Solar Gain by Orientation (Peak)
Monthly Solar Heat Gain Summary
| Month | North | South | East | West | Total | kW |
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Solar Heat Gain Through Windows — HVAC Guide
Solar heat gain through windows is typically the largest single component of peak summer cooling load in residential and commercial buildings. Unlike conduction (which depends on outdoor temperature), solar gain depends on sun position, window orientation, glazing properties, and shading — making it both highly variable and highly controllable.
Understanding SHGC
The Solar Heat Gain Coefficient (SHGC) is the fraction of incident solar radiation admitted through a window as heat. A window with SHGC 0.30 admits 30% of solar energy as heat. The remaining 70% is reflected or absorbed by the glass. Modern triple-pane windows can achieve SHGC as low as 0.15–0.20. Standard clear double-pane windows are typically SHGC 0.65–0.70. Energy Star Canada requires SHGC ≤0.40 for most climate zones. For detailed analysis see the cooling load calculator.
Why West Windows Drive Peak Cooling Load
West-facing windows receive peak solar radiation in the late afternoon (2–5 PM) — exactly when outdoor temperatures are also at their highest. This double peak (solar + outdoor temperature) makes west-facing glass the most critical cooling load driver in most buildings. South-facing windows get intense midday sun but can be effectively shaded with a properly designed horizontal overhang. East-facing windows only get morning sun when temperatures are lower. North-facing windows have the least solar gain of any orientation year-round in the northern hemisphere.
Reducing Solar Heat Gain
- Exterior shading (overhangs, awnings, exterior blinds): 50–70% reduction. Most effective because it stops heat before it enters the glass. Use the cooling load calculator to quantify the savings.
- Low-SHGC glazing (0.20–0.25): 30–50% reduction vs. standard glazing. Best for new windows.
- Interior blinds/shades: 10–25% reduction only — much less effective than exterior shading because the heat is already inside the glass envelope.
- Window film: Applied to existing glass, can reduce SHGC by 30–50% at low cost.
Evaluate the economics with the payback period calculator.
Frequently Asked Questions
For most Canadian homes where heating dominates: south-facing windows can have slightly higher SHGC (0.30–0.40) to capture passive solar heat in winter. All other orientations should use ≤0.25 to limit summer cooling loads. In hot climates like southern Ontario summers, west-facing windows benefit most from low SHGC (0.20–0.25). Energy Star Canada requires ≤0.40 for Zone 4–6. For net-zero homes, triple-pane low-SHGC windows (0.20) are standard on all orientations except south.
A properly sized south-facing overhang can block virtually 100% of peak summer solar gain (when the sun is high) while allowing nearly full winter solar gain (when the sun is low). The overhang depth should be approximately 50% of window height for southern Canadian latitudes (43–55°N). West and east overhangs are less effective because the low sun angle in morning/afternoon can't be blocked by horizontal overhangs alone — vertical fins or awnings work better for east and west. The shading factor in this calculator accounts for these differences by orientation.
Higher latitude (farther north) means lower peak solar angles, which changes the distribution of solar gain by orientation. At higher latitudes, south-facing windows receive more direct winter sun (good for passive solar) but less intense summer sun. West and east windows become relatively more significant at higher latitudes. This calculator uses latitude to compute orientation-specific peak irradiance values for each month using solar geometry equations.