SHGC — Solar Heat Gain Coefficient — is a number between 0 and 1 that tells you how much solar energy passes through a window into your home. The lower the number, the less heat gets in. For most Canadian homeowners in the GTA and Ontario, a higher SHGC is actually desirable in south-facing windows, as it lets free winter solar heat do some of the work your furnace would otherwise handle. Getting this number right during a window replacement project can make a measurable difference on your energy bills.
Key takeaways:
- SHGC measures the fraction of solar radiation that enters through a window as heat, on a scale from 0 to 1.
- In Canada’s cold climate (Climate Zone 2 and Northern Zone), a moderate-to-high SHGC (0.25–0.45) is typically recommended for south-facing glass.
- SHGC works alongside U-factor; both numbers appear on ENERGY STAR™ certified labels.
- North-facing windows see almost no direct sun, so SHGC has little practical effect on those orientations.
- Window frame material, Low-E coating type, and glass layers all directly influence the final SHGC rating.
What Does SHGC Actually Measure?
SHGC is expressed as a decimal. A window rated 0.40 lets 40% of the total solar energy striking its surface pass through as heat. That heat includes both direct solar radiation and the portion of absorbed solar energy that re-radiates inward from the glass itself.
The National Fenestration Rating Council (NFRC) sets the testing protocol, and every ENERGY STAR™ certified window sold in Canada must carry an NFRC label showing both U-factor and SHGC. These two numbers together tell you how well a window holds heat in (U-factor) and how well it captures free solar heat (SHGC).
SHGC vs. U-Factor: Understanding Both Numbers
Homeowners often confuse SHGC with U-factor, but they measure completely different things.
| Metric | What It Measures | Ideal for Cold Climates |
| U-factor | Rate of heat loss through the window | Lower is better (aim for ≤1.22 W/m²K in Climate Zone 2) |
| SHGC | Fraction of solar energy entering as heat | Higher on south/east/west; lower on shaded openings |
| VT (Visible Transmittance) | Amount of visible light passing through | Higher means more daylight indoors |
A window with a low U-factor but a very low SHGC might block the free solar heat gains you need during a Toronto winter. Balancing the two is what separates a good window specification from a great one. For homes looking at energy-efficient window replacement, both metrics belong on the checklist.
From Our Showroom: Best Sellers in the GTA
How Low-E Coatings Change SHGC
Low-E (low-emissivity) coatings are microscopically thin metallic layers applied to the glass surface. They are the primary tool manufacturers use to tune SHGC without changing visible light levels much. Different coatings have very different effects:
- Hard-coat (pyrolytic) Low-E — applied during float glass production; higher SHGC, good solar heat capture, used where passive solar gain is wanted.
- Soft-coat (sputter-coat) Low-E — applied in vacuum; finer control over SHGC, typically lower values; standard on most high-performance IGUs today.
- Cardinal LoĒ-180 — designed for cold climates, retains a higher SHGC to allow beneficial solar gain in winter.
- Cardinal LoĒ-272 — balances solar gain control with strong infrared reflection; a versatile coating for mixed-orientation windows.
- Cardinal LoĒ-366 — triple-silver coating, lowest SHGC of the three; excellent for west-facing glass or rooms prone to summer overheating.
TotalSeal uses Cardinal Glass insulated glass units, which gives homeowners access to this full range of coating options matched to orientation and application.
“Specifying the wrong Low-E coating for a south-facing window in Ontario can cost a homeowner a meaningful amount in heating energy. I routinely see projects where a LoĒ-366 was installed on south glass — a coating that was designed for hot climates — and the homeowner ends up with a colder house and a higher gas bill all winter. Orientation-specific glazing selection is not optional; it’s basic building science.”
— Marcus Delacroix, RHI, Registered Home Inspector, Oakville, ON
Windows and Doors near me
SHGC by Window Orientation in Ontario
Ontario sits in ENERGY STAR™ Climate Zone 2 (most of the GTA) and the Northern Zone (areas north of the French and Mattawa Rivers). Both zones benefit from passive solar heating strategies in winter.
| Window Orientation | Recommended SHGC | Reasoning |
| South-facing | 0.30–0.45 | Maximum passive solar gain in winter; use overhangs to control summer gain |
| East-facing | 0.25–0.35 | Morning sun only; moderate gain useful, less summer overheating risk |
| West-facing | 0.20–0.30 | Afternoon summer sun can cause overheating; lower SHGC preferred |
| North-facing | Any (0.20–0.40) | No direct sun; SHGC has negligible impact; focus on U-factor |
These are starting ranges, not hard rules. Shading from trees, overhangs, and neighbouring buildings all shift the optimal value for any given opening.
How Glass Layers and Gas Fill Affect SHGC
Adding glass layers and changing the gas fill between panes affects SHGC in predictable ways:
- Triple glazing typically reduces SHGC by roughly 10–15% compared with the equivalent double-glazed unit, because the extra pane absorbs and reflects some solar energy before it enters.
- Argon gas fill has minimal direct effect on SHGC but significantly improves U-factor.
- Krypton gas fill, used in narrower triple-glazed cavities, also has little direct SHGC impact but allows thinner overall IGU profiles.
If you are considering tilt-and-turn windows or casement replacement windows with triple glazing, factor in that slight SHGC reduction when choosing your Low-E coating — you may want a coating with a slightly higher SHGC to compensate.
SHGC and ENERGY STAR™ Requirements in Canada
Canada’s ENERGY STAR™ certification for windows is administered by Natural Resources Canada (NRCan) and organised by climate zone. As of 2026, the programme does not set a minimum SHGC threshold — it focuses primarily on U-factor and air leakage ratings. However, SHGC is still printed on every certified label and factors into whole-home energy modelling under the EnerGuide rating system.
When you are applying for the Canada Greener Homes Grant, the windows you install must be ENERGY STAR™ certified. You can learn more about Canadian grant requirements for window replacement before booking your consultation.
“Most homeowners read the U-factor and stop there. SHGC is the number that determines whether your new windows actually help your home stay warm through passive gain or just block it. In my experience reviewing renovation files for building permits in the GTA, very few homeowners or even contractors look at SHGC before ordering. That’s leaving money on the table — literally.”
— Sandra Fortin, BCIN #23571, Building Designer, Mississauga, ON
Practical Tips for Choosing the Right SHGC
When specifying windows for your home, keep these points in mind:
- Get a rough sketch of which walls face south, east, west, and north before talking to your installer — it takes five minutes and directly affects which coating you need.
- Ask your supplier for the full NFRC label data, not just the U-factor. Both numbers matter.
- If you have deep overhangs on south-facing windows, you can afford a higher SHGC because the overhang blocks high summer sun while still admitting lower winter sun.
- Rooms with large west-facing windows, such as a living room or kitchen, typically benefit from a lower SHGC to avoid afternoon overheating in July and August.
- If you are replacing fixed picture windows in a sunroom or addition, passive solar SHGC optimisation can reduce your heating load noticeably.
- When comparing quotes, check whether the SHGC quoted matches the orientation of each opening, not just the overall window model.
How SHGC Affects Window Selection in Practice
Different window styles interact with SHGC differently, mainly because of how much frame area each style uses relative to glass area. A casement window with minimal sightlines delivers more glazed area and therefore captures more solar energy per rough opening than a double-hung with a heavy mid-rail. Similarly, a picture window replacement maximises glass area entirely, making SHGC selection especially important for those units.
Consider your window type alongside SHGC when planning:
- Casement and fixed windows: highest glass-to-frame ratio, SHGC choice has maximum impact.
- Double-hung windows: mid-rail reduces effective solar aperture; still worth specifying correctly.
- Awning window replacement units: good ventilation overlap but frame area is higher than casements.
- Sliding windows: frame interlocks reduce glass area; moderate SHGC impact per rough opening.
- Bay and bow windows: multiple orientations within one unit; consider per-panel SHGC where possible.
Summary Table: Quick SHGC Reference
| Climate Zone | Window Type | Target SHGC | Notes |
| Zone 2 (GTA) | South-facing, unshaded | 0.35–0.45 | Passive solar priority |
| Zone 2 (GTA) | West-facing | 0.20–0.28 | Reduce afternoon overheating |
| Zone 2 (GTA) | North-facing | 0.20–0.40 | Focus on U-factor instead |
| Northern Zone | South-facing | 0.40–0.50 | Higher solar gain priority |
| Northern Zone | Any orientation | ≥0.25 | Cold climate; prioritise heat retention |
FAQ
What is a good SHGC for windows in Ontario?
For south-facing windows in Ontario’s Climate Zone 2, a SHGC of 0.30–0.45 works well. It captures passive solar heat in winter without causing serious overheating in summer, particularly when paired with appropriate roof overhangs.
Does a lower SHGC always mean a better window?
No. In Canada’s cold climate, a very low SHGC on south-facing glass blocks free solar heat gain your home could use in winter. A lower SHGC is better for west-facing or heavily shaded openings where summer overheating is the primary concern.
Is SHGC the same as the U-factor?
No. U-factor measures how quickly heat escapes through a window. SHGC measures how much solar radiation enters as heat. Both appear on the NFRC label of every ENERGY STAR™ certified window and should both be checked before purchase.
Does SHGC matter for basement hopper windows?
Very little. Basement hopper windows receive minimal direct sunlight, so SHGC has almost no practical effect. Prioritise a low U-factor and low air leakage rating for those openings instead.
Can I change the SHGC of an existing window without replacing it?
Not meaningfully. Low-E coatings are applied during manufacturing and cannot be retrofitted. Solar films can be applied to existing glass and reduce SHGC, but they also reduce visible light and may void any existing window warranty. Full window replacement is the only reliable way to get a specified SHGC rating.
