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What could possibly cause a bathroom mirror to fog up? Not the shower or bath, though, I'm sure about that. Because neither has been used for a month. So, what could cause that? And how should I fix that?

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If it's fogging up without a local source of high humidity there are a few other things that could cause it (probably in combination):

  • The mirror being mounted on a cold/badly insulated outside wall.
  • An extractor fan in the bathroom drawing in steam from other sources (cooking, another bathroom etc.) which should be ventilated locally.
  • Damp, for example from leaking plumbing or weather getting in
  • Drying washing or towels over a heater in there
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Well, a bathtub or shower stall are not the only sources of humidity. And humidity is the real reason anyways. But, concerning your problem, the answer could be... it could be anything, really. Even your basement could be the reason, unlikely as it is. What I'm trying to say, perhaps, you should not be looking the source of humidity in the bathroom. Maybe, something else causes your mirror to fog up. In fact, it is entirely possible that the source is not in your home. If you live in an old apartment block, it can be just bad ventilation. It could even be the toilet.

Anyway, if you cannot locate or isolate the source of humidity, there is very little you can do. Perhaps, shaving cream could help. But it would be hard to not leave any stains, although that depends on the cream, I suppose. Anti-fog mirrors are another possibility. They all have different principles of work. Some of them resist fogging because they come with a hot water tank behind the glass. That sounds like it would work, only, such a mirror would be quite unwieldy. And you'd have to get a new mirror. Which makes it a no, I think. Some mirrors have anti-fog coating[1], but you'd still have to get a new mirror. But that also means there is a type of anti-fog coating specifically designed for mirrors.

What does it mean? Well, there might be other ways to treat the surface of a mirror, other than home remedies, at least. I've tried googling it, but other than a Wikipedia article, there was nothing useful on the first page.

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