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Solar heat gain coefficient (SHGC) tells you how much of the sun’s energy enters through a window, while shading coefficient (SC) expresses solar gain relative to a specified reference glazing. They are related, but they are not interchangeable universal ratings. For a commonly cited older convention, the Los Alamos National Laboratory Sustainable Design Guide gives SC ≈ SHGC × 1.15; the reference glazing and rating method must be confirmed before using that conversion.
For a current window or curtain-wall decision, compare products using the reported SHGC under the same rating basis, and evaluate it with U-factor, visible transmittance, orientation, climate and exterior shading. SC is most useful when interpreting older specifications, calculations or literature that explicitly identifies its reference.
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| Name | Best for | Key spec | Checked |
|---|---|---|---|
| Solar heat gain coefficient (SHGC) | Current window product comparisons and energy-performance specifications | Typically 0 to 1; lower values admit less solar heat | November 2026 |
| Shading coefficient (SC) | Older glazing documents or calculations that identify a reference glazing | Relative value; approximately SHGC × 1.15 in the Los Alamos convention | November 2026 |
What does solar heat gain coefficient (SHGC) mean?
SHGC is the fraction of incident solar radiation that becomes heat gain inside through a fenestration product. ASHRAE Terminology defines it as “the ratio of the solar heat gain entering the space through the fenestration area to the incident solar radiation.”
The value includes two components:
- Directly transmitted solar energy: sunlight that passes through the glazing into the room.
- Inward-flowing absorbed energy: solar energy absorbed by the glass, coatings and other parts of the product, with the portion that subsequently flows indoors.
A lower SHGC means less solar heat enters. A higher SHGC means more solar heat enters. Values are commonly reported between 0 and 1, although the number alone is meaningful only when the rating basis and product configuration are understood.
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SHGC is not a constant property at every sun angle. ASHRAE’s fenestration guidance explains that solar-optical behavior varies with incidence angle and wavelength. A window can therefore have different effective performance under morning, midday and afternoon sun, even though its published rating is a single standardized value.
What does shading coefficient (SC) mean?
SC is a relative solar-gain index. Instead of describing heat admission on an absolute 0-to-1 basis, it compares the tested glazing system with a defined reference glazing. The reference is part of the definition, so an SC number cannot be interpreted correctly without knowing which convention produced it.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →SC appears frequently in older architectural specifications, solar-load worksheets and design guides. Some documents use a clear-glass reference; others use a reference defined by a particular calculation or simulation standard. In an ASHRAE Standard 140 addendum, for example, SC is compared with the direct-normal SHGC of the standard’s WINDOW 4.1 reference glazing. That is not automatically the same reference used by every older handbook or spreadsheet.
Because SC is reference-based, “SC 0.40” does not have a universal meaning independent of the document that reports it. Two sources can use the same label while relying on different reference conditions.
What is the difference between SHGC and SC?
SHGC measures the solar heat admitted by the fenestration product under a stated rating method. SC expresses that gain as a ratio to a reference glazing. SHGC is the more useful common language for current product selection; SC requires a reference check.
| Comparison point | SHGC | SC |
|---|---|---|
| What it expresses | Solar heat entering relative to incident solar radiation | Solar gain relative to a specified reference glazing |
| Typical interpretation | Lower value means less solar heat admitted; higher value means more | Meaning depends on the reference and definition used |
| Common context | Current product labels, procurement guidance and energy models | Older literature, legacy specifications and defined calculation conventions |
| Main risk | Comparing unlike ratings, such as center-of-glass and whole-window values | Applying a conversion without identifying the reference convention |
Neither metric is a complete description of a window. Frame area, spacers, operable sections, coatings and the glass-to-frame ratio affect whole-product performance. A center-of-glass SHGC should not be compared casually with a whole-window SHGC.
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How do you convert SC to SHGC?
Under the convention presented in the Los Alamos National Laboratory Sustainable Design Guide, use:
SC ≈ SHGC × 1.15
Rearranging the same approximation gives:
SHGC ≈ SC ÷ 1.15
These are approximate, convention-bound relationships, not an exact universal conversion. They are appropriate only when the SC definition is compatible with the guide’s reference basis. If a specification cites another standard, use that standard’s definition or obtain the corresponding SHGC directly.
The guide illustrates the arithmetic with a glazing entry reporting SHGC 0.28 and SC 0.32. Multiplying 0.28 by 1.15 gives 0.322, which rounds to 0.32. The example demonstrates the stated factor; it does not establish that every product or rating method follows it exactly.
When converting a legacy schedule, record four items beside the result:
- The original SC value and source document.
- The reference glazing or standard named by that document.
- Whether the value is for the glass alone or the complete fenestration product.
- The solar condition, such as normal incidence or another prescribed angle.
If any of those items are missing, label the result as an estimate and avoid using it as a compliance value.
Which SHGC should a window have?
There is no single best SHGC for every building. Select it by balancing climate, window orientation, exterior shading, glazing type and whether useful passive solar heat is desired.
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Hot climates and cooling-dominated buildings
Lower SHGC products generally reduce unwanted solar heat and cooling demand. The U.S. Department of Energy’s residential-window guidance describes low-gain products as those with SHGC below 0.4 for hot climate zones. That category is guidance, not a universal design limit: a strongly shaded elevation or a building with substantial winter heating needs may justify a different value.
Cold climates and heating-dominated buildings
Higher SHGC can admit beneficial winter sun. DOE describes high-gain products as those above 0.55 for cold climate zones. The useful value still depends on orientation, overheating risk, insulation, thermal mass and the building’s heating and cooling balance.
Orientation matters
East- and west-facing glass receives low-angle sun that is difficult to shade with a horizontal overhang. The Los Alamos guide distinguishes those exposures from shaded south-facing windows. In practice, east and west elevations often need stronger solar control, while a properly designed south-facing overhang can permit winter sun and block high summer sun in suitable climates.
Exterior shading can change the decision
Overhangs, fins, canopies, vegetation and operable exterior screens intercept sunlight before it reaches the glass. A higher-SHGC glazing behind effective exterior shading may perform differently from the same glazing in an exposed opening. Model the complete window-and-shading arrangement rather than selecting a glass value in isolation.
How should SHGC be compared across window products?
Compare equivalent ratings from the same type of source. A practical comparison uses the complete product rating, the same standard and the same product configuration.
- Confirm the scope. Check whether each number covers the whole window, including frame, or only center-of-glass performance. ASHRAE load-calculation guidance treats fenestration ratings such as SHGC as whole-product-area values, including frames.
- Confirm the rating method. Record the standard, edition and whether the value is calculated or laboratory-rated. Do not combine a current label value with an unidentified legacy SC without documenting the difference.
- Compare like sizes and configurations. Fixed, operable, opaque-spandrel and curtain-wall assemblies can have different frame fractions and therefore different whole-product results.
- Check incidence conditions. SHGC varies with angle. A single published rating is a standardized comparison point, not a promise that every sun angle produces the same gain.
- Review companion metrics. Use U-factor for conductive heat transfer and visible transmittance (VT) for daylight. A low SHGC does not necessarily mean a dark room, and a high VT does not by itself indicate low cooling load.
- Verify the label or listing. The National Fenestration Rating Council provides consumer-facing rating resources. Use the product’s current label and documentation rather than an old marketing table.
How are SHGC, U-factor and visible transmittance different?
SHGC addresses solar heat, U-factor addresses heat flow caused by indoor-outdoor temperature difference, and VT addresses visible daylight. They answer different questions and should be reviewed together.
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| Metric | Question it answers | Lower value generally means |
|---|---|---|
| SHGC | How much solar radiation becomes indoor heat? | Less solar heat admitted |
| U-factor | How readily does heat conduct through the window assembly? | Less conductive heat transfer |
| Visible transmittance (VT) | How much visible light passes through? | Less daylight transmission |
A low SHGC can reduce summer gains but may also reduce winter solar contribution. A low U-factor helps in both heating and cooling seasons by limiting conductive transfer, while VT affects daylight and electric-lighting opportunities. The best combination depends on the project’s energy model and daylight goals.
Can blinds, shades or exterior screens replace a low-SHGC window?
Shading devices and glazing ratings are separate layers of control. DOE notes that interior blinds or shades can help reduce undesirable solar heat gain, but an interior device acts after sunlight has entered the glazing and may not perform like exterior shading or a solar-control coating.
Use the following order when assessing a real opening:
- Start with fixed geometry: orientation, overhangs, fins and nearby obstructions.
- Select glazing SHGC and VT that fit the climate and daylight needs.
- Add operable exterior or interior shading where occupants need additional control.
- Account for control behavior, including whether shades are deployed automatically, manually or not at all.
Do not describe a shade’s effect as the window’s SHGC. The window retains its tested rating; the shade is an additional system whose performance depends on coverage, gaps, reflectance and operation.
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What mistakes cause SHGC and SC comparisons to fail?
- Treating SC as universal: An SC value without its reference glazing is incomplete.
- Using 1.15 as an exact law: The factor comes from the Los Alamos guide’s convention and should be presented as an approximation.
- Mixing center-of-glass and whole-window data: Frames can materially change the product-area result.
- Choosing by climate alone: Orientation, shading and passive-solar goals can override a broad climate category.
- Using SHGC as a daylight rating: VT is the daylight metric.
- Ignoring angle dependence: The published value represents a defined rating condition; actual solar gain changes as the sun moves.
- Comparing different editions or standards: A number may look comparable while being calculated under a different definition.
- Assuming lower is always better: Lower SHGC can help cooling loads but reduce useful winter gains.
What should a construction specification state?
A clear specification should name the metric, scope and standard instead of listing a bare number. For SHGC, identify the maximum or target value, whether it applies to the complete fenestration product, the rating program or standard, and any required frame or glazing configuration. For a legacy SC requirement, state the reference glazing and calculation convention, then request the manufacturer’s corresponding SHGC where available.
For design review, preserve the source label, product identification, rating date and assumptions used in energy modeling. If a project must demonstrate code or program compliance, the applicable jurisdiction and edition control; an approximate SC conversion should not substitute for the required documented rating.
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In short, specify SHGC for current product selection, retain SC only when its reference is explicit, and evaluate both alongside U-factor, VT, orientation and shading. That approach prevents a deceptively precise number from driving the wrong glazing decision.
Frequently Asked Questions
Is a lower SHGC always better for a house?
No. Lower SHGC can reduce summer overheating, but it can also block useful winter sun in a heating-dominated building. Orientation, exterior shading and the building’s heating-and-cooling balance determine whether a lower or higher value is appropriate.
Why do two documents show different SC values for similar glass?
SC is defined relative to a reference glazing, and the reference can differ by standard or publication. Differences in incidence condition, frame scope or whether the value is center-of-glass can also make apparently similar numbers incomparable.
Can I put an SC requirement on a new window schedule?
You can, but identify the reference glazing and calculation method in the schedule. For new procurement, an SHGC requirement tied to a recognized whole-product rating is usually clearer and easier to verify.
Does SHGC tell me how bright a room will be?
No. SHGC measures solar heat, not visible light. Use visible transmittance (VT), together with room geometry, orientation and shading, to evaluate daylight.
