proper use of greenboard

[econguy]

hi all

i've read on USG's website that water-resistant gypsum board should not be installed over a vapor retarder in a shower or tub compartment (i'm assuming because of the direct and prolonged water contact) OR when covering with an impervious finish, such as ceramic tile, since it would create a double vapor barrier.

this would seem to imply that it's ok to install greenboard over a vapor retarder - like insulation kraft facing - when NOT covering with tile, and when NOT in a tub/shower surround. but wouldn't that also create a double vapor barrier?

[geniescience]

no. It's not all because of a double vapor barrier, since tile and grout do not make a vapor barrier layer.

It's because that drywall has an additive that make it stay rigid longer when wet or damp, making it "resistant" to moisture up to a point and not beyond that point which a shower wall or the bottom edge of a tub surround wall can easily reach. Then the drywall turns to mush.

Before that happens it can become fodder for mold too. Even with the newest treatment.

So greenboard is not all it's cracked up to be. Complaints and problems have been known for years.

david

[leejosepho]

... would seem to imply that it's ok to install greenboard over a vapor retarder - like insulation kraft facing - when NOT covering with tile, and when NOT in a tub/shower surround. but wouldn't that also create a double vapor barrier?

What would be the second so-called "barrier"?

The idea is to be sure the board can breathe somewhere, and ventilation can be crucial to that.

[econguy]

What would be the second so-called "barrier"?

i guess that's why i'm confused as heck on the subject

i'm in the middle of using greenboard for the walls and ceilings in my bathroom, EXCEPT for the immediate tub and shower surround where i'll attach fiberglass panels directly to the (interior wall, non-insulated) studs. i don't plan on installing tile on any of the greenboard, just priming and painting.

the ceiling - which is directly below attic space - as well as the one exterior wall are both insulated with kraft-faced insulation, vapor retarder facing in.

my question is, do i need to remove the greenboard i've already installed on the ceiling/exterior wall just to remove the kraft-facing?

before posting on this forum (but halfway through my job - i had always assumed greenboard was the default for bathrooms) this is what i had read in drywall books, read from USG's website, etc., which is what got me confused to begin with:

Scenario #1:
IF using greenboard as a backer for an impervious finish such as ceramic tile,(which i now know isn't a good idea...use CBU, etc. for the backer), the greenboard should NOT be installed over a vapor retarder like kraft facing since that would cause a double vapor retarder and trap moisture. i'm assuming retarder #1 is the kraft facing, and retarder #2 was the tile/grout (or the other "impervious finish" referred to by USG). this made me assume that it's ok to install greenboard over a vapor retarder when NOT covering with tile

Scenario #2:
IF installing greenboard on the CEILING - even with no tile - it should NOT be installed over a vapor retarder since that would ALSO create a a double vapor retarder. in this case, retarder #1 would the kraft facing, and it implies retarder #2 is the greenboard. but that would mean that in Scenario #1, it's actually a triple vapor retarder (kraft facing is #1, greenboard is #2, and tile is #3)?


Any help on this topic would be much appreciated

[leejosepho]

Scenario #2:
IF installing greenboard on the CEILING - even with no tile - it should NOT be installed over a vapor retarder since that would ALSO create a a double vapor retarder. in this case, retarder #1 would the kraft facing, and it implies retarder #2 is the greenboard. but that would mean that in Scenario #1, it's actually a triple vapor retarder (kraft facing is #1, greenboard is #2, and tile is #3)?

my question is, do i need to remove the greenboard i've already installed on the ceiling/exterior wall just to remove the kraft-facing?

I believe we are hearing all of that the same way, and yes, it seems the kraft facing might not be ideal ... and I am not sure what I would do if I were in your place. More than likely, however, I would leave things as they are and depend upon the exhaust fan to pull excessive moisture from the room and maintain a low level of humidity. I will hopefully be installing the board in my own new bathroom in another week or two, and that is my own general plan.

[geniescience]

personally i wouldn't worry. So far you have been quoting corporate info. Using basic same internet search skills, find out what problems greenboard presents. I don't perceive kraft (paper-based) facing as much of a vapor barrier. Lots of h2o, moisture, humidity, water molecules, can travel through it, slowly. Moisture migrates.

greenboard remains rigid longer when wet, while regular drywall would sag. That is its characteristic property. AFAIK. I used greenboard in a ceiling since it might get wet from a real leak above. Not because it just happens to be in near shower humidity, a steamy environment for all of 20 minutes.

personally i wouldn't worry. So far you have been quoting corporate info. Perhaps USG doesn't know for sure either what they recommend. They have put newer products out that claim to perform better too... perhaps one day it'll be easy to admit that greenboard didn't do anything well enough to be the right product for any of the needs. It's just regular drywall, with an additive.

Would you install regular drywall there, where you are now worried? Yes. Then why not leave the drywall that has a water resistant additive in it? If it gets soggy from a leak above, it'll remain rigid while mold grows in it.

david

[leejosepho]

... why not leave the drywall that has a water resistant additive in it? If it gets soggy from a leak above, it'll remain rigid while mold grows in it.

Quite reassuring, eh?! But yes, we do definitely agree there.