I have a similar problem. My house is a beach house, built on 8' pilings. The area under the house is open. Yesterday was hot and humid and we had the A/C on. I noticed some condensation on the exterior surface of the brownboard under the house today that corresponds to where the HVAC duct work is located. WHat would cause this and what is the solution? I questioned the insulation of the ducts when the builder was enclosing the ducts (I didnt think there was sufficient insulation around the ducts) but the builder blew me off. Is this what is causing the problem?