Solar heat gain coefficient measures the share of incident solar energy entering through a window as heat, including inward heat from energy absorbed by the glazing. Visible transmittance describes visible light passing through. Compare both alongside whole-window U-factor: a window can reduce heat transmission well yet admit substantial sun, or limit sun while making a room darker.
Canadian planning context. Technical examples from other regions are identified in the guide; approval and performance requirements must be checked for the actual building and jurisdiction. Original explanatory diagrams are not construction details.
- SHGC and visible transmittance describe different parts of the solar spectrum.
- Lower SHGC is useful when unwanted sun drives overheating; higher is not automatically better in a cold region.
- Whole-window values include the opaque frame, while glass-only values do not.
- Compare actual glass packages and room conditions rather than a coating trade name.
- SHGC calculations at rating conditions do not predict every sun angle, room temperature or annual bill.
01 /Three things happen when sunlight reaches glass
Incoming radiation can pass through, reflect away or be absorbed by a pane or coating. Energy absorbed in the assembly warms it; some subsequently travels indoors and some outdoors. Solar heat gain therefore includes more than the sunlight you can see through the window. A dark-looking pane can absorb energy and become warm while its light transmission falls. Its total solar heat gain depends on the complete make-up and the resulting inward energy flow.
Visible light occupies only part of the incoming solar spectrum. Near-infrared radiation carries energy but is invisible to your eye. Long-wave thermal radiation exchanged by room surfaces is another range again. Spectrally selective coatings use those differences to achieve combinations that ordinary dark tint cannot: useful daylight with reduced solar heat gain, for example. That does not mean every low-e coating makes the room dark or every clear-looking window admits high solar gain. Ask for the verified ratings rather than inferring performance from appearance.
02 /What SHGC actually measures
Solar heat gain coefficient is a dimensionless fraction, conventionally expressed between zero and one. In an illustrative comparison, 0.30 represents less admitted solar energy than 0.50 under the same conditions. The number includes directly transmitted energy and the inward portion of absorbed energy. It is not a measure of insulation, air leakage, ultraviolet transmission, structural strength or glare.
The U.S. Department of Energy’s purchasing guidance describes SHGC and U-factor as separate central energy properties. Canadian ENERGY STAR’s specification permits qualification through defined U-factor or Energy Rating criteria. Those labels do not select the ideal solar gain for every room. A sunny west bedroom and a shaded north stairwell in the same building can need different balances. A designer can assess this with an annual model that includes shading, internal loads, ventilation and cooling.
03 /Visible transmittance and the opaque-frame effect
Visible transmittance, abbreviated VT or VLT, expresses visible light transmitted. Higher generally means more daylight passes through the rated assembly. It does not state the light level at a kitchen worktop or computer screen. Those depend on sky conditions, obstructions, aperture position, room depth, interior reflectance and other openings. A bright window can also create an uncomfortable contrast with a dark room.
Whole-window VT includes the effect of opaque framing. A glass-only VT can consequently look more attractive than the product value. A wide-frame small window may transmit less total useful light than a slender-frame alternative even with identical glass. An insert replacement can reduce clear glass further if the old frame remains. Compare the delivered daylight opening, not only the rough-opening dimensions or a glass sample held against a showroom light.
| Number | What it describes | It does not directly tell you |
|---|---|---|
| U-factor | Rated heat transmission due to temperature difference | How much sunlight enters |
| SHGC | Fraction of incident solar energy entering as heat | Daylight level at a desk |
| VT or VLT | Fraction of visible light transmitted | Glare comfort or colour fidelity |
| Exterior visible reflectance | Visible light reflected toward outside | Bird-safety performance by itself |
| UV transmittance | Transmission in the specified ultraviolet range | Complete protection against fading |
| Energy Rating | Canadian combined energy metric under its prescribed method | The best glass for every orientation |
04 /Work a transparent solar-gain example
For a limited steady illustration, admitted solar heat can be estimated as incident irradiance × rated area × SHGC. Assume 500 W/m² reaches a 2 m² complete window, with the same area basis used for its rating. An assumed SHGC of 0.50 gives 500 W; 0.30 gives 300 W. The difference is 200 W under these assumed conditions. This calculation illustrates the role of SHGC; it is not a measured room load or a promise of a particular temperature reduction.
Real sun arrives at changing angles. Buildings, screens, trees and dirt modify what reaches the glazing; glass transmission also changes with incidence. A window can face west but be shaded by a neighbour during the hottest afternoon hours. A south window may have useful winter exposure yet be fully shaded in summer. An hourly energy or daylight model deals with these factors more honestly than multiplying a rating by a generic city sunshine figure.
The same care applies to net winter benefit. A window admits useful heat only when the sun reaches it and the building can use that heat. If solar gains overheat a bedroom and require blinds or cooling, a favourable annual heating calculation may still represent a poor room design. Evaluate occupied hours and comfort, not merely an annual total.
05 /Understand g-value and light-to-solar gain
European documents often use g-value for total solar energy transmittance. It describes a closely related physical concept to SHGC, but test or calculation methods, area scope and boundary conditions can differ. Do not treat a glazing-only European g-value as an interchangeable whole-window North American SHGC without checking the underlying record. Ask the supplier to state the method and provide comparable data for the offered assembly.
A light-to-solar gain ratio, often written VT/SHGC, can help compare daylight admitted for a given solar-gain fraction. For invented examples, VT 0.60 with SHGC 0.30 has a ratio of 2, while VT 0.60 with SHGC 0.50 has a ratio of 1.2. That ratio helps identify a daylight-oriented solar-control option. It is not a universal ranking: a project seeking winter solar heating may value the second package, and neither ratio addresses insulation, glare direction, colour, bird collisions or visual quality.
06 /Select by orientation, room use and controllable shading
In Canada’s northern-hemisphere context, south-facing glazing can receive useful low-angle winter sun; high summer sun can sometimes be controlled by a designed overhang. East and west receive lower-angle morning or afternoon sun that a shallow horizontal overhang may not block. North is often dominated by diffuse daylight, but summer sun can still strike northeast or northwest exposures at some latitudes and times. These are starting observations, not a four-number recipe for ordering glass.
A west office used late in the day is different from a west guest room used occasionally. A home without cooling needs an explicit overheating plan; so does a cooled home during a power interruption. Exterior shades, ventilation opportunities and internal heat from people or equipment matter. Consider the consequence when shades remain down for weeks: a low-solar-gain option with good daylight may be more usable than high-gain glass that spends summer covered.
- Record true orientation and major obstructions for each elevation.
- Describe occupied hours, existing glare and hot-weather discomfort.
- Identify exterior shading that can actually be installed and maintained.
- Check daylight and privacy implications before selecting a darker option.
- Where consequences are substantial, request an hourly model rather than a universal SHGC target.
07 /Tint, colour, reflection and fading
A tint can be part of a useful glass design, but it changes colour and view. The effect is easiest to judge with a full-size or representative assembly beside the selected frame, seen in daylight from inside and outside. The appearance of a single sample pane differs from a double or triple unit and from a large elevation. Light, sky and interior brightness also influence whether you notice reflection or transparency.
Fading depends on more than ultraviolet exposure. Visible light, heat, material composition and time all matter. A product with very low UV transmission does not justify a claim that fabrics or art will never fade. For sensitive collections, discuss a conservation strategy that may include controlled light exposure and shading. Reflective glass also deserves a bird-collision review; apparent energy efficiency does not make a reflective exterior visible to birds.
08 /Write an orientation-specific glass schedule
Keep a common base of whole-window thermal and structural requirements, then identify any planned glass variants by elevation and room. Name the glass make-up, coating position, SHGC, VT and appearance requirements clearly. A coloured dot on the drawing is insufficient if the manufacturer’s order codes can be confused. Ask how substituted glass will be checked before production and at delivery.
| Schedule entry | Evidence to request | Question before approval |
|---|---|---|
| Room and orientation | Opening plan, obstructions and shade design | Which discomfort are we solving? |
| Glass package | Exact make-up and coating identifier | Is the offered configuration the rated one? |
| Whole-window energy | U-factor, SHGC, VT, rating method and model record | Are the area scope and units consistent? |
| Appearance | Representative sample or mock-up | Are colour, haze and reflectance acceptable? |
| Special requirements | Safety, acoustics, bird markers or conservation brief | Has the combined assembly been assessed? |
| Substitutions | Written equivalence review | Do changes affect more than thermal performance? |
Retain the approved schedule with the installation and warranty records. If a room remains hot afterward, that record distinguishes a wrong delivered glass package from insufficient shading, changed occupancy or a modelling assumption. Start with orientation and exterior shading and use the ratings lab for heat-transmission arithmetic; the two investigations answer different questions.
FAQQuestions people ask
Is low SHGC always better?
No. It helps limit unwanted solar gain, especially with overheating and cooling concerns. Useful winter sun, daylight, room use and shading can justify another balance.
Does high VT mean there will be glare?
Not necessarily. Glare depends on bright sources, contrast, sun direction and the observer’s view. Glass transmission is only one part of daylight design.
Can I choose different glass on different elevations?
Often yes, if the supplier offers it and the schedule controls the exact packages. Confirm appearance, model ratings, safety requirements and warranty implications for each choice.
Can an ENERGY STAR window overheat a room?
Yes. Certification addresses prescribed energy criteria, while actual room comfort also depends on solar gain, glass area, shading, weather, ventilation and internal heat.
SOURCESSources and further reading
- U.S. Department of Energy — Purchasing energy-efficient residential windows; U.S. purchasing guidance
- NFRC — Residential product certification; North American whole-product energy ratings
- Natural Resources Canada — Canadian ENERGY STAR fenestration specification, version 5.0; Canadian certification scope
- Guardian Glass — Low-e glass and solar-energy behaviour; manufacturer technical explanation
- U.S. Department of Energy — Passive solar home design; northern-hemisphere design explanation
