When does ASTM C1153 recommend starting a thermal roof survey?

• A. Immediately after sunset
• B. At sunset
• C. One hour after sunset
• D. Two hours after sunset

Answer: (C)

The timing is about getting a clear temperature difference to reveal where heat is escaping. After sunset, the roof surface cools, but the interior heat stored in the building and insulation continues to transfer outward. This creates contrast between areas with proper insulation and areas with defects or gaps. Starting about one hour after sunset typically yields a strong, stable delta T that helps the infrared camera show issues like leaks or insufficient insulation. If you go immediately after sunset, there can still be residual heat from the day; too long after sunset and the temperature differences can diminish as everything approaches ambient temperature.