Using 1/2" plywood as a backer for tile

[magoo22]

Good day,

I just joined this form as I have found a great deal of info here. I am currently redoing our master bathroom and will be putting in a custom tile shower, tile floor, tile tub surround and will be tileing a wall behind the tub. I am using Schluter kerdi, pan and curb for the shower and also plan on putting kerdi on all vertical tile surfaces. The one thing that I have not found information here on is if 1/2" plywood is ok behind the kerdi. The tile store's installers I am dealing with recommended that I use plywood rather than cement board or drywall. I understand the drywall reasoning but am not sure on the plywood being better than cement board. They said it would be easier to install than the cement board.
The wall that the tiles will be going on is an exterior wall 12' l X 8'h. 4 ' of the exterior wall will be the shower with a glass enclosure to separate the tub. I will have 2 4' walls running perpendicular to the main wall that will be the end of the shower and tub. I have a good vapour barrier on the exterior wall but I am wondering if having plywood in between the vapour barrier and the kerdi could trap moisture if there is any leakage and ultimately cause the plywood to rot.
Sorry for the long post and thanks in advance for any insight you may offer.

Grant...

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[gueuzeman]

Plywood- bad.

But I'm no kerdi expert, but I expect others to come soon and agree with me, and they'll be able to tell you why.

gueuze- advice lottery player

[jgleason]

Regular drywall over everything and then Kerdi it.

[GabeS]

No plywood for kerdi. Use regular drywall if it's going to be covered with kerdi. It says it in John's Ebook. If you don't believe me then you could download it for ten bucks and see for yourself.:cool:

[cx]

Welcome, Grant. :)

I'd recommend you purchase and download John Bridge's Kerdi eBook before you go any farther. Best ten-dollar Kerdi tool onna market.

In the alternative I'd recommend you visit the Schluter website and familiarize yourownself with the requirements and limitations of the products.

From either source you'll learn that Herr Schluter does not want you should install Kerdi over plywood in any application.

Kerdi really wants to be installed over sheetrock, as Joe indicates. It can be installed over CBU, but it still wants to be installed over sheetrock.I have a good vapour barrier on the exterior wall but I am wondering if having plywood in between the vapour barrier and the kerdi could trap moisture if there is any leakage and ultimately cause the plywood to rot.I recommend you remove or defeat that vapor barrier before you install your wallboard for a Kerdi installation, regardless the type of wallboard.

My opinion; worth price charged.

[GabeS]

Is that a vapor barrier you are talking about on the exterior wall or is that an air barrier (tyvek, tar paper). Air barriers allow vapor to escape to the outside. Vapor barriers don't.

[magoo22]

Thanks for the responses folks,

The outside wall of the house does have tyvec under the siding. The inside of the wall has 6mm poly vapor barrier. So if I understand correctly I should get rid of the 6mm poly and the kerdi will become the vapor barrier for that section of wall? And I will go with regular old drywall instead of plywood or concrete board.

On that note should I also use sheetrock on the tub base and put kerdi over that as well. The end of the tub base will be inside the shower on one end and will be done with kerdi anyways. I think it would only add 16 sqft +/- of kerdi to the job.

[GabeS]

correct about removing the plastic where the kerdi is going. Kerdi acts as a vapor barrier, so there's no need for the plastic and also don't want to create a sandwich for the vapor.