I grew up in a wood house with a wooden bathroom that was well-used each morning. We saw first-hand the effects of rotting wood from moisture. I also rented from landlords that didn't take care of their property, causing black mold to form from excess moisture.
The primary concerns are the following:
- Hydration of permeable substances
- Warping of wood (structure, wooden decor or wooden instruments)
These effects depend on the bathroom construction, the ambient relative humidity, ambient temperature and the humidity generated by the shower.
- - - - - - - - - - - - - - - - - Problems - - - - - - - - - - - - - - - - -
Mildew and Mold
Mildew and mold can each cause a host of respiratory/health issues. They are not something to mess around with. They thrive on moisture and darkness, and if you don't ventilate your bathroom, you may be inviting mildew and mold.
Moisture can seep behind wallpaper and thin or cracked paint to rot the wood behind the surface. Wooden floors can begin to slowly rot. If moisture in the air is insufficient to stay around for long, condensation on some surfaces can trickle down and pool in cracks and crevices to cause more damage. You can generally expect the highest risk of rot to be where the most moisture is -- in the bathroom -- but it could also be in an adjacent room or one connected by an air vent. If the wood in your house is rotting, it can lead to expensive repairs, can attract termites, and can cause structural damage if unchecked for too long.
Wood can warp when exposed to moisture and temperature gradients (more info). This generally won't be a problem for structural wood, but it could be an issue for wooden instruments or other wooden decor -- not only in your bathroom but also elsewhere in your home.
Most people don't have many things in their home near a bathroom that would be subject to rust, but I wanted to include this to be more comprehensive. Things that might rust include knives, cast iron cookware, art & decor, and more. Usually this won't be a problem unless the moisture levels are too high.
- - - - - - - - - - - - - - - - - Causes - - - - - - - - - - - - - - - - -
All of the above problems are caused by excessive moisture in one way or the other. Here are some of the factors that can affect those problems.
If you have lots of wood in your bathroom, rot may be a concern. Moisture can seep behind wallpaper and begin to very slowly rot and/or warp the wood behind it. If there are surfaces where water can condense and pool, these are especially areas of concern. Other materials may also be vulnerable to rot, mildew and mold.
The cooler your house is compared to your shower, the lower the dew point and the more condensation there will likely be on the surfaces of your bathroom and house. The more moisture condensates in the bathroom, the less there will escape into the rest of your home in the short term, but you should expect it to distribute.
Relative Ambient Humidity
If you life in a humid place, you might need to worry about mold/mildew forming from the moisture, or rot developing. If you live in a very dry place, you might be able to use the shower as a humidifier and not even use an exhaust fan. Mold/mildew/rot will not form as quickly in dry environments. Many houses in humid climates will have bathrooms that are better designed to withstand moisture, but don't count on it, especially for older homes.