Condensation will occur on any surface when the surface temperature is at or below the “dew-point temperature” for an air-water mixture. The dew-point temperature depends on the dry-bulb temperature (measured with an ordinary thermometer) and the relative humidity in the space next to the surface. The dew-point temperature is less than or equal to the dry-bulb temperature. The two temperatures are equal when the relative humidity is 100%. Some examples of dew-point temperatures:

Temperature Relative Humidity Dew Point
70°F (70 Degrees Fahrenheit) 50% 50.5 1/4F
70°F (70 Degrees Fahrenheit) 75% 66.6 1/4F
70°F (70 Degrees Fahrenheit) 90% 66.9 1/4F

As you can see, condensation can occur when the outside temperature is cold. Insulation below a roof deck will have an inside surface temperature above the roof surface temperature. The actual temperature of the inside surface depends on the thermal resistance between the roof and the inside surface. The higher the resistance, the closer the interior surface temperature is to the inside air temperature. Maintaining a reasonable inside relative humidity (less than 60%) is important in preventing condensation.