Replies

1. GBA Editor
   Peter Yost|Jan 20, 2019 12:02pm|#1

   Hi Kevin -

   Even back in the day when I was a full-time remodeler (1982 - 1989) and doing lots of bathroom work, we never used the green MR board. It simply does not stand up to moisture exposure. We always used either paperless gypsum board (typically with fiberglass mesh) or cementitious tile backer board.

   In areas with high moisture potential, just does not make sense to invest in quality finish materials and install those finishes on anything other than robust sheathing.

   In tile areas, we always used cementitious board; in bathrooms, kitchens and basements where the finish was simply paint, we would use a paperless gypsum wall board such as USG Fiberock. Yeah, it's heavy and hard to cut. But boy, is it tough.

   It looks as though nowadays USG has tuned Fiberock for flooring; take a look at USG Securock for walls.

   Peter

2. Expert Member
   Malcolm Taylor|Jan 20, 2019 01:41pm|#2

   Kevin,

   Yes you can use green board at 16" oc, and on exterior walls.

   I don't use green board at all. In areas where the surface may be exposed to water splashing I use either Denshield, or a cement-backer. The rest of the walls get regular drywall, the thinking being that the humidity in the room should be dealt with by ventilation. If moisture builds up to the point where it can affect the drywall, it is much too high for all the other materials, and much too high for the house.

3. 
   Roger Berry|Jan 20, 2019 02:26pm|#3

   I have also spent time ripping out failed green board during remodels. The worst mis-use of the material was a sad bathroom with tile applied directly to greenboard with a latex based adhesive. Worse, the job was not the result of a mis-informed weekend warrior. The general lack of water and mold resistance was also reflected in the non-tiled areas.

   About 10 years ago I used Georgia Pacific's DensArmorPlus which is one of the paperless wallboards, for a major basement remodel. It had a sandpaper like feel, which was a little odd to touch, but it taped out and painted just fine. There is also a DensShield material for tiling which I have not used. For a variety of reasons, the basement ran higher in humidity than typically desired, but no one has reported any mold issues since the redo.

   Like Mr. Yost, I put in cement board when tiling. I do, however, recommend a barrier coat over the cement board, like Red Gard at minimum for walls and floors, and full Ditra membrane if you are going to install a steam shower. The Ditra warranty does require non-modified thinset which some tile people will argue about. It remains important to understand that some cement boards, (at least historically) do not block water absolutely.

   Current materials may do better, but I have deconstructed different bathrooms that had extensive water damage behind or under what I think of as cement board, the gray crumbly stuff with fiber mesh embedded in it. It may be worth noting that I did not encountered Hardiboard on any jobs. That may be due to when the material became available or tile people sticking with what they know. Generally the bathrooms I was redoing were 12-18 years old. So original installation was 1978-1995 more or less.

   Tile grout will pass water to varying degrees, even when treated with silicone. Small pattern tiles have a relatively high ratio of grout to tile surface area and thus are prone to passing more water. One bathroom job had 1" mosaic tile on the floor. The subfloor was destroyed pretty much entirely in front of the tub and to a lesser degree by the sink. Both areas subject to wet feet or splashing mess from children. A membrane or barrier coat would have stopped the water from soaking down into the subfloor. This "professional job" was set directly to the subfloor.

   Thinset will also absorb and hold water more than expected and duty cycle can exacerbate the retention. Part of the reason Ditra warranty requires unmodified thinset materials is due to long term saturation, which can cause bonding failure due to softened latex modifiying compounds. For some reason steam showers are particularly aggressive at adding moisture. Bathmats that stay on the floor also tend to keep areas of grout from drying thoroughly between uses.

   Putting plastic film behind the cement board is both useless and dumb. What water doesn't get through the many nail or screw holes, will be held in the cement board, where massive mold colonies will grow on the plastic/cement board faces. I dealt with one bathroom done this way. It was a really disgusting mess to deal with. The cement board was totally saturated over half way up the height and moldy damp the rest of the way. The tile grout of course was black with mold and repeated attempts to clean it were futile. All the studs were spongy at the tub flange level. Needless to say that job quote had to be redone.

   Regarding your question about 12" O.C. ceiling joists - I truly can't imagine why anyone would tell you that. More relevant is the insulation question. Certainly preventing moisture penetration into the wall is necessary. A properly set non-paper wallboard really should not be allowing significant moisture through. Proper venting with a fan is perhaps the best choice to handle that problem. If the wall is under insulated, perhaps others will weigh in on best practices from experience. Generally, I did not find much to note on the non tub/shower walls exposed, whether interior or exterior.

   Non-shower /浴缸墙不应该开心g the bulk water or intense moisture occurring inside the shower/tub enclosure anyway. Non-cementious tile setting materials, like you would use for a kitchen backsplash, should behave fine for those areas. Providing a clean transition between tile set with the two materials is more a design or tactical problem. Don't plan on one continuous plane of tile extending out of the tub area to the main bath. That way there is no "bump" or bend in the tile plane.

   One last thing often ignored or overlooked is the interface between cement board and tile surfaces and painted areas. Typically failures will occur in the 18-24" band above the shower door line or the ceiling if the tile goes all the way up. Ordinary drywall compound will soften if wetted, even through paint if condensation sits long enough. Sealing the line between painted wall and tile with silicone sealant is pointless and results in an annoying little rubber snake of ugliness to deal when the paint bubbles or falls off. Insist on the taping for these areas to be done with setting type compound such as Durabond. Do not top or finish with all purpose. The tapers may fuss, but be insistent. A little more sanding may be required as it is harder, but I have never had a call back for my work. And use a siliconized acrylic very sparingly to tidy up the tile to paint edges. They are still much easier to deal with than paintable silicones down the line.

4. Expert Member
   Zephyr7|Jan 21, 2019 01:30am|#4

   我使用greenboard skylight wells as a little extra protection from condensation. I don’t really trust it for much else. Greenboard is only minimal extra protection, and even that is only on one of the faces. I use cement board for tile, I just don’t trust drywall for that.

   Bill