My wife and I just recently moved into a renovated 1950's mid-century modern home. The master bedroom/bathroom are in a large walkout basement; the walkout portion has walls of (new) windows, as well as some ground-level windows on the other sides. The problem we're experiencing is a pretty large temperature and humidity differential between upstairs and downstairs. Upstairs is usually 3-5 degrees hotter and 40-50% RH. Downstairs it's consistently 50-60% RH and on hot/humid days can get beyond 70% (we live in Northern Virginia). We're wondering what our options are for bringing down the differential--in particular the humidity. We've had success with a standalone dehumidifier (got the basement down to 41%), but really don't want a standalone unit and would rather address the root of the problem rather than treat the symptoms.
As far as we can tell, there are no foundation leaks or signs of mold anywhere. Behind the master bathroom is a utility room with the HVAC and I can actually touch the foundation wall--feels completely dry. There are also some areas in that room where the vinyl plank flooring is not covering the foundation floor--and it feels dry there, too, despite the room's humidity being high. Gutters, with the exception of one, drain at least 5 feet from the house. All of this leads me to believe it's just a matter of humid outside air getting in, though I am surprised at that given all the windows are new.
We've talked to several HVAC guys who made some recommendations (move AC condenser out from underneath deck, re-do the entire ductwork in the house), and we're in the process of contacting some basement waterproofing companies to see what they think, but my gut is telling me that basement waterproofing is not necessary for us (we've had some massive rains recently and no leaks). The HVAC guys don't seem to think the humidity is a problem and that a whole-zone dehumidifier is not necessary. They also didn't seem to think moving from a single zone to a dual-zone would be necessary.
Wondering if anyone has had similar experiences and/or could help me diagnose the source of the problem and potential solutions. We'd really like to do our due diligence first to understand what's going on rather than throwing money at something that won't fix it!
Thanks for your help.