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I'm remodeling my bathroom and shower and I'll be replacing the insulation on the external wall. This wall is the "back" of the shower (opposite from where the water supply lines are). With this in mind, I should be able to just choose some insulation and then plop it in when I have the wall torn down.

The main question and concern I have is about vapor barrier. I will be using cement board with a liquid topical membrane (like Redgard) for waterproofing the shower which means I'm not supposed to put a vapor barrier on the back side of the wall because it is bad to trap moisture. How does this affect insulating? Am I safe to nix the vapor barrier around the shower? What should I do for the rest of the bathroom?

I'm not too concerned with price so if there are other options besides batting insulation (such as foam, etc...) that would solve my problem, I'm willing to hear those ideas as well.

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I'd use unfaced fiberglass, at least if each wall cavity has a place above the shower where the wall can breathe. Fiberglass is very permeable to air, which is why it only works well in still air.

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I really recommend Hardie boards vs. other good competitors. They are oblivious to water - don't hold it and break down. If money is no object, nothing outperforms sprayfoam. If you want something that works well but cheaper - Roxul. I have not used a vapor barrier on any of my HB/redguard installs unless inspector needs to see it then I stick fork holes all over it.

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