What causes mould behind bathroom tiles, and how do I prevent it during a renovation?

Mould behind bathroom tiles is caused by moisture penetrating through grout and tile into the wall cavity, where it becomes trapped in a dark, warm environment with no air circulation — the perfect conditions for mould growth. This is the single most common and most expensive failure mode in bathroom renovations, and it is almost entirely preventable with proper waterproofing during the initial installation.

Here is what happens: grout is not waterproof. Even sealed grout allows some moisture to pass through over time, especially in shower walls and tub surrounds that are exposed to direct water contact daily. If the wall behind the tile does not have a continuous waterproof membrane, that moisture migrates through the grout, through the thinset, and into the substrate — whether that substrate is cement board, drywall, or the green board (moisture-resistant drywall) that was standard in GTA bathrooms for decades. Once moisture reaches the wall cavity behind the substrate, it has nowhere to go. The cavity is dark, warm (heated by the hot shower water on the tile face), and sealed from air circulation. Mould colonies establish within weeks and can grow for years before any visible signs appear on the tile surface.

By the time you see dark spots at grout lines, a musty smell, or tiles that feel spongy or loose, the mould growth behind the wall is typically extensive. Remediation at this stage requires complete demolition of the tile, substrate, and often the studs and insulation behind — a repair that frequently costs $5,000-$15,000 or more in the GTA, often exceeding the cost of the original tile installation.

Prevention During Renovation

The solution is straightforward: install a continuous waterproof membrane behind all tile in wet areas. This membrane catches any moisture that penetrates the grout and tile and prevents it from reaching the wall cavity. The Ontario Building Code requires waterproofing behind shower and tub surrounds, and the three approved methods used in GTA bathroom renovations are:

Sheet membrane (Schluter Kerdi) is applied directly over cement board or drywall using unmodified thinset mortar. The membrane covers every square inch of the wet area, with Kerdi-Band sealing all seams, corners, and penetrations. Tile is set directly onto the membrane. This is the most popular waterproofing method in GTA bathroom renovations — materials cost $3-$5 per square foot for the membrane.

Liquid-applied membrane (RedGard, Mapei AquaDefense) is rolled or troweled onto cement board in two coats, curing into a flexible waterproof film. Mesh fabric is embedded at corners and seams. This method costs $1-$3 per square foot for materials and works well on flat surfaces and simple geometries.

Cement board with separate membrane uses products like DensShield tile backer, which has a built-in moisture barrier on the face surface. When properly installed with sealed seams, DensShield provides waterproofing as part of the substrate itself.

Critical Details That Prevent Mould

The waterproof membrane must be continuous and unbroken — every seam overlapped, every corner sealed, every pipe and valve penetration wrapped with membrane or sealed with compatible sealant. A single missed seam or unsealed corner becomes the entry point for moisture.

Exhaust ventilation is the second line of defence. A properly sized exhaust fan (minimum 50 CFM, ideally 80-110 CFM for showers) removes humid air from the bathroom before it can condense on surfaces and migrate behind tile. In Toronto's humid summers, a humidity-sensing fan that runs automatically is particularly valuable. The fan must vent to the building exterior — never into an attic or soffit, which simply relocates the moisture problem.

Finally, use 100% silicone caulk (not grout) at all changes of plane — where walls meet floors, where walls meet other walls, and around fixtures. Silicone remains flexible and accommodates the slight movement that occurs at these transitions, preventing the cracking that opens gaps for water entry. Replace silicone caulk annually as part of routine bathroom maintenance.