Actually greenboard CAN still be used in tub and shower compartments as a tile backer.
'Best Practice' notwithstanding, wherever the 2003 International Residential Code is in force, it is 100% permissable and Code compliant to use moisture resistant gypsum board as a backer for tile PROVIDED no vapor retarder is used under the backer and all edges are sealed according to the manufacturer, and the material complies with either ASTM C1178 or ASTM 630.
Section R702.4.2 (2003 IRC) GYPSUM BACKER: "Water-resistant gypsum board shall not be installed over a vapor barrier in a tub or shower compartment. All cut or exposed edges, including those at wall intersections, shall be sealed as recommended by the manufacturer."
The reason no vapor barrier can be used behind greenboard is because while greenboard is only "moisture-resistant" and not "waterproof", as already mentioned, and although it CAN withstand occaisional wetting and even saturation...Moisture resistant drywall cannot withstand CHRONIC wetting and can breakdown under such conditions.
Using a vapor retarder such as black paper, or plastic sheathing or other material under greenboard in a shoer compartment can create a "Double Vapor Barrier" which can chronically trap moisture within the drywall sheet. This will contribute to its failure because once internally wetted, moisture cannot escape from within the board because of the double vapor barrier on either side and be allowed to dry.
Eliminating a vapor retarder from behind greenboard can eliminate or reduce damage from leaked or condensed water.
Now, that is NOT to say that one SHOULD use greenboard.
I wouldn't do it and do not recommend the practice.
Concrete backer products are indeed superior for the application.
But it remains completely permissible and Code compliant to use "greenboard" whenever and whereever the 2003 version of the IRC is still or will continue being used.
That said, the 2006 International Residential Code has DISALLOWED the use of tradtional 'greenboard' but still allows other types of gypsum products as tile backers in showers:
R702.4.2 (2006 IRC) CEMENT,FIBER-CEMENT, AND GLASS MAT GYPSUM BACKERS. "Cement, fiber-cement or glass-mat GYPSUM backers in compliance with
ASTM C 1288, C 1325, or C 1178 and installed in accordance with manufacturer's recommendations shall be used as backers for wall tile in tub and shower areas and wall panels in shower areas."
So gypsum products are STILL allowed by Code as tile backers, but they must be 'glass-mat' types conforming to the ASTM standards.
Personally as a builder, I'm in 100% agreement with everyone else that one shouldn't EVER use any gypsum product as a tile backer in a shower, but personal preference aside and as a Code Official, it remains legal in most places in the US to either still use 'greenboard' or 'glass-mat' drywall for that purpose, and I would have to approve the use of greenboard or glass-mat gypsum if any builder or homeowner chose that method of construction. |
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