Monthly Breakdown
| Month | Irradiance (W/m²) | Solar Gain (W) | Solar Gain (BTU/h) | Cooling Load % |
|---|
How to Use the Solar Heat Gain Calculator
Pick the compass direction the window faces. South gets the most winter sun at Canadian latitudes. West gets the most intense summer afternoon sun and typically drives peak cooling load.
Select the nearest Canadian city. The calculator uses location-specific monthly solar irradiance data for that latitude. For cities not listed, pick the closest latitude match.
Measure the total glass area (not the rough opening). Find the SHGC on the window's ENERGY STAR label or spec sheet. A typical Canadian low-e double-pane runs 0.25 to 0.40.
Choose the interior shading device in use and estimate the exterior shading fraction. Exterior shading is much more effective at reducing peak gain than interior blinds, since it blocks sun before it enters the glass.
Understanding Solar Heat Gain in Canadian Buildings
Solar heat gain through windows is one of the largest and most variable components of a building's cooling load. Unlike the steady conductive heat flow through walls and roofs, solar gain is instantaneous and direction-dependent. A single west-facing window in Toronto can contribute 300 to 500 W of heat on a July afternoon when the cooling system is already working hardest.
How SHGC and glazing area interact
Solar heat gain (W) equals irradiance (W/m²) times glazing area times SHGC times shading corrections. Doubling the glazing area doubles the gain. Cutting SHGC in half cuts the gain in half. The two variables multiply, so the combination matters. A large window with a low-SHGC unit can have less gain than a small window with a high-SHGC unit. Use the cooling load calculator to put this gain in context against all the other components.
Orientation effects at Canadian latitudes
At latitudes between 44° and 55°, the sun tracks low across the southern sky in winter and high overhead in summer. South glazing receives its highest irradiance in December and January when the sun is at its lowest angle, which is exactly when you want passive solar heat. In July, the sun is nearly overhead for south-facing surfaces, so summer south gain is surprisingly moderate. West and east glazing see the most intense summer irradiance because they receive direct sun when the sun is at a lower angle in the afternoon and morning. The solar orientation calculator compares annual yield across all orientations.
Interior vs. exterior shading
Interior blinds and curtains intercept solar radiation after it has already transmitted through the glass. The absorbed energy re-radiates as heat into the room anyway, so interior shading is only 30 to 60% effective at reducing the actual cooling load. Exterior shading, including overhangs, awnings, and trees, blocks solar radiation before it hits the glass and is 70 to 100% effective. For west and east exposures with high peak gain, exterior shading is the most cost-effective strategy. The overhang sizing calculator sizes fixed overhangs for south-facing windows specifically.
Connecting solar gain to equipment sizing
The peak monthly solar gain from this calculator is a direct input to cooling load calculations. Add it to the sensible heat from conduction, occupants, lighting, and equipment to get the total room cooling load. Undersizing happens most often when designers use average rather than peak solar values. In Calgary, a sunny July afternoon produces substantially more irradiance than a cloudy Vancouver summer, so the same window contributes a larger cooling load despite Calgary's lower summer temperatures. Always use peak irradiance for equipment sizing, and monthly profiles for energy modelling.
Low-e coatings and SHGC selection
Low-emissivity coatings on Canadian windows serve two purposes. They reduce long-wave radiant heat loss in winter (improving the U-value) and reduce solar heat gain in summer (reducing SHGC). A south-facing window optimised for passive solar heating might specify SHGC 0.40 to 0.50 to capture winter sun while still having some low-e benefit. A west-facing window where cooling load dominates would suit SHGC 0.25 or lower. Some manufacturers offer spectrally selective glass that blocks near-infrared solar radiation while admitting visible light, keeping rooms bright without the heat penalty.
Frequently Asked Questions
SHGC is a number between 0 and 1 that describes how much solar radiation a window allows into a building as heat. A single-pane clear glass has an SHGC near 0.86. A double-pane low-e unit common in Canadian windows runs 0.25 to 0.40. A lower SHGC reduces cooling loads in summer but also reduces useful passive solar gain in winter. Choosing the right SHGC depends on window orientation and your climate zone. The passive solar design calculator helps you optimise south-facing SHGC for winter heating.
South-facing windows receive the most solar radiation in winter when the sun is low, making them ideal for passive solar heating. West-facing windows receive intense afternoon sun in summer and can contribute 40 to 50% of a room's peak cooling load. East-facing windows gain heat in the morning hours. North-facing windows receive almost no direct sun at Canadian latitudes and have the lowest solar heat gain year-round. Use the solar orientation calculator for a side-by-side annual comparison.
Interior shading devices like blinds and curtains reduce transmitted solar radiation but are less effective than exterior shading because the solar energy has already passed through the glass before it's blocked. A mid-tone interior blind typically reduces effective solar heat gain by 40 to 60%. Exterior overhangs and awnings are more effective because they block solar radiation before it reaches the glass. The interior shading correction factor (IAC) in this calculator accounts for this difference. For west exposures with high peak gain, exterior shading gives a much better result.
West-facing windows receive direct afternoon sun in summer when outdoor temperatures are already at their daily peak, which stacks solar and sensible cooling loads simultaneously. This produces the highest peak cooling load of any orientation in Canadian cities. Limiting west glazing, using low-SHGC glass on west exposures, or adding exterior shading are the most effective ways to control this load. The solar heat gain calculator quantifies the peak contribution by orientation so you can compare glazing strategies before selecting equipment. Feed the result into the cooling load calculator for complete system sizing.
Related HVAC Tools
Full room cooling load sizing
Block summer sun on south windows
Size south glazing for winter heating
Compare yield by azimuth and tilt
Hourly shading fractions by month
U-value based winter heat loss