I installed an exhaust fan in the bathroom two months ago but the humidity won't go down. The thing is, mold keeps showing up on the walls and the mirror gets all fogged up. Is the exhaust fan not installed right or do I need something else to control the humidity?
A well-functioning exhaust fan should bring the humidity down quite a bit, so the problem is probably that it's not pulling the air outside correctly. Check if the outlet is clogged, if the duct has any leaks, or if the duct is bent too much, because that kills the airflow. Also pay attention to where you installed it: it needs to be at the highest point in the bathroom so the humid air rises and exits, not in a low corner. If all that checks out, it could be that the exhaust fan is too weak for the size of your bathroom, and then you'd need a more powerful model or supplement it with a window fan, but first make sure you check how it's actually installed.
The exhaust fan only pulls air out but doesn't control how much moisture comes in, so you probably need to improve natural ventilation by opening the window for a bit after you shower or letting air circulate in the bathroom. Also check that the exhaust fan is working properly and that the ducts aren't clogged, because a lot of times people install it but never clean it.
What's missing is properly sealing where the air enters to replace what the extractor pulls out, because if there's no intake, the extractor won't work at full capacity. Check for cracks in the door or a low ventilation grille in the bathroom so air can circulate while it's running.
The extractor is probably working fine, but two months with no results points to something more serious going on: first check that the outlet hose isn't crushed or disconnected (a lot of people leave it inside the ceiling without it actually going outside 😅), and then check if the bathroom door closes properly, because if there's a big gap at the bottom, the air the extractor pulls out just comes back in from inside the house instead of letting dry outside air in. Natural ventilation helps, sure, but if that duct doesn't go properly to the outside with an outlet that prevents return air, no extractor is going to win that battle.
Humidity in bathrooms almost always comes from way more water being generated than the extractor can pull out, especially if you use it for a short time or only while you're showering. What you're seeing here is that the extractor works, but you need to tackle the problem from two angles: first, check that the outlet to the outside isn't blocked or that the duct doesn't have kinks in it (a lot of times it gets crushed and barely pulls anything out), and second, leave the extractor running for 20-30 minutes after you shower because the hot, humid air is still inside for a while. If you open the window right when you finish showering that also helps, but that alone isn't enough if the extractor isn't in top condition.
Years ago I had a similar problem in my bathroom and it turned out to be a combination of things. The extractor was installed correctly, but people forget one crucial detail: if there's no fresh air intake, the extractor can't do anything. It's like trying to vacuum with the windows closed - the thing runs out of "gas". Check that there's enough air flowing in under the door or through cracks - if the bathroom is too airtight, the extractor fights against the partial vacuum it creates.
Now, there's something the others haven't mentioned: the extractor's airflow capacity matters a lot. If you installed one of those generic ones without calculating your bathroom size properly, it just doesn't have enough power. A small bathroom needs roughly 50 to 100 cubic meters per hour, but if yours is large and the extractor is weak, well, that's your problem. Also check where the outlet is: if the duct runs up into an attic or crawlspace without reaching the outside, the humid air condenses right there instead of escaping.
My advice is to check three things in this order: first, make sure the duct is properly connected and completely directed outside (no kinks that crush it); second, make sure air can enter the bathroom without obstruction; third, if all that checks out, you probably need a more powerful extractor or supplement it with a portable dehumidifier while you sort everything else out.
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