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Moisture-Resistant Drywall: Green Board, Purple Board and Cement Board, Which Goes Where in a Bathroom and Which Is a Mistake Behind Tile
Drywall, Taping & Painting
Moisture-Resistant Drywall: What Green and Purple Board Do, Where They Go, and Why Neither One Belongs Behind the Shower Tile
Every bathroom I have opened up that was built in the 1990s had green board behind the tub tile, and about half of them had a black, crumbling mess where the grout had let water through for twenty years. Moisture-resistant drywall is a good product in the right place: the walls of a bathroom or laundry room that get steam and splashes. Inside a tub surround or a shower it is the wrong product, has been for years, and the Building Code and every tile manufacturer agree. Here is what each board is for.
The boards and where they go
| Board | What it is | Where it belongs | Cost over regular |
|---|---|---|---|
| Regular drywall | Paper-faced gypsum | Everywhere dry, which is most of the house, including bathroom ceilings with a working fan. | – |
| Green board (MR) | Gypsum with a wax-treated paper face | Bathroom and laundry walls outside the wet area, behind a vanity, kitchen walls behind a sink. Slows water absorption; does not stop it. | +$0.40 to $0.55 per sq ft |
| Purple board (mould and moisture resistant) | Gypsum with treated paper and a mould-inhibiting core; also sold as M2Tech and similar | Same places as green board, plus basements and anywhere mould is the worry. The better choice for the same money now. | +$0.45 to $0.60 per sq ft |
| Glass-mat board (DensShield, GlasRoc) | Gypsum core with a fibreglass mat instead of paper | A tile backer for showers that is lighter than cement board, and accepted behind tile where cement board is. Still gets a waterproofing membrane. | +$0.90 to $1.20 per sq ft |
| Cement board (Durock, HardieBacker) | Cement and fibres, no gypsum, no paper | Inside the tub surround and shower, behind tile, with a waterproofing membrane over it. The standard. | +$1.20 to $1.60 per sq ft, and it is a different trade |
| Foam board systems (Schluter Kerdi-Board, Wedi) | Extruded foam with a waterproof facing | Showers, as the backer and the waterproofing in one. What I use on my own houses now. | Part of the tile package |
Why green board fails behind tile
Tile and grout are not waterproof. Water gets through the grout lines a little every shower and the backer behind has to either shed it or survive it. Green board’s treated paper slows it down, but the core is still gypsum, and gypsum that stays damp turns to mush and grows mould. Cement board does not care about water; it is the membrane over it, or the foam board, that keeps the water out of the wall. Ontario’s code has not allowed water-resistant gypsum as a tile backer in showers for years, and the tile manufacturers will not warranty tile over it. If you are renovating a bathroom and the contractor plans green board in the surround, you have learned something about the contractor.
The rule: inside the tub surround or shower, cement board or glass-mat board with a membrane, or a foam board system. Outside it, green or purple board on the walls. Ceiling, regular board with a good fan, or MR board if the fan is doubtful. Never green board behind tile that gets wet.
The fan matters more than the board
A bathroom with a 50 cfm fan ducted properly to the outside, run for 20 minutes after every shower, keeps its regular drywall ceiling for 40 years. A bathroom with a fan that dumps into the attic grows mould on purple board. The board is the backup; the fan is the system. If you are spending $60 on moisture-resistant board for a powder room, spend $200 on a quiet fan with a timer first. The fan that vents into the attic is the commonest thing I find in an older house, and it rots the roof sheathing too.
Where homeowners go wrong
- Green board behind the tub tile. Covered above. It is the one that costs you a bathroom in twenty years.
- Cement board with no membrane. Cement board is not waterproof; it is water-tolerant. The membrane (liquid or sheet) is what keeps the studs dry.
- MR board on a ceiling with a bad fan and no primer. The board survives; the paint peels anyway. Prime it and fix the fan.
- Paying the MR premium on the whole house. It is for wet rooms. The bedroom does not need it.
- Letting the drywaller tape the cement board with paper tape and compound. Cement board joints get alkali-resistant mesh and thinset, and that is the tile setter’s job.
Questions people ask
Do I need moisture-resistant drywall in a bathroom?
On the walls outside the tub and shower, it is cheap insurance and most builders use it. On the ceiling, regular board with a proper fan is fine; MR board if the fan is weak. Inside the shower, no drywall of any kind.
Is purple board better than green board?
Purple board (mould- and moisture-resistant) resists both water and mould growth; green board resists water only. They cost about the same now, so purple is the better buy. Neither goes behind shower tile.
Can you tile over moisture-resistant drywall?
On a kitchen backsplash or a dry bathroom wall, yes. In a tub surround or shower, no; the code and the tile manufacturers call for cement board, glass-mat board or a foam system with waterproofing.
How much does moisture-resistant drywall cost?
About $0.40 to $0.60 a square foot more than regular board installed, so $60 to $120 for a typical bathroom. Cement board in the surround is $1.20 to $1.60 more and usually priced with the tile.
Price the bathrooms with the house
The drywall installation calculator has a line for moisture-resistant square footage. The tile backer goes in the bathroom budget; the bathroom pages cover that side.
Sources and further reading
- CGC Inc.: the maker of most of the board and compound in Ontario; the product sheets give thicknesses, span limits for ceilings and the right compound for each coat.
- CertainTeed Gypsum Canada: the other common board in Ontario yards; their installation guides and Level 5 primer-surfacer data are on the site.
- Addressing moisture and mould in your home: Health Canada. When wet drywall can dry and when it comes out, and the 48-hour rule.
