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Community and Q&A

What type of board in bathrooms

Chris Johnston | Posted in Interior Design on

I am building a new home and have a question about wall and ceiling board in bathrooms. My confusion is over where to install normal drywall, where to install greenboard or moisture resistant drywall, and where to install non-paperfaced board such as hardibacker 500 cement board. Any advice appreciated.

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  1. Riversong | | #1

    Greenboard (MR drywall) in bathrooms (requires 16" oc ceiling framing) and backer board on tiled surfaces. In areas with uncontrolled moisture or wicking moisture (such as sub-grade concrete walls), non-paper faced drywall is wise. There's already too much cement in a typical home and no need to use it for wall board as well.


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  2. Robert Susz | | #2

    Green board is fine and I see it in every new home I go to, and many existing homes I go to (that have severe moisture problems).

    I have been spec'ing, and just installed in my own home, regular, run of the mill paper faced drywall.

    I do this for three reasons:
    1) Tinting it green doesn't fool the mold
    2) The cheapest fan I would ever install is a Panasonic 90cfm fan
    3) Tile should never be installed directly to cement backerboard with thinset mortar. I use Kerdi membrane systems by Schluter. (maybe spelled wrong)

    My new shower has a curved wall and I actually did that with 4 layers of 1/8" luan plywood. Then applied the Kerdi on top.

    Moisture from showers should be ventilated.

    Tile should have a membrane.


  3. Riversong | | #3

    Robert S,

    MR drywall has sodium pyrithione or other mold inhibitors in the gypsum and the paper facings - it's not just green tint. The gypsum and paper facings are also more water resistant.

    Cement backerboard is specifically designed for direct application of tile with either thinset or mastic. ½" board and thinset is best for floors, while ¼" board and mastic is fine for walls. The only place a membrane would be necessary is in a tiled shower base.

    Floors and showers have been successfully tiled for generations on full floated mortar bed. Cement backer board is the modern alternative to a full mortar bed. But I would recommend the fiberglass-faced cement boards and not the cellulose/cement backer boards for floors.

  4. Robert Susz | | #4

    Hi Robert,

    I am aware of the ingredients added to MR board. It still won't make up for an inadequately ventilating bath fan with a steam shower. The first time I used the regular drywall was in my own bathroom in 1994. Not a hint of moisture or mold after 16 years. That's more than can say for the greenboard bathrooms with the junk fans after 4 to 8 years.

    Also, based on the number of leaky, rotted, or otherwise failed tile shower enclosures I have investigated, I decided that I like rolled membrane waterproofing. maybe when a "shower" was a pathetic showerhead in a rectangular box it was all good. Showers have changed, we now have curves, seats, and multiple heads. Membrane waterproofers will work here where others are failing.

    I have to admit I am frankly surprised you would suggest otherwise. I expected you to advocate for slabs of solid slate or something. 8-) The "old" method relies on mortar for waterproofing. If this is a bad idea for cultured stone, why is it a good idea here?

    The only real criticism anyone can level at membrane systems is that they are "expensive and uneccesary." But after 11 years in Building Science it seems that these tend to be the type of products I seem to like. 8-)

    The walls of the shower is the one place that gets a tropical rainstorm at least once a day. many times with limited drying potential. I see no reason why a Weather Resistant barrier should not be used there.


  5. Riversong | | #5

    Robert S,

    I don't know who you think you're arguing with, but it certainly isn't me.

    I said that membranes would be appropriate for a shower base, but that they are unnecessary outside the shower and backer board/thinset is more than adequate for tile installations (you said never to do this, which is patently absurd).

    And it's also reductio ad absurdem to say that MR drywall is inadequate if the rest of the moisture management systems is inadequate. Nothing will resist mold if moisture is not properly evacuated, but that's irrelevant to this discussion.

    And to suggest that a normal enclosed shower is "pathetic" belies your bias against simple functionality and towards wasteful extravagance. In a program of extravagance, high-priced membrane systems make perfect sense. Those who prefer simple know how to build a shower that doesn't leak without resorting to high-tech "solutions".

  6. Robert Susz | | #6

    My reference to pathetic shower heads was directed towards 60's and 70's era water conserving units before manufacturers really figured it out.

    I'll admit I'm slightly offended you think I'm biased towards extravagance. I'm not sure why you interpreted it that way. None of my showers have more than 1 Moen showerhead.

    But my point is that better stuff exists these days, what's new is not necessarily bad. I find tile bonded with thinset to cement backerboard (on walls) to be unacceptable in a shower enclosure for reasons stated above. Everywhere else in the house is fine stuck to drywall. The kerdi system uses a membrane on the base as well. That would be really crazy to not use a membrane on the floor.

    I certainly don't advocate for extravagance, but I'll stick with membranes on the floor and walls of showers. regardless. Otherwise you're left with mortar and caulk to handle the walls. I'm not comfortable with this.


  7. Robert Susz | | #7

    I guess I should have clearly stated in my previous post that I most of my comments related specifically to the use of membranes on the shower wall system. And once you go to mebranes, the underlayment doesn't need to be moisture resistant anymore.

    I only encountered 1 shower base that did not have a membrane under the mortar bed. It was a slab on grade house. His tile guy told him he didn't need it, but it turns out he did.

    As a side issue though. I don't like the mortar bed on top of the membrane because it seems this creates a large wet reservoir under the tile. This would slow drying.


  8. Garth Sproule | | #8

    Good point about the mortar bed being a reservoir...I have done showers with and without Kerdi. The Kerdi system along with their drain assembly is the way to go. Not that expensive either considering that ordinary wallboard is all you need for a backer board for the walls.

  9. Chris | | #9

    I am a little confused after the discussions between the Roberts. My shower will have marble walls. Can I attach it with adhesive directly to backer board, or do I need a membrane? Also, the shower base is a heavy solid piece marble unit with obviously a hole cut for the drain. Do I need to put backer board or a membrane under it?

  10. Riversong | | #10

    And once you go to mebranes, the underlayment doesn't need to be moisture resistant anymore.

    I'd like to see that non-mold-proofed substrate after an air-conditioned summer of condensation on the backside of the membrane when installed on an outside wall.

  11. Robert Susz | | #11

    Robert, you assume my walls have air leakage?

    Seriously though, mold will grow on anything. Green board, cement board or not. Just ask Terry Brennan. 8-) In your air conditioning scenario, mold will also grow on the rest of the walls that are not greenboard. Particularly if their perm rate has been lowered by things like paint, wallpaper, artwork, etc.

    Chris, are you using a cast "marble" base? We have a manufacturer locally that makes these. Or are you using a solid slab of actual, solid marble? Just curious, as I haven't seen a real piece of marble yet.

    My answer is that I would use a membrane system under the base and the walls. Though details may vary based on what your base looks like. The Kerdi flange can be recessed into your subfloor and the membrane bonded to it. Then a thin bed of mortar and then your base. I suspect you can get your drain provisions to integrate with the kerdi flange properly without much issue.

    Thank You Garth.


  12. Brett Moyer | | #12

    Robert R,
    So are you saying green board is ok in shower and baths? I think other experts would disagree with you.
    The following was taken from an article written by Joe Lstiburek:
    "And then what do we do? We color the paper green like we are going to fool the mold. And then where do we put the greenboard? In a shower. Let’s put green paper in a shower and cover it with “lick and stick” tiles—there’s a good idea. Coloring the paper green to protect it from mold is like putting lipstick on a pig—at the end of the day you still have a pig... The waxes added to protect the paper (on green board) from liquid phase water—to meet a ridiculous ASTM spec—are “mold food.” When products are optimized to meet arbitrary ASTM specifications rather than optimized to meet the “real world” problems result. Of course paper in a shower regardless of color is pretty dumb."

  13. Riversong | | #13

    Brett: So are you saying green board is ok in shower and baths?

    Not I, but Robert S did say that, as long is its buried behind an impermeable membrane, which makes it vulnerable to backside condensation during the AC season.

    Robert S: you assume my walls have air leakage?

    Not at all, it requires only vapor diffusion to create a summer-time condensation problem behind impermeable inside membranes.

    Condensation, moisture accumulation, and mold are not problems with gypsum walls in air-conditioned houses except when they are covered or backed by vapor-impermeable membranes.

    It's just as foolish to put MR drywall behind a Kerdi membrane as it is to use 6 mil poly in a hot climate house.

  14. Chris | | #14

    Defininitely a cast marble base. I'm not rich enough to have real marble :-)

  15. Robert Susz | | #15

    Chris, The details for the kerdi drain system are available online. And there are many videos on youtube. Integrating a membrane system with your base shouldn't be a problem.


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