Hanging
Bathroom and kitchen drywall: where moisture board goes, and where not
Green board is for splash and steam, not for water. Knowing where the line falls between gypsum and tile backer is most of what there is to bathroom drywall.
Three kinds of wall in a bathroom
A bathroom has three zones, and each takes a different board:
| Zone | What it sees | Board |
|---|---|---|
| Tub and shower walls, to the top of the tile | Direct water | Cement board, fiber-cement or another approved tile backer |
| The rest of the walls and the ceiling | Steam and splashes | Moisture- and mold-resistant gypsum board |
| Walls outside the bathroom | Normal room air | Regular gypsum board |
The International Residential Code (R702.4.2) requires cement, fiber-cement or glass-mat gypsum backers, or another approved backer, as the base for tile in tub and shower areas. It also says water-resistant gypsum board is not to be used in places directly exposed to water or continuous high humidity, nor over a vapour retarder in a shower or tub compartment (R702.3.7).
What moisture-resistant board is
Moisture-resistant, or mold-resistant, gypsum board has a core and facing treated to absorb less water and resist mold growth. The old version, with green paper facing, is still called green board; newer versions have paper or glass-mat facings and are rated for mold resistance under ASTM D3273. It weighs a little more than regular board, about 54 lb for a 4 × 8 sheet of 1/2 in.
It is for rooms with steam and occasional splashes: bathroom walls away from the tub, kitchens, laundry rooms, basements. It is not waterproof and not a tile backer for wet areas. Under water, the gypsum softens whatever the facing.
Counting a bathroom
An 8 × 6 ft bathroom, 8 ft ceiling, one door, one window, a tub with a tiled surround
- Walls: 28 ft × 8 ft = 224 sq ft, less 35 sq ft of openings
- Tub surround in cement board: 60 sq ft, 5 sheets of 3 × 5 ft with 15% waste
- Remaining walls in moisture-resistant board: 129 sq ft, 5 sheets of 4 × 8 with 15% waste
- 140 screws for the gypsum board, 1 pail of compound for a Level 4 finish
A walk-in shower instead of the tub, with 63 sq ft of tiled walls, gives the same order: 5 sheets of each. The moisture-resistant board calculator splits the room between the two boards.
Waste runs higher in bathrooms, 15% here, because the rooms are small and full of cut-outs: the window, the door, the vanity plumbing, the fan. The whole house count is in the sheet count method, and the drywall sheet calculator handles the rooms around it.
The joint at the tub
The board stops about 1/4 in above the tub flange or shower pan lip, and is never bedded in the flange. The tile backer comes down over the flange to the same line, and the tile covers the joint. Water that gets behind the tile should run down the face of the flange into the tub, not wick into the board.
Where the cement board meets the gypsum board, at the edge of the tile, the joint is taped with alkali-resistant mesh and setting compound under the tile, and finished with ordinary compound beyond it.
Waterproofing
Cement board is not waterproof either: it does not soften, but water passes through it. Behind a tiled shower, a waterproofing layer goes either behind the backer, as a membrane or sheet on the studs, or on its face, as a liquid-applied or sheet membrane under the tile. Follow the backer maker’s system, and do not use both a vapour retarder behind and a membrane in front, which traps water in the wall.
Ceilings
The bathroom ceiling sees the most steam. Moisture- and mold-resistant board is the usual choice; some products have limits on ceiling framing spacing, and the older water-resistant board had tighter ones in the IRC, so check the board maker’s data for ceilings on joists 24 in apart. A good exhaust fan, ducted outside, protects the ceiling more than any board.
Kitchens and laundry rooms
Kitchens need no special board by code. Moisture-resistant board behind the sink and range, and in the whole room in a busy kitchen, is cheap insurance; the backsplash tile goes on it directly, since a backsplash is not a wet area. Laundry rooms are similar: moisture-resistant board, and a pan under the washer.
Finishing
Moisture-resistant board is finished like regular board with ordinary joint compound. Behind tile outside a wet area, Level 2 is enough: tape and one coat give a flat, sealed surface for the thinset. Painted walls get Level 4 and a primer, then a bathroom paint with a mildew-resistant additive. The joint compound calculator gives the pails, and the guide to levels of finish explains the levels.
Screws for moisture-resistant board are the usual drywall screws, from the screw calculator. Cement board takes its own corrosion-resistant screws, about 8 in apart, by its maker’s instructions.
Common mistakes
- Green board in the shower. It is not a tile backer for wet areas.
- Board bedded in the tub flange. Keep it 1/4 in above.
- A vapour retarder behind the shower backer and a membrane in front. Trapped water.
- Paper tape under tile on cement board. Use alkali-resistant mesh and setting compound.
- No fan. Steam does more harm than splashes.