I copied this over from “Feedback on FHB” because I’d like some more input. I’ve seen some strong opinions on this subject, and the new issue of FHB povides more food for thought:
On p. 91, Tom Meehan states there is no need for a vapor barrier behind the cement backerboard in a tiled shower. I’ve seen comments here on BT that a VB is essential. To me, the need seems obvious on an exterior wall in a heating climate, but what about interior walls? Assuming the joints in the board are finished with fiberglass tape and thinset mortar, and the tiles and grout are intact, only a limited amount of vapor, and no liquid water, could get through. Assuming (again) the house is well insulated and there are no cold surfaces for condensation, how much of a problem could this vapor cause? Has anyone seen real evidence of a problem that goes beyond theory?
Al Mollitor, Sharon MA
Replies
The problem is not so much having a vapor barrier as having a water barrier. Cement board is impervious to water ie: water will not damage it. But it is water permeable; water can pass thru it. No matter how good your tile instalation is, eventualy water seeps thru micro cracks (or not so micro)in the grout. And slowly your framing begins to rot. Having a vapor barrier virtualy eliminates this issue. BTW there are new products(last ten yrs) that do not require a vapor barrier because they are already water proof. One that comes to mind is Gergia-Pacifics 'Dens glass gold'. It is a sort of glorified sheetrock that has a silicone treated core and waterproof membrane. You just have to apply a water proof mastic to the seems and you are done. Comes in 4x8 sheets instead of 3x5 and it is lighter and easier to cut.
Hi Mojo, I'm no expert around here but I just finished tiling a tub surround in my bathroom last week and spent the extra $$ on the Dura-Rock instead of the aqua board (is there anyone around here that would actually recommed aqua board instead of cement backer??).
I had one exterior wall to deal with and 2 interior walls. Filled the joints as you said with mortar and tape but for the minimal cost of adding some 6 mil poly to all the walls between the substrate and the studs - why not? Just a little more insurance. By the way, whomever installed the old tub surround (circa 1960) did a great job - not one drop of water damage - for once a nice surprise!
Out of curiosity, are you using any sort of grout sealer? I bought the 'Tile Lab' Teflon treatment...not sure how good this product is.
Good luck...Mike
What is aqua board??
Thanks.
Rich Beckman
Another day, another tool.
Sorry Rich, I was referring to the water-resistant drywall when I mentioned aquaboard. Hope this helps...
Just got the latest issue of JLC(Dec 2003) and on page 86 are several new products that address your question. One is only 60c a sqft.