Help with humidity intrusion
So I have a true split level built in the early 60s. MBR en-suite occupies the first landing. Rest of the rooms ove the “main” side of the house. Previous owners did a number of renovations, some of them poorly. MBR and the MBR bath both have cathedral ceilings not original to the house. This leaves a small ‘L’ shaped attic over the closet/dressing hall along the exterior wall which extends over the stairs. We’ve had an issue with humid air entering the stairwell wall and first floor closet which are the back and side of the MBR bath. The HVAC duct for much of the second floor also occupies this space as the return is on the other side of the stairs. Mildew then grows on the inside of the drywall and the house smells musty. Replaced the upper drywall in the stairwell to remove mildew and had the vent stack reblocked tightly to close obvious air intrusion from the L attic. This has helped, but one can still smell mustiness when the humidity outside is high.
I’m wondering if adding a fan to the 14”x18” attic vent might help? The roof already has ridge vents and soffit “guides” (?) to aid in the air flow from the soffits into the small attic space. I’m at a bit of a loss as to what might be causing the continued humid air intrusion and while the two contractors we’ve hired have made improvements, there is clearly still an issue and more to do. Would just moving this small amount of attic air around more help keep it out of the house?