I have a lake house in North Florida which I only use on weekends. When I'm not there I turn the central AC system (not a heat pump) up to low 80's, but then turn it down to mid - high 70's on weekends when I'm there. After being in the house all weekend I've noticed that the sheet metal (aluminum?) supply plenum (which is not wrapped or insulated) coming out of the air handler box in the attic has lots of condensation dripping from the bottom of the plenum onto the insulation on the attic floor. However, when I first get to the house on Friday afternoon (when the temp has been turned up all week) the air handler plenum is dry as a bone with no condensation in sight. What's up with this and how do I fix it?

insulate the duct work/plenum......also might help to vent the attic,,,,get as much hot air out as possible,with a good thermostat controled attic fan,,,,cold air flowing thru a hot duct system will sweat