The solar heat gain coefficient describes how much of the sun’s energy a window lets into a building as heat. Formally, it is the ratio of solar heat gain entering a space through a window to the solar radiation striking that window. It counts two things: solar energy transmitted straight through the glass, and solar energy absorbed by the glazing and then released inward. The same window that trims a winter heating bill on a south wall can push a west-facing suite into uncomfortable temperatures by mid-July, which is why the number deserves more attention than it usually gets at the specification stage.
Reading the Number: 0 to 1
SHGC is a simple ratio. A value of 0 would mean no solar heat enters through the window, and a value of 1 would mean all of it does. As an illustration, a window with an SHGC of 0.25 lets roughly a quarter of the incident solar energy become indoor heat, while a window with an SHGC of 0.50 lets in roughly half.
The direction is worth stating plainly, because it is often misread. A higher SHGC means more solar heat gain. A lower SHGC means less. Neither is automatically better. Which one serves a project depends on the climate, the orientation of the glass, and what the building is trying to achieve.
What Goes Into the Number
SHGC combines two separate heat paths:
- Solar radiation transmitted directly through the glazing into the space
- Solar radiation absorbed by the glass and then re-emitted, conducted, or convected indoors
Several product choices shift the result. Low-e coatings are the main lever, since they can be tuned to reflect part of the solar spectrum while still admitting visible light. Tints, the number of panes, and the frame also affect the final figure. That last point matters when comparing products: SHGC can be reported for the centre of the glass alone or for the whole window including the frame. The two are measured on different bases, so products should always be compared using whole-window values.
SHGC vs. U-Factor vs. Visible Transmittance
SHGC is one of three performance numbers typically listed for a window, and each measures something different:
| Metric | What it measures | Better when |
|---|---|---|
| SHGC | Solar heat entering through the window | Depends on climate and orientation |
| U-factor | Rate of conductive heat loss through the window | Lower |
| Visible transmittance (VT) | Daylight passing through the window | Depends on daylighting goals |
These metrics trade off against one another. A coating that lowers SHGC can also reduce visible transmittance, which means less daylight and potentially more reliance on electric lighting. A good selection balances all three rather than optimizing one in isolation. On terminology, the European g-value describes essentially the same concept as SHGC, and the older shading coefficient, which compared a window against a sheet of clear glass, has largely been replaced by SHGC in North American practice.
High or Low SHGC? It Depends on the Wall It Faces
This is where SHGC stops being a definition and becomes a design decision.
Much of BC is heating-dominated for most of the year. On south-facing glass, winter sun arrives at a low angle and can offset a meaningful share of heating demand. A moderate or higher SHGC on those windows allows the building to use that free heat. This passive solar strategy is a common part of the design approach for Net Zero homes, where every reduction in heating load makes the renewable energy target easier to reach.
East- and west-facing glass behaves differently. Morning and afternoon sun comes in low and nearly perpendicular to the glass, and exterior shading struggles to block it. A high SHGC on these elevations can drive summer overheating, and the effect is stronger in modern buildings than in older ones. An airtight, well-insulated envelope holds heat very effectively, which is exactly what is wanted in January and exactly what causes discomfort in July if solar gain is not controlled.
Multi-unit residential buildings are the sharpest case. Large glazing ratios, single-aspect suites, and limited cross-ventilation mean that SHGC selection becomes a comfort and livability issue, not just an energy one. For this reason, orientation-specific glazing, with different SHGC values on different elevations, is often the practical answer rather than a single product across the whole building. Current BC Building Code provisions related to overheating and cooling should be checked for the specific project and permit date before glazing is finalized.
Where SHGC Shows Up on a BC Project
SHGC appears at three points in a typical project.
- Product ratings: Window performance values in Canada are rated under CSA A440.2, and NFRC 200 is the North American procedure for determining SHGC. Current standard editions and labelling requirements should be confirmed before publication or specification.
- Energy models: SHGC is a direct input in energy modelling for BC Energy Step Code compliance. It affects thermal energy demand intensity (TEDI) through winter solar gains, and it affects cooling loads through summer gains.
- Specification review: The SHGC listed on the window supplier’s specification sheet should match the value used in the energy model. When a product substitution happens late in a project and the model is not updated, the modelled performance and the built performance can drift apart. This is one of the more common sources of discrepancy between a compliance report and how the building actually behaves.
Common Misreadings
- “Lower SHGC is always better.” Not in a heating-dominated climate. On south-facing glass in much of BC, a very low SHGC gives up useful winter heat and can increase heating demand.
- “SHGC and U-factor measure the same thing.” They do not. SHGC measures solar heat coming in, while U-factor measures heat escaping by conduction. A window can perform well on one and poorly on the other.
- “Any two SHGC values can be compared.” Only if they are measured on the same basis. A centre-of-glass value will usually look different from a whole-window value for the same product, so mixing the two leads to misleading comparisons.
Conclusion
SHGC is a single number, but choosing it is an orientation and climate decision rather than a search for the lowest value on the market. Monolith Housing Solutions supports design teams with Step Code energy modelling and envelope performance guidance, and it uses infrared thermal imaging to show how glazing and frames actually perform once installed. This work is led by founder Shahin Arvandi, a Registered Energy Advisor, Certified Passive House Designer, and Level II Infrared Thermographer, who reviews glazing selections against both energy targets and overheating risk.
Frequently Asked Questions
The Definition
- What does SHGC stand for? SHGC stands for solar heat gain coefficient, the fraction of solar radiation striking a window that becomes heat inside the building.
- What is a good solar heat gain coefficient? There is no single good value. The right SHGC depends on the climate, the orientation of the window, and whether the goal is to capture winter solar heat or limit summer overheating.
- Is SHGC the same as the g-value? They describe essentially the same concept. SHGC is the term used in North America, while g-value is common in Europe, and the two are calculated under slightly different conditions.
Interpreting Values
- Is a lower SHGC better? Only where solar heat is unwanted, such as on east- and west-facing glass or in cooling-dominated climates. On south-facing glass in a heating climate, a higher SHGC can reduce heating demand.
- What does an SHGC of 0.3 mean? It means roughly 30 percent of the solar energy striking the window enters the building as heat, through direct transmission and through heat absorbed and released by the glass.
- What is the difference between centre-of-glass and whole-window SHGC? Centre-of-glass SHGC measures only the glazing, while whole-window SHGC includes the effect of the frame. Whole-window values are the better basis for comparing products.
Comparing Metrics
- What is the difference between SHGC and U-factor? SHGC measures solar heat entering through a window, while U-factor measures how quickly heat escapes through it by conduction. Both matter, and they are independent of each other.
- How is SHGC different from the shading coefficient? The shading coefficient compared a window’s solar heat gain against a reference sheet of clear glass. SHGC has largely replaced it as the standard North American metric.
- Does a low SHGC reduce natural light? It can. Coatings and tints that lower SHGC may also lower visible transmittance, so both values should be reviewed together.
- Climate and Application
- What SHGC is best for BC homes? It varies by elevation. Many BC projects benefit from a moderate or higher SHGC on south-facing glass and a lower SHGC on east- and west-facing glass to manage overheating.
- Should south-facing windows have a higher SHGC? In heating-dominated climates, often yes, since low winter sun on south glass can offset heating demand. Summer shading should still be considered for those windows.
- Can low-SHGC windows prevent overheating in condos? They reduce one of the main causes, especially on east and west elevations, but overheating also depends on shading, ventilation, glazing ratio, and mechanical cooling.
