Certainly a dehumidifier would make a difference. Also maybe look into putting the bathroom fan on a timer switch. I have had one for over ten years now. You can set it to run for x number of minutes (try 15) after you leave the room. Then you don't have to worry about leaving it on all day but it will run longer and remove more moisture. However, if you live in a damp climate the dehumidifier is probably the only solution that will work.

Dehumidifier indeed. Go buy one and marvel at the amount of water you are going to collect in it daily. Also be prepared for your electric bill to increase.

We had one in our basement and it made a significant difference.

I need to see the floor plan of the bedroom and bathroom and closet space there is no way to help with no context . If you are redoing these spaces we need to see a to scale floor plan A concrete wall needs insulation so a bit more info as to where you live is also helpful

A: Thank you for such a detailed response. By jumper duct, do you mean a vent opening in a shared wall of the closet and master bedroom? I had thought of that, but wasn't sure how large it would need to be. We live in Florida and are 2 blocks from the ocean, so we always have high humidity. The closet is colder in winter and hotter in summer than the bath or bedroom, but I would say the dampness and mildew problem are about the same all year. I'm sure the block wall has not insulation, as that's seldom done here in Florida, and the other interior walls would not be insulated either. Our attic space, however, is very well insulated.

If the vent opening is what you're recommending, would you place it closer to the ceiling or floor?

Patricia Colwell Consulting:

(I'm not sure how to reply on here, so hopefully you get notifications)

We are simply, re-tiling shower and floors, replacing vanity, etc. No walls being removed.

I will try to take some photos tomorrow of existing layout as that will not be changing.

As stated in my reply to A, above, we live on the on the East coast of central Florida so humidity is constant. Further details are also in that response.

Hopefully, you can add insight once I post the photos and layout/measurements.

Yes, a jumper duct is just a vent that connects two rooms. As far as the size, that would need to be calculated and I’m not even sure if it would be sufficient depending on the amount of heat loss/heat gain of the closet. You may need a supply duct from your HVAC system. This is more of a building science problem. There are people who specialize in identifying such problems. They would come to your house and really evaluate the situation. They do not generally do the repairs, but they may have lists of contractors to make repairs once the problem is identified. I would recommend against just having an HVAC company come in and evaluate the cause of the problem because they are experts in only one area and they may overlook the actual cause if it is not related to their area of expertise. For instance, it may be an insulation problem. From afar, I will try to help you determine what the problem might be. I have a few more questions about the house.

1. How big is the closet? Square feet? Dimensions? How high is the ceiling? Is it a flat ceiling or a cathedral ceiling?
2. How big is the concrete wall that is an outside wall inside the closet? How long? How high? This is an area of heat loss/heat gain which is why I ask. Is there a window in the closet?
3. Is there an attic above the closet? If so, how much insulation is in the attic? R value? Newer homes will have a card near the attic access with the R value written on the card as well as the date of installation. Is there any missing insulation about the closet or other installation defects above the closet?
4. Where is the insulation located in your attic? On the ceiling (directly above the drywall) or up on the roof (directly under the roof rafters?)
5. Where is your HVAC system located? In the attic? I’ve seen in Florida condos that they are oftentimes located in a utility room and there are no ducts in the attic. Instead, the ducts are located inside soffits under the ceiling. This is actually a good thing if the attic insulation is on the ceiling and the attic is basically an oven.
6. When was your house built? Is it a single family home or a condo or what?
   7.Are you sure there’s no insulation on the outside wall? Oftentimes, they use rigid insulation on the outside of these walls and then cover it with stucco so you won’t see it. Newer homes would have this, but older homes might not. That’s why I’m asking when the house was built.
7. Is there any mold or mildew on any walls or ceiling in the closet? If so, what is on the other side of the wall?

Patricia Colwell Consulting:

I have tried to post a photo or pdf of the floorplan and cannot make it work. I have never been able to attach photos to this site for some reason.

A:

Sorry for the delayed reply. Busy calendar lately.

I cannot attach a floor plan for some reason, but I will try to answer the questions as best I can.

1. If you look from the back exterior wall it is shaped like a barn with an angled roof. So dimensions are: concrete block wall - 6' 7 3/4", wall to left 5' 6 3/4", angled doorway - 38", front wall - 2' 9", angled wall - 2', right wall - 6' 1/4". Flat ceiling 7'10".

2. 6' 7 3/4" x 7'10" h. No window.

3. Hip roof, so limited attic space above closet. I don't see anything about the insulation in the attic and I haven't crawled that far over to see. However, I do remember the home inspector remarking as to how well insulated the ceilings are.

4. Blown in on ceiling.

5. HVAC unit is in garage. Ducts are in ceiling/attic which is basically an oven in Florida.

6. 1990. SFR. Not positive. It is block with stucco. Not sure what codes were as to foam board or vapor board at that time.

7. None on walls, ceiling or floor. Just shows up on shoes and other items in closet. The side wall 5' 6 3/4", is shared with the shower. No moisture on carpet or wall. Walls are sanded plaster.

Wish I could get the attachment to load, but from reviews of this site, I'm not the only one.

Thank you for your time and assistance.

Buy three $10-12 humidity and temperature gauges (like these: https://www.amazon.com/ThermoPro-TP50-Digital-Thermometer-Temperature/dp/B01H1R0K68 ) Add one to the closet where the mildew occurs. Put another one in the bathroom. The third goes in the bedroom. Check several times a day to see what is going on.

I suggest moving all the clothes and shoes out of the closet for a couple weeks or so. Air circulation is much less in a full closet than an empty closet. Hang the clothes on a temporary rod in the bedroom or put them in another room while you are checking out the situation.

BTW, houzz will only let jpgs get uploaded. No pdfs. The houzz site wasn't letting anything get uploaded at all for a few days. You could try again.

The answer was discovered in demolition. It appears there was no type of membrane installed when the shower pan was formed and poured. As it was a sunken shower the sand and mortar was level with the surrounding slab which allowed the moisture to wick into the slab of the adjacent closet. After the pan was removed, we ran the dehumidifier for several days and eliminated a lot of water. I feel that when the new pan is properly installed it will eliminate the problem. Thank you all for your input.

Thanks for posting a follow up. Mystery solved. I hope your new installation solves the problem.