R702.3.7

Section R702.3.7 is titled 'Water-resistant gypsum backing board' and covers the requirements for gypsum board used as the backer for non-absorbent finishes. Standards which the gypsum board must conform to, clarification that it is allowed for use on ceilings, disallowance of a class I or II vapor retarder between the gypsum board and framing, and a requirement to seal edges not protected by the face paper is established. The purpose of this section is to prevent premature degradation of water-resistant gypsum board and moisture from becoming trapped in the wall.

The model shows a partial bathroom including the tub area which has been surrounded by water-resistant gypsum board, including the ceiling, (which is labeled as being compliant with one of the listed standards), and ceramic tile. Notice that the class I vapor retarder (plastic polyethylene sheet) has not been installed behind the water-resistant gypsum board in the tub area.