Spent the weekend sealing the basement and finally saw the humidity drop

I had a similar struggle with the air quality downstairs, though my main issue was the drafty hollow-core door at the top of the stairs. Even after I got a Frigidaire 50-pint unit running, that damp smell would still find its way into the kitchen. I ended up swapping that door for a solid-core one and added a sweep at the bottom. It made a massive difference in keeping the zones separate so the HVAC didn't have to work overtime. I'm curious if you noticed any backdrafting issues after cutting that return into the handler. When I looked into doing that near my furnace, a local guy warned me about pulling odors or combustion gases if the seal wasn't perfect. Instead of a return, I just installed a louvered panel in the door to help the air move without messing with the internal pressure of the ductwork too much. It kept my humidity around 45% all summer without spiking the power bill.

I went thru a similar phase of sealing every crack, but my biggest breakthrough came from looking at the exterior Bilco door rather than the interior one. My old steel bulkhead was leaking a surprising amount of humid air every time the wind shifted. I ended up framing a secondary insulated exterior door at the bottom of the concrete steps using an exterior-grade fiberglass slab. It created an airlock of sorts that dropped my basement's RH from 65% down to a steady 48% even during the July humidity spikes.

Instead of relying on the HVAC system for circulation, I actually took the opposite approach and isolated the basement air entirely. I installed a dedicated Santa Fe Advance90 unit with a condensate pump so I never have to empty buckets. I found that when I had the HVAC return open down there, it was actually pulling too much moisture into the rest of my ductwork during the summer months when the AC wasn't running enought to strip the humidity. Keeping the basement as its own closed-loop enviornment with high-capacity filtration has kept the dust down and saved my storage bins from that old house scent.

Cutting a return without sealing the door is a rookie move. I went with a Masonite SolidCore slab and a Pemko 303CS gasket set to ensure a true seal. Without that, the negative pressure from your air handler just pulls damp air thru the jamb. I also swapped my garage entry for a Steelcraft L-Series steel door. Kept my basement at a steady 42% RH even in August. If you’re messing with ductwork, you have to treat that basement door like a high-performance thermal break or you’re wasting your time.

Have you guys ruled out the threshold on that walk-out door though? Even if the sump is sealed and you've got a solid slab, I’ve seen humidity levels stay high becuase of a missing thermal break. If that metal sill is sweating, you're fighting a losing battle regardless of the ductwork. Also, check the hinge-side gaps—people always focus on the sweep but forget the vertical air gap. Are you sure you aren't just pulling moisture thru the jamb now that the return is creating negative pressure?

I had the same moisture issue til I looked at the actual door frame. Everyone focuses on the slab, but my jamb was sitting on a wet concrete floor and wicking water right up into the wood. I ended up cutting the bottom two inches off the casing and replacing it with PVC trim blocks. I also used a heavy-duty silicone bulb seal around the entire perimeter instead of those cheap foam strips. It stopped that swampy air from leaking past the door stop. If you’re still seeing 50%+, check if your weatherstripping is acutally compressing when you latch the handle.