Moisture-Resistant Drywall: Green Board vs Purple Board vs Cement Board — Where Each Belongs (2026)

Three Products, Three Jobs

"Moisture-resistant drywall" covers three different things that get confused at the lumber counter, and the confusion is why so many showers have a soft wall behind the tile.

The common misreading: "moisture-resistant" heard as "waterproof." Green board's paper resists humidity; its gypsum core absorbs water like any drywall, softens, and grows mold from the wet side. It is for damp air, not wet surfaces.

What the Code Says

The International Residential Code, which most US jurisdictions adopt with local amendments, draws the line clearly. Water-resistant gypsum backing board (green board) is not permitted as a tile backer in tub and shower areas, or on ceilings where framing spacing exceeds what the board is rated for. Tub and shower walls that receive tile get cement board, fibre-cement board, or glass-mat gypsum backer made for wet areas — and modern tile standards add a waterproofing membrane over or behind the backer. A shower built with green board behind the tile is the one that comes apart soft, black and crumbling twenty years later; the shower mold guide describes exactly that discovery.

Outside the wet area — the rest of the bathroom, a laundry room, a basement finish — moisture-resistant or mold-resistant gypsum is appropriate and, in humid rooms, worth the small premium.

Which Board After Water Damage

A wall being replaced after a leak, a flood or a mold cut-out is the moment to upgrade:

• Bathroom, basement, laundry, garage, kitchen behind the sink: mold-and-moisture-resistant board (the purple-type products). It gives the next wetting less to work with — a treated core and a facing mold cannot eat.
• Behind tile in a tub or shower: cement board or glass-mat backer with waterproofing — never gypsum of any colour.
• Bedrooms, living areas that were wet once from a burst pipe: standard board is fine once the cause is fixed; mold-resistant board is cheap insurance if the room runs humid.

None of it substitutes for the sequence that actually prevents a repeat: fix the water, dry the cavity to a normal moisture reading, then hang the board. Wet drywall: save or replace covers the drying decision; mold-resistant board over a damp cavity grows mold on the studs instead of the paper.

Cost

Moisture-resistant and mold-resistant gypsum typically run modestly above standard drywall per sheet — a difference of a few dollars a sheet at retail that disappears into the labour of any real job. Cement board costs more and is harder to cut and hang, which is why it is reserved for the wet area itself. For a bathroom remodel the upgrade is a rounding error against tile; for a whole basement finish it is a few hundred dollars that decides how the walls behave in a humid July.

The Limits

• Not waterproof. Any gypsum board that stays wet fails. Mold-resistant board resists surface growth; it does not stop water.
• Joints, fasteners and cut edges are the weak points — seal and paint properly, and keep the bottom edge off a slab that sweats.
• Ventilation still decides the room. A bathroom without a working exhaust fan grows mold on the ceiling paint regardless of the board behind it — the fan sizing rule matters more than the sheet.
• Humidity still decides the basement. Under 50%, or the board is the least of it.

Bottom Line

Green board for humid rooms, purple-type mold-resistant board for bathrooms, basements and any wall that has been wet, cement or glass-mat backer behind tile in tubs and showers — that is the whole map, and the code enforces the last line. Buy the better board when you are replacing a wall anyway; fix the water first, because no drywall is designed to be a sponge.