Solar Heat Gain Coefficient (SHGC): What the Number on Your Window Label Means
You probably have a room like this. The back bedroom bakes every summer afternoon, while the den stays chilly even on a bright winter day. A lot of that comes down to one number on the window label: the solar heat gain coefficient shgc. Below, you'll get a simple explanation of the term. After that, we'll walk through how SHGC compares with U-factor and visible transmittance, which ENERGY STAR targets fit each US climate zone, and why the best number can change from the west wall to the south wall. Replacing windows soon? We cover that day, too.
What Is Solar Heat Gain Coefficient?
Think of the solar heat gain coefficient as a share. It tells you what portion of the sunshine striking a window turns into warmth in the room behind it. Ratings run from 0 up to 1, and a smaller rating keeps more of that warmth outside.
So what is solar heat gain, exactly? It's the extra warmth a room picks up when sunlight shines in through the glass. The Department of Energy's Building America window guide says most windows land somewhere from 0.15 to 0.80 on that 0 to 1 scale.
What the number actually counts
If you've ever typed "what is solar heat gain coefficient in windows" into a search bar, there's one more detail worth knowing, and it comes from a glass maker. Cardinal Glass's SHGC glossary entry explains that the number counts solar energy that passes straight through, plus energy the glass absorbs and then releases indoors. Don't mix it up with solar power, either. The rating describes warmth passing through a pane, and rooftop panels play no part in it.
SHGC vs U-factor vs visible transmittance
Look at a new window and you'll usually find an NFRC sticker listing several ratings. Because they're printed together, people often confuse them. Here's what each one tells you, with typical ranges from the DOE Building America Solution Center (BASC).
Rating on the NFRC label | What it tells you | Better is... | Typical range (DOE BASC) |
Solar Heat Gain Coefficient (SHGC) | How much solar heat gets through | Lower in hot climates; higher can help in cold ones | 0.15 to 0.80 |
U-factor | How fast heat escapes through the whole window | Lower | 0.15 to 1.1 Btu/h·ft²·°F |
Visible Transmittance (VT) | How much visible light gets through | Higher for more daylight | 0.20 to 0.80 |
Air Leakage (AL) | Air that passes through the unit | Lower; most codes and standards require 0.3 cfm per sq ft or less | N/A |
Condensation Resistance | How well it resists water forming on the inside surface | Higher (scale of 0 to 100) | N/A |
When you compare the window solar heat gain coefficient with U-factor, think of it as heat coming in versus heat leaking out. BASC also notes that a U-factor of 0.20 or lower likely points to a triple-pane window. You'll often see the NFRC label next to an ENERGY STAR label, which shows the US regions where that window qualifies.
Windows are only one piece of a tight, comfortable house. A home energy audit shows where heat really gets in and out, so you can tell whether the glass is even the main problem.
Once the label makes sense, the next step is picking a target for where you live. That target depends on your climate zone.
What SHGC Do You Need? ENERGY STAR Targets by Climate Zone
ENERGY STAR sets window limits for four US climate zones. In hot zones, it caps SHGC to keep summer sun out. In the coldest zone, it sets a minimum instead, because some winter sun helps heat the house.
ENERGY STAR Version 7.0 window targets
The table below covers ENERGY STAR Version 7.0, which took effect on October 23, 2023, according to EPA's Version 7.0 letter. The zone values match Cardinal Glass's Version 7.0 guidance.
ENERGY STAR zone (Version 7.0, effective Oct 23, 2023) | IECC climate zones (DOE BASC mapping) | Window U-factor | Window SHGC |
Northern (prescriptive) | CZ 5 to 8 | 0.22 or lower | 0.17 or higher |
Northern (equivalent performance options) | CZ 5 to 8 | 0.23 or 0.24 | 0.35 or higher |
Northern (equivalent performance options) | CZ 5 to 8 | 0.25 or 0.26 | 0.40 or higher |
North-Central | CZ 4 | 0.25 or lower | 0.40 or lower |
South-Central | CZ 3 | 0.28 or lower | 0.23 or lower |
Southern | CZ 1 to 2 | 0.32 or lower | 0.23 or lower |
How the Northern trade-off works
The Northern rows hold a simple trade-off. A window can have a slightly higher U-factor if it lets in more winter sun to make up the difference. That's why the solar heat gain coefficient for windows in Minnesota looks so different from the target in Florida.
Finding your climate zone
Don't guess your zone from your state, since many states span more than one. The ENERGY STAR climate zone finder lets you pick your state and county.
Matching glass to climate is one of the core ideas in passive house design for homes, where orientation and glass are chosen together. If the label on your current windows misses your zone's target, replacement may be on the table. And replacement day takes some planning of its own.
Replacing Windows for a Better SHGC: Planning the Project Day
New windows usually go in one opening at a time, and the crew may need to work around outlets, alarm sensors, or wiring near the frames. Planning the day keeps the house usable while openings are covered. A few checks before the crew arrives prevent most surprises.
Before the crew shows up
Start with the paperwork, since mistakes are cheaper to catch on a quote than on a wall. Before install day, run through these points with your installer:
Confirm that the NFRC values on the quote (SHGC, U-factor, and VT) match your zone and each wall's direction.
Ask whether the crew follows the maker's installation instructions. BASC notes that some warranties require an installer certified by the manufacturer.
Ask how the rough openings will be flashed and air sealed, which BASC lists as key steps.
If your home was built before 1978, ask for proof of lead-safe certification. EPA's lead-safe renovation rule requires anyone paid to disturb painted surfaces in those homes to be certified. EPA says about three-quarters of US homes built before 1978 still contain some lead-based paint, so this check matters for older houses.
Keeping lights, phones and Wi-Fi on if a circuit has to be off
During the install, your electrician may cut power to a room so they can safely work near an outlet or an alarm wire. That's a problem if you're working from home that day. You still want light, a charged phone, and internet for your calls.
The load here is small. Say your Wi-Fi router draws 24W (an example value; your router's adapter label lists its rating), and an LED work light draws 40W (also an example; check the light's label). That's 64W in total. A 1,024Wh battery at an assumed 85% efficiency gives 1,024 × 0.85 ÷ 64, or about 13.6 hours.
That math covers lighting and communication only, not saws, heaters, or other tools. For that kind of short, low-power job, one compact power station is enough. If that sounds like your project day, the EcoFlow DELTA 3 Classic is sized for it.
To be clear about its role, the power station doesn't change a window's SHGC, improve its insulation, or turn solar heat into electricity. Check your real loads and hours first, and skip sizing up to a bigger unit just to have one.
Phones and laptops are easy to plan for, but internet is the piece most people forget. The right battery backup for your router keeps work calls going.
Window Direction and SHGC: South, West, East and North
A west window takes the hot late-afternoon sun, while a north window gets very little direct sun at all. So each wall can get its own rating. Overhangs, awnings, and shades change things as well.
Window faces | What it deals with | SHGC and shading approach |
West | Late-afternoon sun and glare (DOE BASC) | Lower SHGC, especially in hot climates; DOE BASC suggests limiting west windows |
South | Strong sun that changes angle by season | Overhangs, awnings, or pergolas (DOE BASC); in cold zones, a higher SHGC may help |
East | Morning sun | Usually treated like west in hot climates |
North | Little direct sun | SHGC matters less; focus on U-factor |
Choosing glass for each side of the house
Keep in mind that ENERGY STAR ratings apply to a whole climate zone, not to one side of your house. You can ask your installer about glass chosen for each orientation. Where west-facing glass can't be avoided, BASC also suggests adjustable shades, inside or out, so you can block the glare during the hours it hits. A west-facing kitchen in Phoenix and a north-facing bedroom in the same house may not need the same solar gain coefficient.
For hot west-facing glass, some homeowners weigh window film versus full replacement before buying new units. Film can be a smaller first step, while replacement changes the whole unit.
SHGC Mistakes That Lead to the Wrong Window
SHGC is easy to read and easy to misread. Most mistakes come from treating "lower" as always better or mixing SHGC up with other ratings. Here are the ones to watch for:
"Lower SHGC is always better." Not in the Northern zone, where ENERGY STAR sets a minimum SHGC.
"SHGC and VT are the same thing." VT tracks daylight, while SHGC tracks warmth. A pair of windows can look equally bright yet pass very different amounts of heat.
"Dark tint means low SHGC." Don't judge by color. Check the label instead.
"SHGC has to do with solar panels." It doesn't. The rating only covers sunlight passing through a pane.
"U-factor covers everything." U-factor is heat loss through the window, while SHGC is heat gain from the sun. You need both.
Comparing quotes from different brands
Still stuck on "What is SHGC?" when you read a quote? Go back to the label and the zone table above. The SHGC (solar heat gain coefficient) means the same thing on every NFRC label, so you can compare brands side by side.
Window upgrades often happen alongside other projects. With a little forethought, eco-friendly home renovations can be planned so each one supports the next.
Conclusion
The solar heat gain coefficient tells you how much of the sun's warmth a window lets into your home, from 0 (none) to 1 (all of it). Hot zones want low numbers, the Northern zone sets a floor, and window direction and shading fine-tune the choice from there. Before you sign anything, find your ENERGY STAR zone, then read the SHGC, U-factor, and VT on the NFRC label of every window you're quoted. And if a room will be without power for a few hours on install day, an EcoFlow DELTA 3 Classic can keep the lights and Wi-Fi on while the crew works.
FAQs
What is a good solar heat gain coefficient?
There's no single best number, because it depends on where you live. Under ENERGY STAR Version 7.0, windows in the Southern and South-Central zones need 0.23 or lower, North-Central windows need 0.40 or lower, and Northern windows need at least 0.17.
Is a lower SHGC better?
In hot climates, it usually is, since less solar heat means less work for your air conditioner. In cold climates, a higher SHGC can let in useful winter sun, which is why ENERGY STAR sets a minimum for the Northern zone.
What's the difference between SHGC and U-factor?
SHGC tells you how much heat a window picks up from sunlight. U-factor tells you how fast heat escapes through the window, and a lower U-factor is better in every climate.
Where do I find a window's SHGC?
Look at the NFRC label on the window or ask for it on your quote. The label lists U-factor, SHGC, and visible transmittance, and it often shows air leakage and condensation resistance as well.
Does SHGC affect solar panels?
No. SHGC only describes how much of the sun's heat passes through window glass. It has no effect on solar panels or how they generate electricity.
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