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Region: Santa Cruz, California, USA.

I'm gutting a bathroom down to the studs and would like a reminder on the proper order of layers for them to ensure that it's moisture-resistant. As far as I know, it's usually:

1) Insulation. 2) Some sort of moisture barrier (a thick plastic) 3) Water-resistant drywall.

Is there anything else that's needed? Do I need to add a specific type of plastic? Do I need "backer board"?

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Confirming we are talking about your bathroom, not the shower/tub area.

In honesty it doesn't matter. Purple, green, regular drywall will all eventually act the same given moisture. Does it taking more time really help? Most building codes require purple/green drywall. So put that up to pass inspection. If you want to not have to worry about mold issues then you need to create a barrier.

Two heavy handed oil based primer coats are the most effective way to do this (Killz for example - tons of brands that can do this). This is why you see 100 year old bathrooms in great shape. The oil based paint creates a moisture barrier. Paint whatever you want over top of it. Depending on size of bathroom (smaller ones I do this for sure) I will either go oil based everywhere and for sure on all ceilings.

So to answer your question the steps are - insulation, drywall, oil primer, paint. You don't put a plastic barrier behind drywall unless you are worried about moisture from the other side.

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Not a sales pitch, I just saw some of this go in:

[Purple Drywall] is ideally suited to environments like bathrooms and kitchens that are vulnerable to the damaging effects of moisture... offers the advantages of traditional moisture-resistant drywall with added mold and mildew resistance in the core and paper

You need backer-board if you're planing to tile. Painted over the backer board should be a waterproofing membrane (my preference is Pro-Red). Tile in a vanity doesn't really need either of those; tubs and shower areas need both. Backer-board is the same as a tub's 3'x5' footprint. It sits on top of the screws holding the tub flange to the studs. It does not sit inside the lip of the tub. The tile closes this gap, but be sure to leave room for a healthy gout line, no less than an 1\8" regardless of the scheme. (flip your 1/16th spacers sideways)

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For our bathroom remodel, we went with normal drywall and Schluter Systems. While more expensive, I believe the water barrier for our shower, which rotted out before 7 years on a new build had elapsed, is now awesome. Before any tile went in, we filled our new floor pan 6" deep with water and let it sit over night. Not any moisture showing around the shower before we started closing walls!

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Insulation and vapor barrier are meant for exterior walls to keep moisture from coming in. (Well, you can add insulation for sound proofing reasons but that's another issue.)

You should use blue board (not green contrary to popular belief) especially around the tub shower area, but even that is discretionary in most cases. All that blue board does is resist mold; it is NOT water resistant. If you have significant water issues from a leak, poor ventilation, or other causes, it will crumble and fail the same as any drywall. It will last longer, though, so it's not a bad idea.

The best thing you can do is make sure to install a quality fan with proper ducting. Make sure your CFM is adequate and you should have no issues. If you want to go a step further you can apply a natural clay texture such as American Clay to the walls. It's naturally hydroscopic so it will pull an amazing amount of moisture out of the air and it looks pretty sharp too.

Insulation is a MUST in bathrooms, because they are protecting rest of the house from water--especially in case of any malfunction. Keeping that in mind insulation should be as close to the finishing layer as possible.

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