Quick answer
Solar heat gain coefficient (SHGC) is a window rating, from 0 to 1, for the share of the sun’s heat striking a window that ends up inside the house, whether it passes straight through the glass or is absorbed and then released indoors.
A low number blocks more solar heat; a high number lets more in. It is printed on the NFRC label next to U-factor, and energy codes cap it in warm climates.
Quick facts
The key facts about Solar Heat Gain Coefficient (SHGC), with sources:
- Scale
- 0 to 1; low numbers resist solar heat, high numbers allow it 1
- Where printed
- The NFRC label, beside U-factor and visible transmittance 1
- ENERGY STAR 7.0
- ≤0.23 Southern and South-Central, ≤0.40 North-Central, ≥0.17 Northern 5
- Low vs high solar gain (NRCan)
- Below 0.30 is low solar gain; above 0.30 is high 2
- Paired with
- U-factor (heat loss) and visible transmittance (daylight) 1
Key takeaways
- SHGC measures how much of the sun’s heat a window lets in, on a scale of 0 to 1.
- Lower SHGC keeps homes cooler in summer; higher SHGC can help heat a home in winter.
- Hot, sunny climates get a code cap of 0.25; the coldest zones have no cap at all.
- ENERGY STAR’s Northern zone even sets a minimum SHGC, so a window is not too dark to the sun.
- Compare windows only by the NFRC label value, never by a glass brochure number.
SHGC in everyday terms
Stand by a sunny window in July and you feel heat. Some of it is sunlight passing straight through the glass. Some is the glass and frame warming up and giving that heat off into the room. SHGC adds both together and expresses them as a fraction.
The National Fenestration Rating Council (NFRC), which certifies window ratings in the United States, says SHGC measures how much solar heat a product allows in, on a range of 0 to 1, and that "low numbers resist solar heat, high numbers allow solar heat" 1.
Natural Resources Canada defines it as the fraction of incident solar radiation the window admits 2.
A window with an SHGC of 0.25 lets in about a quarter of the solar heat that strikes it. One rated 0.50 lets in about half. Neither is "good" or "bad" by itself. What you want depends on your climate and which way the window faces.
SHGC is one of three numbers on the label that do different jobs. U-factor covers heat flowing through the window from a warm side to a cold side. Visible transmittance covers daylight 1. SHGC covers the sun’s heat.
How solar heat gets through a window
Sunlight carries energy across a wide band: ultraviolet, visible light and near-infrared. A window handles each part in a few ways.
- Direct transmission. Some solar energy passes straight through the glass into the room and warms whatever it lands on.
- Absorption and re-release. Glass and frame absorb part of the energy, warm up, and release some of that heat inward. SHGC counts this share too 2.
- Reflection. Coatings reflect part of the energy back outside before it gets in.
- Coatings change the mix. Low-emissivity (low-E) coatings, thin metal layers on the glass, come in many types. NRCan notes their solar performance varies greatly 2, and one glass maker sells separate coatings for "high solar gain" and for "solar control" 5.
- The frame counts. NFRC rates the whole window, so frame and spacer area lower or raise the label value compared with the glass alone 1.
- TransmittedPasses straight into the room
- Absorbed, released insideCounted in SHGC
- Absorbed, released outsideNot counted
- ReflectedBounced back outdoors by glass and coatings
SHGC is the share that ends up inside: the transmitted part plus the absorbed heat released indoors. Source: [2] [1]
The result is one number for the whole product. Two windows with the same glass can have different SHGC values if their frames differ in size or material.
Low, moderate and high solar gain windows
| Type | Typical SHGC range | Best fit |
|---|---|---|
| Low solar gain | Below 0.30 2 | Hot, sunny climates; west- and east-facing glass |
| Moderate solar control | Around the middle of the scale | Mixed climates with both heating and cooling |
| High solar gain | Above 0.30 2 | Cold climates; south-facing glass meant to collect winter sun |
| Tinted or reflective glass | Often low | Glare control; may cut daylight too |
| Skylights | Rated separately | Roof glass gets intense sun; the 2021 IECC treats skylights in zones 0–3 with a special exception 3 |
Lower is not always better
Unlike U-factor, where lower always means less heat loss, the right SHGC depends on climate. ENERGY STAR’s Northern zone actually requires an SHGC of at least 0.17, so the window still collects some free winter sun 5.
Example: what two SHGC values mean for one sunny window
This is an illustration of the ratio, not a measurement. It uses round numbers to show how SHGC scales solar heat. The SHGC values are real code and ENERGY STAR limits 35.
| Step | Window A | Window B |
|---|---|---|
| Solar heat striking the window (example units) | 100 | 100 |
| SHGC on the NFRC label | 0.23 (ENERGY STAR Southern limit) | 0.40 (ENERGY STAR North-Central limit) |
| Solar heat ending up inside | 100 × 0.23 = 23 | 100 × 0.40 = 40 |
| Difference | — | About 74% more solar heat than Window A |
| Good for | Cutting cooling load in hot sun | Collecting winter sun in a heating climate |
Solar heat inside = SHGC × Solar energy striking the window
SHGC- From the NFRC label, 0 to 1
Solar energy striking the window- Depends on sun angle, direction, shade and season
A simple ratio for comparing products. It ignores overhangs, screens, blinds and nearby trees.
The same arithmetic works for any window: solar heat inside equals SHGC times the solar energy hitting it. Real heat gain also depends on shading, orientation and time of day, which is why an HVAC load calculation uses each window’s SHGC and direction.
Where SHGC appears: the label, the quote and the permit
On the NFRC label. New windows carry a temporary NFRC label with SHGC beside U-factor, and often visible transmittance and condensation resistance 1.
On a quote. A good window quote lists the U-factor and SHGC for each opening. "Low-E glass" alone does not tell you the SHGC, since low-E coatings vary widely 2. Our quote comparison tool helps line up bids on the same ratings.
On the permit. Where the energy code applies, inspectors check the label against your climate zone. One Maryland county’s 2021 IECC form for additions and alterations lists an SHGC of 0.40 for zone 4 4. The permits and approvals guide explains how that check fits in.
In ENERGY STAR listings. A certified window shows which ENERGY STAR climate zones it qualifies for, based on its U-factor and SHGC 6.
Match SHGC to each wall
You do not need the same glass everywhere. NRCan notes that mixing glazings, low solar gain on some walls and high solar gain on south walls, can give the best results 2. Ask your supplier whether a mixed order is possible.
What the right SHGC does for a home
Low SHGC helps
Limits of the SHGC number
SHGC is measured for the window alone, at a standard condition. It does not include roof overhangs, awnings, trees, screens or blinds, all of which can cut solar gain a great deal. A west window under a deep porch roof may never see the sun the rating assumes.
It also does not tell you about daylight or glare. A low-SHGC window can still be bright if it has a high visible transmittance, or it can be dark if it uses a heavy tint. Check both numbers on the label 1.
Finally, SHGC does not cover heat flowing through the glass by temperature difference. That is the U-factor’s job. A window can block sun well and still lose heat on winter nights.
What drives the cost of a given SHGC, and how long it lasts
We do not publish window prices. SHGC is mostly set by the glass coating, which is usually one of several options in a product line, not a separate item.
Coating type. Solar-control coatings with more silver layers tend to sit higher in a line’s glass options than basic low-E. One glass maker offers separate high-solar-gain, moderate and "ultimate" solar-control coatings 5.
Mixed orders. Different glass on different walls can add ordering steps.
Climate target. Meeting a 0.23 limit in the South may push a buyer toward a different glass package than meeting 0.40 in the North-Central zone 5.
Installation. SHGC does not change labor, but full-frame replacement and new openings do. See the cost guide and the whole-home renovation guide.
Coatings sealed inside insulated glass are protected and generally last the life of the glass unit. If the unit’s edge seal fails and it fogs, the glass unit is replaced, and the coating with it.
How to pick and check SHGC for a window project
You can do the choosing yourself. Installing new openings or changing structure is work for a licensed contractor under a permit.
- Find your IECC climate zone from your building department.
- Look up the code cap for your zone: 0.25 in zones 0–3, 0.40 in zone 4 (except Marine) and zone 5, none in zones 6–8 under the 2021 IECC table 34. Confirm the edition your town enforces.
- If you want ENERGY STAR, check its zone limits too 56.
- Decide by wall: lower SHGC for hot west and east glass, higher for south glass in cold climates 2.
- Ask every bidder to list U-factor and SHGC per window in writing.
- Check NFRC labels at delivery against the quote and photograph them 1.
- Keep the labels or photos for the inspection.
Keeping solar control working
A window’s SHGC does not wear off with normal use, but a few habits keep it performing.
- Clean coated glass gently. Use the maker’s recommended cleaners on any exposed coated surface.
- Watch for fogging between panes. A failed seal can mean the glass unit needs replacement.
- Use shades and overhangs. Exterior shading cuts solar gain beyond what the rating covers.
- Be careful with aftermarket films. Adding film to coated or insulated glass may affect the window warranty; ask the window maker first.
- Trim or plant trees thoughtfully. Deciduous trees shade in summer and let sun through in winter.
Signs your windows have the wrong SHGC
- Rooms that overheat on sunny afternoons even with the air conditioning running.
- Fading floors, rugs or furniture near west or south windows.
- Cold, dim rooms in winter in a cold climate with very low-SHGC glass on south walls.
- Uneven temperatures between sunny and shaded sides of the house.
- High summer bills tied to large glass areas.
Window openings and safety
Changing window sizes or adding openings can affect structure and egress. Enlarging an opening may need a new header, and bedroom windows may have to meet emergency escape rules. Use a licensed contractor and get a permit; see our egress window entry before changing a bedroom window.
Before replacing windows, ask an energy auditor or HVAC contractor to look at shading and the whole house. Our home energy audit entry explains what they check.
SHGC rules: the 2021 IECC and ENERGY STAR
The energy code. The 2021 International Energy Conservation Code (IECC), Table R402.1.2, sets a maximum glazed fenestration SHGC of 0.25 in climate zones 0 through 3 and 0.40 in zone 4 (except Marine) and zone 5 and Marine 4, with no requirement ("NR") in zones 6 through 8.
Footnotes say the SHGC column applies to all glazed fenestration, that there are no SHGC requirements in the Marine zone, and that in zones 0 through 3 skylights may be excluded if their SHGC is 0.30 or less.
We read these values in an ICC committee report that reproduces the 2021 table 3 and in Montgomery County, Maryland’s 2021 IECC form for additions and alterations 4.
Codes change by edition and states amend them, so confirm the edition your building department enforces.
ENERGY STAR. ENERGY STAR is voluntary.
Its Version 7.0 window criteria took effect October 23, 2023, and, as reproduced by a glass maker, set SHGC at 0.23 or less in the Southern and South-Central zones, 0.40 or less in North-Central, and at least 0.17 in Northern (with higher minimums on the Northern equivalent-performance path) 5.
ENERGY STAR publishes the official tables on its product criteria page 6. The federal 25C tax credit that once rewarded ENERGY STAR windows expired on 31 December 2025.
Ratings. SHGC values used for code and ENERGY STAR come from NFRC certification 1.
SHGC vs other window ratings
| Rating | What it measures | Better when | Main difference from SHGC |
|---|---|---|---|
| SHGC | Share of solar heat let in, 0 to 1 1 | Depends on climate | — |
| U-factor | Heat flow by temperature difference | Lower | Covers winter heat loss, not sun |
| Visible transmittance | Share of daylight let in, 0 to 1 1 | Higher for daylight | Covers light, not heat |
| Condensation resistance | Resistance to inside condensation 1 | Higher | Covers moisture, not heat |
| Low-E coating | A glass feature, not a rating | — | One of the things that sets SHGC 2 |
SHGC myths
- Myth A lower SHGC is always better.
- Reality In cold climates, higher solar gain can help; ENERGY STAR Northern requires at least 0.17 5.
- Myth All low-E glass has the same SHGC.
- Reality Low-E coatings vary greatly in solar performance 2.
- Myth SHGC and U-factor measure the same thing.
- Reality SHGC covers the sun’s heat; U-factor covers heat flow by temperature difference 1.
- Myth Low SHGC means a darker room.
- Reality Not necessarily; visible transmittance is a separate rating on the label 1.
- Myth Every zone has an SHGC limit in code.
- Reality The 2021 IECC sets none in zones 6–8 or the Marine zone 3.
How climate zone sets the SHGC you need
Climate zone is the deciding variable. Homes in zones 0 to 3, across the Gulf Coast, the Southwest and the Southeast, face a 2021 IECC cap of 0.25 3.
Zone 4 (except Marine) and zone 5 have a cap of 0.40, while zones 6 to 8 have none 34.
ENERGY STAR draws its own four zones and asks for 0.23 or less in the South but at least 0.17 in the North 5.
Within any zone, the direction each window faces changes the best choice 2.
When SHGC should drive your window choice
SHGC matters most when you cool more than you heat, when a room has large west- or east-facing glass, and when a permit will check the label in zones 0 to 5. In those cases, stay at or under the code cap and consider ENERGY STAR for your zone.
In cold climates, look for a higher SHGC on south-facing glass to collect winter sun, paired with a low U-factor. If you are adding a kitchen or sunroom with lots of glass, ask your HVAC contractor to include the new windows in the load calculation.
- If Hot climate or strong west sun
- Then Choose low SHGC, at or under 0.25 in zones 0–3
- If Cold climate, south-facing glass
- Then Consider higher SHGC to collect winter sun
- If Mixed climate
- Then Mix glass by wall direction
- If Quote lists only "low-E"
- Then Ask for the NFRC SHGC and U-factor for each window
Questions about Solar Heat Gain Coefficient (SHGC)
What is a good SHGC for windows?
It depends on climate. In hot zones 0 to 3, the 2021 IECC caps SHGC at 0.25, and ENERGY STAR asks for 0.23 or less in its Southern and South-Central zones.
In cold regions a higher SHGC can help with winter heating; ENERGY STAR’s Northern zone requires at least 0.17. Compare values on the NFRC label.
Is a lower or higher SHGC better?
Lower SHGC is better for blocking summer heat, and higher SHGC is better for collecting winter sun. NFRC says low numbers resist solar heat and high numbers allow it. Hot climates generally want low SHGC; cold climates may want higher SHGC, especially on south-facing glass.
What is the difference between SHGC and U-factor?
SHGC measures how much of the sun’s heat a window lets in. U-factor measures how fast heat flows through the window because of a temperature difference between inside and outside. A window can be good at one and average at the other, so check both numbers on the NFRC label.
Where do I find a window’s SHGC?
It is on the NFRC label on the new window, next to U-factor and visible transmittance, and in the NFRC certified products directory. Ask your contractor to list the SHGC for each window on the written quote, and photograph the labels before they are removed.
Does low-E glass lower SHGC?
Often, but not always. Low-E coatings come in many types, and Natural Resources Canada notes their solar performance varies greatly. Some coatings are designed for high solar gain and some for strong solar control. The NFRC label value tells you the result for the whole window.
What SHGC does the energy code require?
Under the 2021 IECC, glazed fenestration must have an SHGC of 0.25 or less in climate zones 0 through 3 and 0.40 or less in zone 4 except Marine and zone 5. There is no requirement in zones 6 to 8 or the Marine zone. Your building department confirms the edition it enforces.
What does an SHGC of 0.25 mean?
It means about 25% of the solar heat striking the window ends up inside the home, counting both energy passing straight through and heat absorbed by the glass and released indoors. The other 75% is reflected or released back outside.
Do I want the same SHGC on every window?
Not necessarily. NRCan suggests mixing glazings, such as low solar gain on some walls and high solar gain on south walls, can give the best results. In a hot climate, low SHGC everywhere is common. In a cold or mixed climate, direction matters more.
Does SHGC affect HVAC sizing?
Yes. Solar gain through windows is part of a home’s cooling load, so window SHGC, area and direction feed into an HVAC load calculation. Replacing high-SHGC windows with low-SHGC ones, or adding large windows in a remodel, can change the cooling load the system has to meet.
Sources
- National Fenestration Rating Council, Consumers: understanding the NFRC label, retrieved .
- Natural Resources Canada, Low-Solar and High-Solar Gain Glazings (modified 2024-12-23), retrieved .
- ICC, IECC Residential Committee Action Report on 2021 IECC public input changes (updated July 26, 2022), Table R402.1.2, retrieved .
- Montgomery County (MD) DPS, 2021 IECC Residential Energy Compliance for Additions and Alterations form, retrieved .
- Cardinal Glass Industries, U.S. ENERGY STAR 7.0 glass guidance (criteria table, effective Oct 23, 2023), retrieved .
- ENERGY STAR, Residential Windows, Doors and Skylights Key Product Criteria, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: The definition and label reading come from NFRC and Natural Resources Canada.
Code values were read in Table R402.1.2 as reproduced in an ICC committee action report and in a Maryland county’s 2021 IECC compliance form; ICC Digital Codes blocked automated access on the retrieval date.
ENERGY STAR 7.0 values come from a glass maker’s reproduction of the criteria table, because the official ENERGY STAR table is an image without a text layer; the official page is linked. The worked example uses illustrative units. No prices are given.
Suggest a correction. We fix errors and say what changed.