, you’ll let in too much solar heat and risk raising your cooling costs.
In the Northern zone, however, the prescriptive SHGC is 0.17 or higher.
Balancing Both the U-Factor & SHGC
There’s a bit of flexibility for those who fall in the Northern zone. The Energy Star Program allows for two routes to compliance.
If you can’t meet the recommended U-factor in the prescriptive path, you can opt for the equivalent energy performance route. To do that, however, you’ll have to balance the scales with higher SHGC window values.
Suppose the U-factor is 0.23 or 0.24. In this case, you need an SHGC rating of 0.35 or higher. This way, you let in more of the sun’s heat to compensate for the heat loss due to the difference in the U-factor.
Similarly, a U-factor of 0.25 and 0.26 is okay if they’re matched with SHGCs of 0.4 or higher.
Benefits of Energy-Efficient Windows
Here’s why energy-efficient windows are well worth the investment:
- Lower Energy Bills. Controlling the household’s heat loss/gain through windows can translate to 7–15% annual savings on energy bills.
- More Comfort. The right windows will keep you cozy year-round.
- Reduced Condensation. Energy-efficient units can minimize the difference between the inside glass surface temperature and the room air temperature.
FAQs
Is U-Factor or SHGC more important?
None is inherently more important than the other. It all depends on your location and energy usage patterns.
The larger your heating bill is, the more you need low U-factor windows. Low SHGC rating for windows, however, is often more important for homeowners in warmer climates.
What is a good U-Factor for windows?
Good insulation and resistance to heat flow automatically mean lower values. A high-performance, double-pane window, for instance, might have a U-factor of 0.3 or lower.
How do I know which rating is right for my home?
You can use the NFRC Consumer Guide’s window selection tool to find the best U-factor and SHGC ratings for your home.
Just type your ZIP code. The tool will identify your climate, and you can check the Energy Star recommendations for that zone.
Alternatively, you could use Energy Star’s climate zone finder to check the recommended criteria and the ratings required for tax credits in your county.