What kind of vents? Forced air heat vents or ??? Musty can come from lack of ventilation, lack of air movement or it can come as the result of a long term but slow water leak or drip. Or both. Along the hallway and in the bedroom you can carefully spring the baseboard off and cut small inspection holes. Keep the holes smaller than the baseboard and you can just patch without worrying about fine finish and replace the base. What you are looking for is mold and mildew on the back, paper face of the drywall and/or moisture in the cavity. Does the bedroom area back onto the hallway or use a connecting wall? Hope so because then it is one position traveling to two locations, most likely. Hope, hope. It could be a roof vent that has decided to leak, down into a wall cavity. It might be flashing over a window. Find it with the least damage possible and then begin taring stuff out til you get to the origin. Good luck. Just a good flashlight and a nose will tell you quite a bit.

-Humidity upstairs is 36%, basement is 33% If these readings are accurate, then the current humidity is not an issue and is actually too low for most people. It is interesting the basement has a lower humidity that the house. I suppose it is because the dehumidifier is in the basement. When I removed the existing wallpaper in several of the lower rooms, I found black mold or mildew. This is concerning. There was a problem in the past that was so bad mold grew behind the wallpaper. After caulking inside the windows, I had the walls sealed and repainted. It is better now, but one room still smells musty when it is closed for a few days. How old is this house? What type of siding does it have? Has it ever been replaced? Given the wallpaper mold perhaps there was water seeping in through outdoor siding and widows that became trapped in interior walls and insulation. You might have to open a section of the interior wall of the room that still smells to see if there is mold and wet insulation in the wall.

Hi Stefanie, I would be glad to help if I can. I got an air purifier and it seemed like it helped a little as long as it stayed on. We ended up moving our bedroom to the one next to it and turning the musty room into a studio. I took the air purifier out to put into the new bedroom and the stinky room got really bad and thick. I happened to ask my exterminator when he came by for a routine spraying so he checked the attic above the room and said everything was normal critter-wise but theres no fan so it gets very hot. He said he used to do moisture/mold remidiation and would take a look to see if anything explained it. We have a tree with large branches that hover above that room . It looked like rainwater was pooling in the gutter above the window and if you looked under the brick, there was a lot of moss and black staining. He said to trim the branches, treat the black stains, and get a de-humidifier for that room and see what happens. Since taking his advice, I have noticed a big decrease in smell. I am still cautious as to whether this will permanently solve it but its enough of a difference to where the room is bearable to be in now.

Hi, Sonya. I spent a good bit of time on the outside of the home caulking around all entry points, including things like the gas line, the cable TV line, etc. And while we had the wall open, I used some expanding foam to seal around those entry points from the inside. That had been suggested by a home inspector/moisture remediation person that we had out to the house. We live in a wet climate (more than 60 inches of rain per year), and the inspector felt that at times when we were getting a lot of rain, some moisture was entering through those points and probably causing the smell. We have since moved from that house but haven't heard of any issues from the new homeowner, who we still see periodically.