Roof decking condensation.

[David A. Andersen]

Adding heat, does not change the amount of moisture in the air (it only allows a greater amount of moisture capacity of the air). The dew point temperature does not change with the addition of sensible heat.

My analogy on this is; when you have an issue such as this, you need to address the source of the issue not the issue.

Adding ventilation in the attic (where it is stated that several sources of ventilation are currently installed) will work. However is it really necessary to put a 4 x 4' agricultural ventilator fan in the gable? If you move enough air, fast enough you'll prevent condensation on the roof. You will also be sucking all the air out of the house.

The water under the roof is coming from latent heat in the air. It's your job to locate where this latent heat is coming from. It can be coming from either interior or exterior of the house. What were the previous weather conditions to this inspection? I get condensation in my barns and shop all the time when climatic conditions change (because there is no heat in these buildings). If I prevented ventilation in these buildings, I wouldn't get condensation.

Latent heat in the air comes out of the air in the form of condensation when it comes in contact with an object below the dew point temperature of that air.

Building envelope leakage is most prevalent in homes of this era. A test should be conducted to evaluate the building envelope.

The house was just winterized one week previous. Somebody could've dumped the hot water heater in the basement and it is just now drying up. High temperature and high vapor pressure will move towards the attic if there is a passage opening.

The house is vacant, so you don't have to worry about occupant activities adding to moisture (such as taking showers, cooking etc.).

Your job should be quite easy. Just find the source of latent heat and block the passage to the attic.

If the moisture is coming from the exterior, this is just a temporary thing and there is nothing you can do about it. Adding more ventilation will make it happen even more.

Adding additional ventilation is not the appropriate remediation for this issue.