If you look at my
Mini-Loo project, you will see that my whole bathroom is only slightly larger than the shower you want to enclose, so the Mini Loo actually is an enclosed shower. The lessons I have learned there apply to what you want to do.
In my case, I didn't want a ceiling exhaust fan unless is was absolutely necessary. I have been using the Mini Loo for about a year and a half and things are working out well.
In the summer, when the air is warm and relatively dry, I just open the little Mini Loo double hung window (which I intentionally mounted very high on the wall), and convection does it's thing. In the winter, when it is cold and humidity is higher, I set a fan on the floor and let it run (also with the window open) until everything is dry... then I close the window.
I was warned by many people who were in possession of good sense and much enthusiasm, that not having an exhaust fan would result in the Mini Loo becoming a hideous mold garden.
However, previously I had read about the concerns that must be addressed in building a home for someone who is extremely allergic to the presence of mold... I learned that mold needs three things to grow: warmth, moisture, and food (not so different from the three things required for a fire to burn).
Quote:
In an initial experiment that began in January 2004, researchers used a small panel of wallboard -- which is very porous -- soaked in water and injected with non-toxic fungal spores. In one month's time, those spores germinated as the wallboard was kept in a high-humidity environment. Mold thrives in damp wallboard because of its paper-based encasing, DeJesus explained.
http://www.gtri.gatech.edu/casestudy/detect-hidden-mold
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So, in these houses, non-organic studs in the walls are used (steel studs), and non-organic wall board is used (sheet rock is covered on 2 sides with paper which is pre-digested wood pulp, ready to become a mold explosion).
[EDIT: I just came across a gypsum wall board that is covered in a non-cellulosic material and should be better at mold prevention] There is a type of wall board that is not covered with paper... it's not even gypsum based (gypsum isn't really the problem, it's the paper), it's something else. The green sheet rock is almost no better than regular sheet rock. In my opinion, the previously mentioned type of wall board, or something similar should be use in all bathrooms.
Here is a Fema paper on post-flood mold remediation. Essentially a bathroom, and even more a shower stall, repeats the mold-growing conditions that flood victims are faced with... only on a daily basis.
But back to the Mini Loo... In building the Mini Loo, I knew that I couldn't prevent warmth, or moisture, but I could prevent food (anything organic). So the ceiling of my Mini Loo is cementitious tile backer board, Hardibacker in my case.
The Mini Loo is warm and moist, but there is no mold food... therefore, no mold at all.
Regarding the fan... where is your fan exhausting to? Is it dumping the moisture into your attic? Could be a future problem if it is.
Also, for your fan to properly exhaust a sealed shower, you will need to provide air in. In the case of my Mini Loo, it has a curtain door which obligingly opens sufficiently at the bottom after showers, to allow moisture to naturally convect out the window.
I think that the humidity sensor idea is really great, just great. The ground moisture sensor idea will not work, it uses a different principle. You need an airborn moisture sensor... they can be found for cheap on ebay... and the Arduino stuff you already know how to do.
To recap...
Since you will be concentrating your moisture and mold producing conditions into a smaller space, you need to:
- Change out shower ceiling sheet rock to tile backer board, or equivalent
- Provide source for air in
- Go with a moisture sensing exhaust fan
-AC