Vapor barrier with new shower

Replacing a tiled shower with a prefab type shower (exact model not known yet). Two walls are exterior. The tile was installed directly over the lathe and plaster. There wasn't any mold or water damage when I tore out the shower.

My question is should I add a vapor barrier when I install the new insulation?

Thanks

Where are you located, your climate makes the difference?

So, "prefab shower" means what, pan only or entire stall/enclosure? If the latter, I shouldn't think you'd need any
more barrier than the impervious plastic wall of the shower itself. If pan only, of course you'd want a moisture
barrier behind the tile, and in hotter climes a vapor barrier on the outside of the wall structure would be called for.

Don't install poly sheeting unless local code requires it. Vapor barriers in an Ohio climate are pretty much bad-news from the 1980s, and if the wall assembly had managed to dry well enough to avoid mold with the tile adhered to the plaster (wicking moisture into the wall toward the lath through the grout seams) it should be able to handle whatever you through at it (within reason). Some sort of vapor retarder would help if you've stripped it to the studs though. Fan-fold 1/4" XPS (siding underlayment) has a semi-impermable polymer facer on it (~0.6-0.7 perms) which would block any liquid moisture and slow vapor diffusion during the long-shower extremes, yet still allow the assembly to dry toward the interior (the way it could through the plaster & tile.)

If you can accomodate the space, half-inch un-faced XPS is about 2.3 perms (and R2.5) and 1-inch is about 1.2 perms (and R5). Putting R5 of thermal break on 2x4 studs raises the whole-wall R value by about 50%. (From about R10 to about R15, after the thermal bridging of the framing is factored in.)

kreemoweet said:

So, "prefab shower" means what, pan only or entire stall/enclosure? If the latter, I shouldn't think you'd need any
more barrier than the impervious plastic wall of the shower itself. If pan only, of course you'd want a moisture
barrier behind the tile, and in hotter climes a vapor barrier on the outside of the wall structure would be called for.

Click to expand...

The shower will be a complete unit that mounts to the studs. I was worried about trapping moisture between the plastic sides and the vapor barrier.

Thanks