Bathroom exhaust fans are essential to removing moisture
It seems a lot of home buyers believe the bathroom exhaust fan is used primarily to rid the area of those odors you don't want lingering around your home after taco night. That is only partially true.
During a home inspection we hopefully don't have to check it for how long a odor lingers, we want to make sure it is extracting the moisture from those hot steamy showers (which also extracts those odors).
Moisture from showers without proper ventilation will find its way on and even into the walls. The steam will turn to condensation when it cools. Warm air finds cooler air meaning it can and will travel through the wall to get there.
Drywall is porous and can allow for moisture to condensate on the ceilings and walls and even get into places we can not see during your home inspection and may allow for fungal growth.
Turning on the bathroom fan during showers and baths can drastically reduce the chances of fungal growth or excessive humidity. Leaving the fan on for 20 minutes after running water in the bathroom and opening the door is suggested by most manufacturers. If you are forgetful like me, it is recommended to have a fan timer installed which will do this for you.
Just like your HVAC air filters you should keep an eye on the exhaust fan grill to make sure it isn't being clogged by dust.
Most building codes require the bathroom exhaust fan to be operational or having a 3x3 window to allow for proper air flow.
It is also required the bathroom exhaust vents to the exterior of the home and not to the attic.
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