Bathroom mold: why it grows and how to stop it

Why the bathroom

Mold needs moisture, something to feed on and time. A bathroom supplies steam from showers, standing water around the tub and sink, soap film and skin cells as food, and warm still air. Without ventilation it stays damp for hours after each use.

Where it grows, and what each location means

Caulking around the tub and shower. Black spotting inside the silicone itself. Cosmetic at first, but failed caulk lets water behind.

Grout lines. Surface growth on soap film. Cleans off.

Ceiling. Scattered spots from condensation. A ventilation problem.

Painted walls, especially above the shower. Condensation again.

The wall outside the shower, at floor level. A warning sign. Water is escaping the enclosure or getting through the tile.

Around the toilet base. A failed seal or a sweating tank.

Under the sink. A slow leak at the trap or supply lines.

Behind or under a vanity. Usually a leak that has gone unnoticed.

The room on the other side of the shower wall. If paint is bubbling or the baseboard is swollen there, the shower is leaking into the wall.

Cleaning surface mold

1. Wear gloves and eye protection, and ventilate the room.
2. Wash tile, grout and painted surfaces with water and dish detergent, using a stiff brush on grout.
3. Rinse and dry.

Do not mix cleaning products. Bleach combined with ammonia or acidic cleaners produces dangerous gases.

Replacing moldy caulk

Mold inside silicone cannot be scrubbed out.

1. Cut out all the old caulk with a utility knife or a caulk removal tool.
2. Clean the joint and let it dry completely, ideally overnight.
3. Apply a bathroom-grade silicone with mold inhibitor.
4. Let it cure fully before using the shower.

If the joint is wet inside, or the tile is loose, there is water behind and caulk alone will not solve it.

Signs that water is behind the tile

• Tiles that sound hollow when tapped, or move
• Grout that keeps cracking in the same place
• A musty smell that cleaning does not shift
• Staining or soft drywall on the adjoining wall or the ceiling below
• Mold that returns within days in one spot

At that point the surround needs opening. The wet backing board and any moldy framing are dealt with, and the shower is rebuilt on a proper waterproof base.

Ventilation that works

• An exhaust fan ducted to the outdoors, not into the attic or a soffit cavity
• Enough capacity for the room size
• Run it during the shower and for at least 20 minutes after. A timer switch makes this automatic.
• Keep the grille clean
• Leave the shower door or curtain open afterwards so that the enclosure dries
• Squeegee the walls
• Hang towels to dry outside the bathroom if it has no window

Bathrooms with no fan

Many older homes have only a window. Opening it helps in summer and does little in winter. Adding a properly ducted fan is one of the most effective things you can do for the whole house, since bathroom moisture also contributes to attic mold.

When to call a professional

• More than a small area is affected
• The mold is on or in drywall and keeps returning
• There are signs of a leak into the wall or the ceiling below
• Someone in the home has asthma, allergies or a weakened immune system

Pink, orange and black: what the colours mean

Not everything that grows in a bathroom is mold.

• Pink or orange slime on shower curtains, around drains and at the waterline of the toilet is usually a bacterium that feeds on soap residue. It wipes off easily and returns quickly where surfaces stay wet.
• Black spots on caulk and grout are usually common moisture molds that root into the porous surface.
• Grey or white fuzz on the ceiling or on wood is mold growing on a surface that stays damp.
• A white, crusty deposit on tile or grout is usually mineral scale from hard water, not growth.

The colour does not tell you the species or the risk, and laboratory testing is rarely needed to decide what to do. The questions that matter are how large the area is and whether water is getting behind the surface.

Habits that make the most difference

• Squeegee the shower walls and door after use. It removes most of the water that would otherwise evaporate into the room.
• Leave the shower door or curtain open so that the enclosure dries.
• Hang towels and bath mats to dry, and wash them regularly.
• Run the fan during the shower and for at least 20 minutes afterwards. A timer switch does this without anyone remembering.
• Keep the bathroom door open after use, if the fan is weak.
• Fix dripping taps and running toilets, which keep surfaces wet.

Basement bathrooms

A bathroom in the basement has two extra problems. The walls and floor are cooler, so condensation forms more readily, and the fan duct often has a long run to the outside, which weakens the airflow. A dehumidifier in the basement and a fan sized for the length of the duct both help. Mold low on a basement bathroom wall can also come from moisture in the foundation, not from the shower.

Renovating a moldy bathroom

If you are opening walls anyway, use the chance to do it properly: cement board or a waterproof membrane behind tile instead of ordinary drywall, a correctly sloped shower base, and a fan ducted to the outside. In older houses, have suspect materials such as old flooring, drywall compound and pipe insulation tested for asbestos before demolition.

More: mold removal and remediation.