The direction your windows face significantly affects how much heat enters your room. Understanding this is crucial for proper AC sizing.
Why Window Direction Matters
Different window directions receive different amounts of solar radiation throughout the day:
South-Facing Windows (Northern Hemisphere)
- Receive direct sun most of the day
- Generate the most heat gain
- Heat gain consistent year-round
- Worst for cooling needs
West-Facing Windows
- Receive afternoon/evening sun
- Hottest time of day (2-5 PM)
- Generates intense, direct heat
- Very challenging for AC
East-Facing Windows
- Receive morning sun
- Heat gain early in day
- Moderate solar gain
- Less intense than west
North-Facing Windows (Northern Hemisphere)
- Receive minimal direct sun
- Only indirect/reflected light
- Minimal solar heat gain
- Best for cooling
Solar Heat Gain Adjustment Factors
Adjust your basic BTU calculation by window direction:
| Direction | Adjustment | Reason |
|---|---|---|
| North | -10% | Minimal solar gain |
| East | +5% | Morning sun moderate |
| South | +10-15% | Constant sun exposure |
| West | +15-20% | Afternoon peak heat |
Window Type and Shading Effects
Single vs Double Glazed
- Single-pane: Maximum heat transfer
- Double-glazed: Reduces solar gain by 15-20%
- Low-E coating: Reduces solar gain by 25-30%
Shading Impact
- No shading: 100% solar gain
- Light curtains: Reduces by 25%
- Heavy/blackout curtains: Reduces by 50-70%
- External shading (awnings): Reduces by 75-90%
- Shade trees: Reduces by 40-60% (temperature dependent)
Example Calculations
Room: 300 sq ft with large west-facing window
- Base load: 300 × 25 = 7,500 BTU
- West direction: 7,500 × 1.15 = 8,625 BTU
- With heavy curtains: 8,625 × 0.70 = 6,037 BTU (effective)
- Result: 1 ton AC needed
Same room with north-facing window
- Base load: 300 × 25 = 7,500 BTU
- North direction: 7,500 × 0.90 = 6,750 BTU
- Result: 0.75 ton AC would suffice
Strategies to Reduce Solar Heat Gain
External Shading (Most Effective)
- Awnings and overhangs: Block 75-90% of sun
- Shade trees: Block 40-60%
- External blinds: Block 70-80%
- Reflective films: Block 40-80%
Window Treatments
- Blackout curtains: Block 50-70%
- Cellular shades: Block 40-60%
- Light-colored roller shades: Block 30-50%
Building Modifications
- Plant deciduous trees (lose leaves in winter)
- Install permanent overhangs on south/west sides
- Add reflective window film
- Upgrade to low-E windows
Peak Cooling Needs Considerations
Peak cooling typically occurs around 3-5 PM when:
- Outdoor temperature peaks
- West-facing windows get intense afternoon sun
- Inside temperature has risen throughout the day
- AC has been running all day without break
Ensure your AC can handle this peak load, not just average load.
Seasonal Variations
Solar heat gain changes seasonally:
- Summer: Highest sun angle, maximum heat gain
- Spring/Fall: Moderate heat gain
- Winter: Low sun angle, minimal heat gain (useful for solar heating)
Conclusion
Window direction and sun exposure can increase AC cooling requirements by 10-20%. Consider your room's orientation when sizing your AC. External shading like awnings is the most effective way to reduce this additional cooling load.