We have own this rental house for about 12 years. First a single guy lived there for four years, then a small family of three, now a family of four and we are for the first time experiences some issues. The home is a single family home, on a full basement, we had a complete basement interior French drain and sub pump put in two years the same year this family moved in. Now they are experiencing rust on the bathroom fixatures, shower rod, metal light switches, vanity door knobs, floor vent. all of which we changed out today. ALSO notice the new high powered exhaust fan grill was closed up, not sure what had collected on the small vent holes but it was difficult to remove what was in the clogged holes. THE fan works and it's two years old, as it was replaced at move in, but no moister from the bathroom could get pulled into the fan. after cleaning the fan vent cover we hope this helps solve the water issue in the bathroom. HOWEVER in the kitchen, the refrigerator is also getting rust spots on the exterior sides, I have never seen such. It's a fairly new refrigerator also. I do not pay the water bill there, but I do pay the sewer bill (based on water usage) and since this family moved in the sewer bill has doubled and remains the highest sewer bill of any of the properties we own. NEXT week we will go back and visit the home and work with the family to figure out why the rust (which must come from moisture) is happening in the house. One thing I want to look at is if they have disconnected the dryer from venting outside. What else, where else, can I start looking, what questions can I ask them about their life style and living in the home. This has never been an issue with any other of the folks living in the house. Matter of fact the metal switch plates have been there some 10 years plus, with no rust, until now. Now sure if it's the house of the life living skills of the people who live there, but it's not normal and we are working together for a solution.

Humidity is a pain to deal with. The apartment I just cleaned had mildew in the bathroom. All over. It was obvious that they never turned on the fan, which is wired separate, when they took showers. Then there is the cooking... lots of boiling water and that steam gets stuck to everything. Then the tenant didn't open windows to air the place out either. I had mildew on all the window frames. The vent for the dryer goes right thru the wall. I pulled out a softball sized bunch of lint. No chance the dryer was venting thru there.

Sometimes being a shut in isn't cool.

Oh also the stool was leaking from the tank to bowl connection. You could see where tenant was wiping it up. Probably thought the young son was a horrible shot.

Check and see if they are the type that all take 2+ showers every day. Also, do they leave the bathroom door open when they do? Maybe when the bath fan vent cover became clogged they started leaving the bath door open because the mirror and everything were "too fogged up". Then the moisture from all the showers went into everywhere.

Naturally, they never cleaned the vent cover.

Make a note in your maint schedule for this place to check that vent cover every time you go there but at least monthly. See if the vent was clogged from just dust or maybe cat hair/dander/fluff?? Check the dryer hose connection and clean out the dryer vent to the outside for lint clogging it.

Also, address the excessive sewer charges and tell them from now on they pay it.

Do they leave the bathroom door shut all the time?

Cleaning out that vent should help, but if they take lots of showers and when done, turn off the lights and shut the door then the moisture remains. I had this happen at my rental a few tenants back. It was not the house but the tenants.

I had a condo where over the course of a summer I had similar rust stains on bathroom fixtures and random rust spots on outsides of the refrigerator and washing machine. It had not happened before and I don't want it to happen again. It occurred in multiple condos across multiple buildings in the complex. The HOA management team told us that it was a combination of some weather condition that year as well as blocked vents. Owners were responsible for ensuring all vents were cleaned, including HVAC, bath, and kitchen vents.

Take a look at an automatic fan sensor switch which I've installed in almost all my rental properties for this reason. It can be set for whatever humidity level you need for that property. Here is the link to the DewStop fan switch that I found at Home Depot:

homedepot.com/p/DewStop-Condensation-Fan-Control-Humidity-Sensor-with-Countdown-Timer-in-White-120-Volt-50-60Hz-HS-C00-W7/206424892

Did you recently install a heatpump? We had this in one cinderblock house, not enough insulation and it molded inside horrible.

Another house we just bought a dehumidifier for the tenant to use after trying several other things.