What is the difference between green board, purple board, and standard white drywall for GTA bathroom use?

Standard white drywall should never be used in GTA bathrooms, green board (moisture-resistant) is the minimum acceptable choice for bathroom walls not in direct water contact, and purple board (mould-resistant) is the superior option that most experienced Toronto drywall contractors now recommend for all bathroom applications. Understanding the differences between these three products can save you from a mould disaster that is all too common in GTA bathrooms.

Standard white drywall has a paper facing on both sides and a regular gypsum core. Paper is an organic material that mould feeds on readily, and the standard gypsum core absorbs water like a sponge. In a GTA bathroom, where hot showers generate substantial moisture daily and Toronto's humid summers keep background humidity elevated for months, standard drywall behind or near a shower will absorb moisture, grow mould inside the wall cavity, and eventually deteriorate. The paper facing will turn black, the gypsum core will soften and crumble, and you will face a costly tearout and replacement. Standard white drywall costs $14–$20 per 4x8 sheet, but using it in a bathroom is a false economy that will cost you thousands in remediation.

Green board (moisture-resistant drywall) has a water-resistant paper facing (the green colour) and a moisture-resistant gypsum core treated with wax and silicone additives. It resists moisture absorption significantly better than standard board, making it suitable for bathroom walls and ceilings that are exposed to humidity but not direct water contact. Green board has been the standard bathroom drywall in GTA construction for decades. At $20–$28 per 4x8 sheet, it is a modest upgrade over standard board. However, green board still has paper facing — and paper, even treated paper, remains an organic food source for mould. In bathrooms with poor ventilation (a common problem in Toronto's older homes where bathroom exhaust fans may be undersized, poorly ducted, or nonexistent), green board can still develop mould on the paper surface over time.

Purple board (mould-resistant drywall) represents a significant upgrade over green board. Products like CGC Mold Tough use fibreglass mat facing instead of paper, completely eliminating the organic food source that mould needs to grow. The gypsum core is also treated with mould-inhibiting compounds. Purple board resists both moisture and mould growth, making it the best drywall choice for GTA bathrooms, laundry rooms, and any high-humidity area. At $24–$32 per 4x8 sheet, the premium over green board is only $4–$6 per sheet — a negligible cost increase for dramatically better mould protection. For a typical GTA bathroom renovation requiring 12–18 sheets, the total premium for purple board over green board is only $50–$100.

Critical limitation for both green and purple board: neither product is suitable for use in areas of direct water exposure — shower surrounds, tub surrounds, and behind sinks where water splashes regularly. These areas require cement board (Durock, HardieBacker, or equivalent) as a tile substrate. Cement board is completely inorganic, impervious to water, and provides the rigid, stable substrate that tile needs. Installing any type of drywall — including green or purple board — as tile substrate in a shower is a code violation and will eventually fail, leading to tile delamination, water infiltration into the wall cavity, and structural damage. Cement board runs $25–$40 per 3x5 sheet.

For a complete GTA bathroom renovation, here is the recommended board selection: Purple board (or at minimum green board) for all walls and ceilings outside the shower/tub area. Cement board for all surfaces inside the shower or tub surround, extending at least 6 inches beyond the edge of the tub or shower pan. A 6-mil polyethylene vapour barrier or a liquid-applied waterproofing membrane (like Kerdi or RedGard) behind the cement board and extending onto the adjacent drywall by at least 2 inches.

Toronto's climate makes bathroom moisture management especially important. During winter, GTA homes have very dry indoor air (15–25% humidity) from constant furnace operation, but bathrooms are humidity spikes in this dry environment — each hot shower sends a burst of moisture into the walls and ceiling. During summer, background humidity is already high (60–80%), and bathroom moisture simply adds to an already challenging environment. In lakefront GTA neighbourhoods like the Beaches, Mimico, and Port Credit, where ambient humidity is elevated year-round from Lake Ontario, purple board is particularly worthwhile.

The bottom line for GTA homeowners: the $50–$100 extra cost of purple board over green board for a bathroom renovation is one of the best investments you can make. Mould remediation in a GTA bathroom — removing mouldy drywall, treating framing, and reinstalling new board — costs $2,000–$5,000 or more. Purple board virtually eliminates the drywall component of mould risk in your bathroom.