Completely confused about vapor barriers for bathroom

[beckypeep]

My house was built in 1955 so it does not have a vapor barrier and I am completely confused about what I should install. Below is some information about my bathroom:

• the bathtub is on an exterior wall
• the house is located in the San Francisco Bay Area (temperate weather)
• after removing the tile and blown in insulation there was significant moisture on the exterior wall behind the insulation as well as mold.

I have read conflicting information on what I need to install. I have done minor “fix it” things around the house and now I am worried I am in over my head.


Thanks so much,

Becky

[Dana]

Adding 1/4" of fan-fold XPS (sold as siding underayment at some home-center box-stores) between tile-backer and the studs keeps the bulk of the moisture from showers etc in the backer & tile, but still allows some vapor permeation to occur so as not to lock water from exterior moisture drives in the wall cavity- it can still dry (slowly) in either direction, and does not create a moisture trap the way poly sheeting would.

Was the mold primarily on the exterior sheathing, or primarily on the interior edge of the studs? (Sometimes it's hard to tell if it's everywhere.) What type of siding & insulation does this house have?