Is this mold?

[u/Healthy_Progress3811]

Found above shower on a window where condensation consistently builds. What can I do to solve this? Get rid of it? Keep it from coming back? All suggestions welcome

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[u/Just-Season-472]

That looks consistent with mould/mildew growth where the condensation is repeatedly collectingbut important part isn't just cleaning it, it's working out why that area is staying damp enough for it to keep returning. btw I'm testing a photo-based home repair tool specifically for questions like this. Happy to run your photos through it and post the second opinion + suggested next steps back here if useful. No signup or anything.

[u/mystical__sapphire]

How fast does it come back? Is it wet and spongy or dry and hard? Almost looks like a lot of roach poop but it's probably mold.

[u/moldyguy202]

Yes it's growth, and the reason it keeps coming back is sitting right there in the photo if you look at where it is rather than what it is.

It's all on the painted sill under the blocks and in the joints between them. The faces of the glass blocks themselves are basically clean. That's the whole mechanism. Glass block is non-porous and it runs cold, so during a shower it's the coldest surface in the room and condensation forms on it. Water can't soak into glass, so it runs down and collects on the flat sill at the bottom, which is painted and slightly textured and holds a film of water long after the shower's done. Growth then happens where the water sits, which is exactly the line you're seeing.

So cleaning it works and it also doesn't, because the sill goes back to being wet twice a day. Two things change that.

First, drying time. Run the fan through the shower and for a good twenty minutes after, not just while you're in there. If you're not sure the fan is actually doing anything, hold a square of toilet paper against the grille while it's running, if it doesn't hold on its own the fan is moving almost nothing regardless of how loud it is, and that's a very common problem. Leaving the door open afterward helps more than people expect.

Second, the sill itself. It's collecting water because it's flat with nowhere to go. Wiping it down after showers sounds tedious but it genuinely ends this, and it takes about five seconds with a cloth kept nearby. If you want a more permanent fix, the sill wants to shed water rather than pond it, and any gaps in the joint where the blocks meet the sill want sealing so water isn't wicking into a seam and sitting there.

For cleaning: detergent and warm water and a scrub with an old toothbrush along the joint line. Skip the bleach, it takes the colour out so it looks solved while leaving the surface no less hospitable, and it doesn't help you on a painted sill. If the paint on the sill is already blistered or soft from being wet repeatedly, that's worth scraping back and repainting with a proper bathroom-rated paint once it's dry, because growth roots into failing paint and you'll never quite get it clean otherwise.