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There are the seams around the door frame and tile in the bathroom. I'm not sure if the black dirt in is mold or mildew, but at least it's not very clean. The relative humidity is 50%, but the bathroom has not been used for a long time.

I would like to seal these seams to keep water out, and wonder what I should do. Shall I clean the black dirt in seams, which it quite difficult, and how? After cleaning, Is caulking with silicone the right thing to do? Shall I fill some large seams with something else before caulking? Thank you.

Edit: I removed the tile and some door casing paint in the first photo, and some paint and cover (paper maybe, it is the right material to be used here?) over the drywall, and photos are added. I have tested with tissue paper and confirmed the door casing is dry. It's the bathroom in the basement, close to the exterior foundation wall. The shower door sweep is missing, I guess water came out from the gap when it was in use.

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  • Please see the red lines in the photos. Thank you.
    – bobby_yan
    Commented Sep 15, 2022 at 21:07
  • You probably want a paintable caulk. Pure silicone isn't. Look for a mildew-resisting kitchen & bath product.
    – isherwood
    Commented Sep 15, 2022 at 21:09
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    Is caulking with silicone the right thing to do? No. That's a popped tile. Fix it. And then it's missing a piece of wood trim. Make one. How to clean and fill seams around tile? with a scrapper and grout. But that's a can of worms that will pull that tile completely off. So as a stop-gap can you just calk it? Sure. But I wouldn't even touch it unless you were prepared to gut the whole thing if we have to.
    – Mazura
    Commented Sep 16, 2022 at 16:29

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There are cleaners to remove the mildew, however that area has signs of water infiltration and the result is the mold. The water infiltration needs to be corrected first. Any other work to clean / or caulk and seal will be fruitless. 50% humidity is pretty high and another indication of water leaking/ seeping. If you don't have an exhaust fan in the bath you need one even after the problems are solved.

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  • I think it was due to basement flooding long before I moved in. Let's presume the water problem can be solved, what should I do besides that? Thank you.
    – bobby_yan
    Commented Sep 15, 2022 at 21:12
  • The effected porous surfaces should be removed. (Things like the drywall and door casing. ) the framing under that should be inspected for damage and a solution of 50% chlorine bleach 50% water should be applied to any surfaces showing mildew. Any wood that is damaged so it cannot serve it's purpose should be replaced or treated and have additional supports added. This may not be so bad or could be a big deal that you need a professional to handle once you get to see what is behind all the damaged wood and drywall and possible tile.
    – RMDman
    Commented Sep 15, 2022 at 21:47
  • Do I need to remove the tile in the first photo to check behind? Is there anyway to check behind the drywall and the damaged wood easily? The door casing wood seems as hard as the normal one. Thank you.
    – bobby_yan
    Commented Sep 16, 2022 at 2:18
  • Yes, you should remove the tile and check behind it. A wide putty knife will probably make that pop right off the wall. that should tell you how high any water caused damage. It looks like it is next to a tub or shower. The water could have come from that as well. Good Luck
    – RMDman
    Commented Sep 16, 2022 at 11:20
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    You could look at that tile funny and it will fall of. Is that really what you want to get into... because at that point caulk isn't part of the equation anymore, knowing how to tile is.
    – Mazura
    Commented Sep 16, 2022 at 16:31

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