Some advice please! 2 years ago I had installed a new a/c system in the unconditioned space in my attic. Contractor used rectangular plennum with 1 inch duct liner inside of metal plennum.
Using remote themometers last winter I took measurements of room temp and temp inside the plennum (removed a register grill and stuck themometer inside the flexduct as far as I could reach). With registers open the temperature inside the plennum was 3 degrees higher than the room temp. With registers and intakes closed the plennum temp was 1 degree higher. I then sealed the registers with plastic and the plennum temp dropped another 25 degrees.
In the spring when unit was serviced it was noticed that the 5" filter at the air handler was moldy and the metal on it was rusted. This winter I sealed registers as before but left off plastic on two unheated rooms. Last week I noticed icicles hanging from ceiling below air handler over carport (Virginia). Water was dripping out of air handler and filter was soaked, so I removed the plastic from the registers.
What can I do to prevent this condensation and at the same time reduce the heat loss through the plennum? The unit is resting on a floor in the ceiling. I measured 290 square feet of outside surface area of the plennum. How is this not like having an additional 30 foot long outside wall with only R4 insulation?
I am considering better sealing all registers and intakes with tape. Also I could line the outside of the plennum with 2" polyisocyanurate board (additional R8). Any suggestions? Thanks for the help.