Solar Heat Gain Coefficient (SHGC): The Math of Comfort

A number between 0 and 1. That is all SHGC is. And yet it is responsible for more uncomfortable, overheated rooms in Toronto than any other single window specification.

The mistake is almost always the same: someone reads that high SHGC means "free winter heat" and orders the same passive solar glass for every window in the house. By August, their west-facing bedroom is uninhabitable. Their Enbridge bill went down. Their Hydro bill went up. Net result: zero.

SHGC is not a setting you max out. It is a tool you match to orientation, season, and overhang geometry. Here is how it actually works.

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What Does SHGC Actually Measure?

Solar Heat Gain Coefficient (SHGC) — a dimensionless number between 0 and 1 representing the fraction of incident solar radiation that passes through a window assembly and enters the building as heat. It accounts for both directly transmitted solar energy and the portion absorbed by the glass and re-radiated inward.

Plain version: an SHGC of 0.40 means 40 cents of every dollar of solar energy hitting that window gets into your home as heat. An SHGC of 0.70 means 70 cents gets in.

That 70-cent window is not "better." It is higher-gain. Whether that is good or bad depends entirely on where the window faces and what month it is.

[Image Idea: Diagram showing solar radiation arrows hitting a window cross-section, with percentages splitting into "transmitted heat," "absorbed and re-radiated," and "reflected — blocked." Label SHGC value at the bottom.]

SHGC vs. Visible Light Transmittance

One thing that trips people up: SHGC is not the same as how bright a window looks. Visible Light Transmittance (VLT) measures only the visible spectrum. Solar radiation includes near-infrared energy your eye cannot see but your skin can feel. A window can be visually clear and still block a significant chunk of infrared heat — which is exactly what low-e coatings do.

Cardinal LoĒ³-366, for instance, has a visible transmittance of 66% — most people would call it a clear window — but its SHGC of 0.31 means it is blocking roughly 69% of the sun's heat energy. You get the daylight without the oven.

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The Scale: What the Numbers Mean in Practice

SHGC Range	Category	Heat Admitted	Typical Application
0.60 – 0.80	High gain	60–80%	South-facing passive solar windows with overhangs
0.40 – 0.59	Moderate gain	40–59%	Mixed climates, south-facing without overhangs
0.25 – 0.39	Low gain	25–39%	East, west, and north exposures; sun-belt climates
< 0.25	Very low gain	< 25%	Commercial buildings with high internal heat loads

Natural Resources Canada classifies anything above 0.30 as "high solar gain" and anything below 0.30 as "low solar gain" for residential purposes — though those cutoffs are more regulatory shorthand than a precise design guide.

For a Toronto home, the meaningful thresholds are:

• South windows: Target SHGC ≥ 0.50, ideally 0.60–0.70
• East and west windows: Target SHGC ≤ 0.30
• North windows: SHGC is irrelevant. Minimize U-factor instead.

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Why Toronto's Climate Makes Orientation-Specific Glass Worth the Conversation

Toronto sits at 43.7°N latitude. That latitude determines everything about sun angle — and sun angle is what makes SHGC matter.

In January, the sun climbs only about 24° above the southern horizon at noon. Low, oblique, and hitting south-facing glass almost perpendicularly — perfect for heat collection. In June, the same sun peaks near 70°. Steep angle, long days, and a south-facing window that was your furnace in January becomes a liability.

This seasonal swing is larger in Toronto than in Vancouver (milder winters, less heating benefit) and more dramatic than most of continental Europe. Our 2,400+ heating degree-days mean passive solar gains on the south side genuinely reduce Enbridge bills. Our humid summers mean overheating on east and west exposures is equally real.

An older Etobicoke bungalow with a big south-facing picture window in the living room is a passive solar opportunity. That same bungalow with a west-facing addition full of high-SHGC glass is an air conditioning problem.

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The Condition Most People Miss: You Need the Overhang

Here is where the "free winter heat" pitch goes wrong.

High SHGC glass on south-facing windows works in winter because the sun is low. But in summer, the sun is high — and if there is no overhang shading the glass, that same high-SHGC window admits peak summer solar radiation just as enthusiastically as it admits January sun.

The passive solar system only works when overhangs (or awnings, or deep soffits) complete the equation.

How Overhang Geometry Works at Toronto's Latitude

At 43°N, the geometry breaks down like this:

• Winter solstice noon sun angle: ~24° above horizon
• Summer solstice noon sun angle: ~70° above horizon

A properly sized overhang positioned above a south-facing window exploits that 46° difference. In December, the low sun clears the overhang and strikes the glass directly. In June, the steep sun is blocked by the overhang before it reaches the glass.

The rule-of-thumb sizing: overhang depth equal to roughly 45–55% of the window height. For a 1.5-metre-tall window, that means a 0.7–0.8-metre overhang projection. Exact geometry depends on how far the overhang sits above the window head, roof pitch, and whether neighbouring structures or trees cast additional shade.

Pro Tip: There is a complication with the spring-fall shoulder seasons. The sun sits at the same angle on March 21 as it does on September 21 — but September is significantly warmer. An overhang sized to admit March sun will also admit September sun. If you run AC into late September, consider exterior blinds or retractable awnings to handle the shoulder season manually.

[Image Idea: Side-view diagram of a Toronto house showing sun angle in December (low, passing under overhang and hitting window) versus June (steep, blocked by overhang). Label the 24° and 70° angles and overhang depth measurement.]

What If You Have No Overhang?

Then the passive solar argument weakens considerably. Without shading, high-SHGC south glass creates overheating from April through October — roughly half the year. In those cases, a moderate SHGC of 0.40–0.50 on south exposures is a safer compromise: you still capture meaningful winter gain without the summer penalty being catastrophic.

A Scarborough semi with a shallow 10 cm soffit and full south exposure is not a passive solar candidate. It is a moderate-gain candidate with good blinds.

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The Glass Products Behind the Numbers

Theory is useful. Actual glass specs are more useful.

Cardinal LoĒ-180 — The Passive Solar Workhorse

Cardinal LoĒ-180 carries a U-factor of 0.26 and an SHGC of 0.69 in a standard double-pane argon-filled unit. One silver layer. Eighty percent visible transmittance. It was designed specifically for cold climates where winter solar collection is a priority.

This is the glass we specify for residential window replacement on south-facing elevations in Toronto — living rooms, sunrooms, south-facing bedroom picture windows — when the homeowner has overhang coverage.

SHGC 0.69 means 69% of incident solar radiation becomes interior heat. On a clear January day in Toronto, a 1.5 m² south window with LoĒ-180 can deliver 300–400 watts of passive heating during peak sun hours. That is not trivial.

Cardinal LoĒ³-366 — The Solar Control Glass

Cardinal LoĒ³-366 runs at U-factor 0.24 and SHGC 0.31 in double-pane argon. Three silver layers. Sixty-six percent visible transmittance. It blocks 95% of UV radiation and the majority of near-infrared heat while remaining optically clear.

This is the right specification for west and east exposures — anywhere solar heat rejection matters more than solar collection. A North York post-war semi with west-facing bedrooms. A downtown condo with floor-to-ceiling east-facing glazing. A Mississauga new-build where the family room faces southwest.

LoĒ³-366 on the west side of a home can cut afternoon radiant heat loads dramatically — the difference between sleeping comfortably and running a window AC unit all night.

[Image Idea: Side-by-side cross-section of LoĒ-180 and LoĒ³-366 showing the number of silver coating layers and the solar radiation percentage passing through each. Caption: "Same clear appearance. Very different heat behaviour."]

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How SHGC Interacts with the ENERGY STAR ER Score

If you have shopped for windows in Ontario recently, you have seen the ENERGY STAR label with an ER (Energy Rating) number. The ER formula is a weighted combination of U-factor and SHGC, and it rewards high solar gain — because the formula assumes a south-facing window in a heating-dominated climate.

That assumption is baked in. The ER formula was designed to give a single score for a window without knowing where it will be installed.

The result: a window with high SHGC scores well on ER even if you plan to put it on the west side of your house, where it will spend every summer afternoon pumping heat into your living room.

Canada moved to a single national ENERGY STAR standard in recent years — no more regional zones. That simplifies certification but further abstracts the label from installation reality. An ENERGY STAR rating is a floor, not a specification. You still need to know your orientations.

Quotable: The ER score on a window label tells you how well it performs facing south in a Canadian winter. It tells you nothing about how it performs facing west in a Canadian August.

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Building the Right Specification for a Whole House

A real window replacement project — the kind where residential window replacement touches six to twelve windows across multiple orientations — should use at least two different glass packages.

Here is a simplified decision framework:

Window Orientation	Priority	Recommended Glass	Target SHGC	Target U-Factor
South (with overhang)	Solar gain	Cardinal LoĒ-180 or equivalent high-gain	≥ 0.60	≤ 0.28
South (no overhang)	Balanced	Mid-gain low-e	0.40–0.50	≤ 0.28
East	Heat control	Cardinal LoĒ³-366 or equivalent	0.25–0.30	≤ 0.28
West	Heat control	Cardinal LoĒ³-366 or equivalent	0.25–0.30	≤ 0.28
North	Insulation	Triple pane	Not critical	≤ 0.20

Most contractors skip this conversation and spec the same glass product on every elevation. It costs the same to order differently — it is a line-item change on the purchase order. The difference is whether someone asks the question.

If you are comparing quotes for glass-only replacement or a full window changeout, ask each contractor: "Are you specifying different glass packages for different orientations?" If the answer is no or a blank look, that is useful information.

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What Passive Solar Design Actually Requires (Not Just Windows)

High-SHGC south glass is one leg of a passive solar strategy. The full system needs three things working together:

1. Thermal mass. South-facing solar gain heats rooms quickly. Without thermal mass — a concrete slab, tile floor, or masonry wall — the room overheats during the day and goes cold at night. Wood-framed interiors with carpet have almost no thermal mass. The sun energy that comes in at noon has nowhere to store.

2. Overhang or shading. Covered above. Non-negotiable for preventing summer overheating.

3. The right U-factor alongside the right SHGC. A window with SHGC 0.69 and U-factor 0.50 lets heat in during the day and leaks it back out at night. You want both numbers dialed — high SHGC and low U-factor on south glass. Cardinal LoĒ-180's U-factor of 0.26 passes that test.

The Ontario Building Code's supplementary standard SB-12 governs energy performance for new construction, but it does not mandate orientation-specific glass selection — it sets compliance paths based on assembly U-values and air leakage. Passive solar optimization sits above the code minimum. It is a design decision, not a regulatory requirement.

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A Note on Toronto-Specific Housing Stock

Most passive solar literature is written for new construction with controlled orientations. Toronto's existing housing stock is messier.

Semi-detached homes in the Annex or Roncesvalles are often rotated 10–20° off true south. Downtown infill lots have neighbouring buildings that shadow south exposures for three hours each afternoon. Scarborough and Mississauga suburbs from the 1970s and 1980s frequently have the largest windows on the east or west facades — developer preference for "curb appeal" glazing, not solar optimization.

Before committing to high-SHGC glass on a window you believe faces south, check it with a compass. A window that faces SSW (200°) still captures good winter gain. A window facing WSW (250°) is effectively a west window for solar purposes and needs solar control glass.

We walk every property with a compass before we quote orientation-specific glass. It takes five minutes. It prevents the wrong glass from going into a house for the next thirty years.

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Frequently Asked Questions

What is a good SHGC for windows in Toronto?

For south-facing windows in Toronto, an SHGC of 0.50 or higher — ideally 0.60–0.70 — captures meaningful free winter heat. For west and east exposures, choose 0.25–0.30 to limit summer overheating. North windows need no solar gain consideration; prioritize a low U-factor instead.

Does a higher SHGC always save money on heating?

Not automatically. High SHGC on the wrong orientation — west or east — drives up cooling costs in summer and erases any winter heating savings. You also need a properly sized overhang on south glass, or the summer sun will overwhelm the benefit.

What SHGC does ENERGY STAR require for Canadian windows?

Canada uses a single national ENERGY STAR standard rather than climate zones. The certification focuses primarily on the Energy Rating (ER) score and U-factor rather than mandating a specific SHGC value, because the right SHGC depends on window orientation — something a single product label cannot account for.

Can I put high-SHGC glass on a west-facing window?

No. West-facing glass takes the harshest afternoon sun from roughly 3 PM to sunset all summer. High SHGC on the west side causes significant overheating and spikes AC bills. Use low-SHGC solar control glass (SHGC ≤ 0.30) on west exposures.

How big does an overhang need to be to shade a south-facing window in Toronto?

At Toronto's latitude of 43°N, the winter sun sits at roughly 24° above the horizon at noon while the summer sun peaks near 70°. A rule-of-thumb overhang depth of 45–55% of the window height provides full shade at the summer solstice while allowing full winter sun through — but exact sizing depends on window height, roof pitch, and any nearby obstructions.

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