Calculate attic temperature from the outdoor temp, roof color, sun and ventilation, or solve backward, and see the ceiling heat gain and AC cost it drives.
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Attic Temperature Formula
The calculator uses a simple rule-of-thumb model for attic heat gain from sun exposure. It assumes the maximum attic temperature rise above the outdoor temperature is 60°F, which is about 33.33°C.
- T_attic = estimated attic temperature
- T_outside = outside air temperature
- SG = solar gain factor
- DeltaT_max = assumed maximum attic temperature rise, 60°F or 33.33°C
If you leave the attic temperature blank, the calculator estimates it from the outside temperature and solar gain factor. If you leave the outside temperature blank, it works backward from the attic temperature. If you leave the solar gain factor blank, it estimates how strong the attic heat gain is based on the temperature difference.
The calculator converts Fahrenheit entries to Celsius internally, performs the calculation, then converts the result back to the unit you selected.
Solar Gain Factor Reference
| Solar Gain Factor | Approximate Rise Above Outside | Typical Meaning |
|---|---|---|
| 0.00 | 0°F, 0°C | No attic heat rise in this model |
| 0.25 | 15°F, 8.33°C | Low solar heat gain |
| 0.50 | 30°F, 16.67°C | Moderate attic heating |
| 0.75 | 45°F, 25.00°C | High solar heat gain |
| 1.00 | 60°F, 33.33°C | Full assumed maximum rise |
| Greater than 1.00 | More than 60°F, more than 33.33°C | Outside the normal range of this rule-of-thumb model |
Common Temperature Conversions
| Temperature | Equivalent | Use in Calculator |
|---|---|---|
| 60°F | 15.56°C | Cool outdoor condition |
| 75°F | 23.89°C | Mild outdoor condition |
| 90°F | 32.22°C | Hot outdoor condition |
| 120°F | 48.89°C | Possible hot attic temperature |
| 150°F | 65.56°C | Very hot attic temperature |
Example Attic Temperature Calculations
Example 1: Calculate attic temperature
Suppose the outside temperature is 90°F and the solar gain factor is 0.70.
The estimated attic temperature is 132°F.
Example 2: Calculate solar gain factor
Suppose the outside temperature is 85°F and the attic temperature is 121°F.
The solar gain factor is 0.60.
FAQ
What is a normal attic temperature on a hot day?
On a sunny hot day, an attic can often be much hotter than the outdoor air. In this calculator, a solar gain factor of 0.50 means the attic is estimated to be 30°F above the outside temperature. A factor of 1.00 means the attic is estimated to be 60°F above the outside temperature.
What does a solar gain factor above 1 mean?
A solar gain factor above 1 means the attic temperature rise is greater than the calculator’s assumed maximum rise of 60°F. That can happen if the inputs describe an unusually hot attic, a measurement issue, or a situation where this simple rule-of-thumb model is not accurate enough.
Can the attic be cooler than the outside temperature?
Yes, but that is not a solar heat gain condition. If the attic temperature is lower than the outside temperature, the calculated solar gain factor will be negative. This may happen during changing weather, at night, with strong ventilation, or when the attic temperature measurement does not reflect peak sun conditions.
