[hayden55]

Yeah I would agree. RH doesn't mean much in the attic. You just want to compare it to outside of the attic and see that it is pretty much about the same as it means it is capable of drying itself (as long as it isn't leaking or moldy up there). A lot of the times your walls and ceilings and ducts will sweat depending on what you're doing because they will hit dew point on the cold outside surfaces. When I say that i just mean a cold surface will condense water out of the air onto it. So more or less the walls just dry through and keep chugging to a certain point (range of 30-60% inside is pretty good for that). Inside though is really where the humidity and dew point matters. Things really start to hit dew point on exterior surfaces when you get high humidity inside which will lead to wet moldy walls etc...
You can see the dew point, wet surface, mold thing really well on windows typically as they aren't able to dry through and have very little insulation so they are always a cold surface in the winter for water to condense on.
Like I said as well about drying through I can take an hour shower since I haven't fixed the bathroom vent yet and go in the attic and you can see the moisture in the loose fill fiberglass. Its not much though and it drys through so theres no mold.
Redpoint I would say paint the basement with a good epoxy waterproofer, insulate it with foam board or fiberglass whatever your heart desires, play with a top vs bottom damper in your duct, maybe even try a radiant barrier up top on the rafters and play with your venting in the attic for temp reduction and see how it goes. That should reduce the heat gain a bit in the top and replace the air more often so it stays closer to thermostat temp up there, and insulating the basement will get rid of the ground cooling effect down there. Should bring them closer in heating and cooling loads but probably won't be perfect. The hvac system might be able to manage it though!
Also, air sealing with a foam gun and caulk will help you a bit as it sounded like your hvac was unable to keep up sometimes. Less air changes means less outside air coming in needing to be conditioned.