Vapor Barrier

1) Which is better for a vapor barrier in a bathroom? Regular drywall with a 4 mil plastic vapor barrier or greenboard? My understanding is that you shouldn't use both greenboard and a vapor barrier.

2) Only exterior walls and ceilings below attic spaces need the vapor barrier right?

3) How about behind shower tile. Does 4 mil plastic and cement board work?

First off, forget the greenboard. Outside of the shower/tub area, regular drywall is fine. It is no longer in the national codes, but your area may not have updated. there are some newer fiberglass coated sheets (i.e., no paper which was the problem - green board still had paper, but it was treated) that work better, but are still probably overkill. Keep in mind, you really should use an exhaust fan or open the window (one or the other is required to be at least present in a bathroom, but doesn't dictate you have to use them!).

Plastic should go behind the cbu in a tub/shower area, lapping over the pan or tub flange. Roofing felt works as well, as well as a surface membrane (such as a paint on or sheet material). A vapor barrier is generally required in exterior or attic spaces and interior rooms, and again, plastic is fine for this.

Drywall Thickness

Does national code say 1/2" is fine for ceilings?

Unless you want a fire rating (which may be required), 1/2" is fine. Greenboard, if used, required 12" supports on a ceiling, another reason to forget about it.

How would I know?

Is fire rating required?

That's up to local codes. Often required for multiple family dwellings, not often for single family homes. A question for your local inspector.

unless I am confused a vapor barrier and MR sheetrock serve two completly different purposes. I agree with who ever said forget the greenboard its not necessary in most instances and the new purpleboard is better anyhow

Lou

Ditto what Lou said - go purple, you'll never go back.

I don't know where you got that bit about greenboard no going with a vapor barrier, but it's BS.



Personally don't care for plastic sheeting as a vapor barrier, it puts the barrier in the wrong location - behind & right up against the sheetrock. Think about it, is that really where you want condensation to happen?

It also doesn't seem to stop moisture migration very well - there's always tears, punctures, and the seams...

It's pretty good at concealing leaks, though: meaning, if your roof has a problem, you only find out about it after it's much much much too late...


Any remodeller can tell you horror stories about plastic vapor barriers. Unless you live in Canada or AK, it's more of a hazard than an advantage. Look into vapor-barrier primer, instead - every reputable contractor I know of, switched over years ago.


Where are you located?

Location

I'm in Minnesota.

The no green board with a vapor barrier came from the permit office, but I'm pretty much skeptical of everyone at this point. Can't get the inspector to return my calls.

The only place I'm going to use the vapor barrier is on a small section of ceiling where the new bathroom is. The rooms adjacent to it are plaster and lathe, with no barrier. I'll have a sloped ceiling that will have spray foam insulation, which I don't think requires one. The other place would be behind the shower tile.

nelsonba said:

I'll have a sloped ceiling that will have spray foam insulation, which I don't think requires one.

Click to expand...

Closed-cell foam (ie. polyurethane) doesn't. Open-cell (demilec, Icynene) does.

The typical way to vapor-barrier open-cell foam is... a coat of paint sprayed directly onto the foam.

nelsonba said:

Is fire rating required?

Click to expand...

Fire rated sheet rock is required in garages, I believe... and it's 5/8" instead of 1/2".

Don't quote me on this, it comes from old brain cells and second hand knowledge from a fly by night subcontractor.

Rancher

Around here, they require the fire rated stuff between units and on the top floor ceiling (but not within) in a multiple family dwelling (like the townhouse I live in).