As I indicated in an earlier thread, I've always suspected that my AC sucked warm humid outside air into the cabin and, therefore, also ejected cooled air out of the cabin through every orifice available to it. I've now confirmed that and fixed it.
The AC compartment, at the foot of the mid-berth in my 258, is not closed. It shares volume with the vented line locker immediately above and an in-wall cup holder. That volume is uninsulated, of course, and it gets heated by sun on the port side - and it has a source of outside air which allows that air to enter the AC return.
I can't stop the heating very easily, but I did stop the AC from inhaling it by building a thin plywood baffle that fits tightly around the AC evaporator inlet, gluing 2"x3" foam strips around its periphery, which seal the gap between the evaporator and the return grille, effectively making a plenum that the grille supplies and the evaporator draws from. Very little or no compartment air now makes it into the evaporator.
I used to be able to feel cold air spilling out of gaps around the companionway door when the AC was running - and even the anchor locker was cooled by cabin air blowing through two bulkhead wiring holes. No more.
