Is this a good idea – 6 mil poly in the shower?
netrate
| Posted in Construction Techniques on
I found this thread and at first thought it was a smart idea (post #17):
http://www.johnbridge.com/vbulletin/showthread.php?t=97468&page=2
But now that I think about it, I wonder if this would cause mold problems because the walls can’t breathe. the picture is attached. He has put 6 mil poly sheets along the walls from ceiling to floor behind the concrete wonderboard. If this a good idea or not?
Replies
I can't even guess about condensation. But it's a waste of time. The poly will leak like a seive at every penetration. If this is the only water barrier it won't do any good at all. I prefer roofing felt with plenty of laps on a full float, because it seals around staples and nails. Properly installed Denshield is rated as a moisture barrier without anything behind it.
I am using wonderboard and then after that is done, RedGuard (overkill I know) to finally seal it. I just thought it might be worth investing in the 6mil poly for extra protection on top of everything else.
If you use it I think it would be wise to put it under/behing everything else, not on top of it.
So you don't think this would be a mold problem or even a breathing problem underneath? I was worried, similar to insulation, if condensation wouldn't have a chance to evaporate and then mold would develop.
Well, if you put it on top of everything else, yes.
Condensation on anything!
You only get condensation on a surface that is below the "dew point"
Think about your windows, in the summer there is no condensation because its warm outside.
In the winter there is condensation at times, because at times the outside temperature causes the room side of the glass to be below the "dew point" of the air inside the room.
The way you stop condensation is simple. You back the plastic/glass/steel whatever with a decent sheet of insulation, this keeps the plastic etc warm, with both sides above "dew point" And there is no condensation.
Your question is really all about keeping the water out of the walls - and that has already been answered.
it depends...
If you're not using a topical membrane (like RedGard or Hydroban or Kerdi or...) over the cement board, then yes, it is a good idea to put 6-mil poly behind the cement board.
But you don't want to put poly behind the cement board and then another membrane on the front of the cement board.
If you are using a tile backer board that has a water or vapor-impermeable surface built in to it, again, that is a membrane. So you don't want another membrsane (the 6-mil poly) behind that backer board.