What type of drywall should be used for a Toronto condo bathroom that has high humidity but no exterior wall?

Mould-resistant (purple board) drywall is the best choice for a Toronto condo bathroom with high humidity, even when there is no exterior wall involved. While moisture-resistant green board is the traditional choice for residential bathroom walls, purple board's fibreglass facing provides superior mould protection in the consistently humid environment of a condo bathroom — and the modest price premium is well worth it for the long-term protection it provides.

The distinction between these two products matters. Green board (moisture-resistant drywall) has a water-resistant paper facing and a moisture-resistant gypsum core. It resists moisture better than standard drywall, but because it still has a paper facing, it can support mould growth if exposed to sustained high humidity — and condo bathrooms are among the most humidity-prone spaces in any GTA home. Purple board (mould-resistant drywall, such as CGC's Mold Tough or Georgia-Pacific's DensArmor Plus) replaces the paper facing entirely with fibreglass mat, eliminating the organic food source that mould feeds on. In the GTA market, green board runs $20-$28 per 4x8 sheet while purple board runs $24-$32 — a difference of only $4-$6 per sheet that adds perhaps $30-$50 to the total material cost for a typical bathroom.

The fact that your condo bathroom has no exterior wall actually simplifies one aspect of the project — you do not need to worry about the vapour barrier and insulation assembly required on exterior walls, or about condensation forming on cold surfaces inside the wall cavity. However, a condo bathroom without exterior walls still faces serious humidity challenges. Hot showers generate enormous amounts of moisture vapour, and in a condo bathroom, that moisture has nowhere to go except into the walls and ceiling. Toronto's humid summers (with outdoor humidity often exceeding 60%) compound the problem by reducing the moisture gradient that would otherwise help dry out bathroom walls.

For the ceiling in a condo bathroom, purple board is even more important than on the walls. Ceilings directly above showers and bathtubs receive the most concentrated moisture exposure, and ceiling surfaces are more prone to mould because warm, moist air rises and condenses on the ceiling surface. Use 5/8-inch mould-resistant board on the ceiling for both moisture protection and sag resistance.

There are critical areas where no type of drywall is acceptable. In shower surrounds and tub surrounds — any surface that will receive direct water contact or tile — you must use cement board (Durock, HardieBacker, or equivalent) as the substrate. This is a non-negotiable requirement. Drywall, including green board and purple board, is not designed to withstand direct water exposure and will eventually fail behind tile, leading to leaks, mould, and costly repairs. Cement board runs $25-$40 per 3x5 sheet in the GTA.

Condo-specific considerations add complexity to bathroom drywall projects. Party wall fire ratings must be maintained — if any of your bathroom walls are shared walls with adjacent units, those walls must maintain their fire-resistance rating (typically 1-hour, requiring 5/8-inch Type X drywall). You cannot simply replace fire-rated drywall with non-fire-rated moisture-resistant board. The solution is to use Type X mould-resistant board (which combines fire rating with mould resistance) or to maintain the fire-rated Type X layer and add moisture-resistant board on top. Your condo building's management or engineering department can confirm which walls have fire-rating requirements.

Ventilation is the other critical factor for bathroom moisture control, and it matters more than the type of drywall you choose. A properly sized bathroom exhaust fan — minimum 50 CFM for small bathrooms, 1 CFM per square foot for larger ones — ducted to the building's exhaust system (never into the ceiling cavity) is essential. Many older Toronto condos have undersized or poorly maintained exhaust systems, and upgrading the fan is one of the most effective investments you can make to protect your bathroom drywall.

For a typical Toronto condo bathroom renovation, the drywall scope (purple board on walls and ceiling, cement board in shower/tub areas, taping, finishing to Level 4) runs approximately $1,500-$3,500 depending on the bathroom size and complexity. If you need help finding a drywall professional for your condo bathroom project, Toronto Drywall Installers can match you with local contractors for a free estimate.