Mold on the ceiling: causes and removal

Two different problems that look similar

Condensation mold is a scatter of small dark spots, often spread across a bathroom ceiling or gathered in a corner where the ceiling meets an outside wall. The drywall is hard and flat. It forms because warm, moist air meets a cool surface again and again.

Leak mold is a defined patch, usually with a brown or yellow water stain, sometimes with bubbling paint or sagging. It sits under a bathroom, a roof penetration or plumbing. The drywall may be soft. The growth is fed by water coming through from above.

Telling them apart matters, because the first is a cleaning and ventilation job and the second is a repair.

Bathroom ceilings

A hot shower puts a large amount of water vapour into a small room. If it has nowhere to go, it settles on the coolest surface, which is usually the ceiling.

Fixes that work:

• An exhaust fan that is actually vented outdoors, not into the attic
• Running the fan during the shower and for a good while afterwards
• A fan sized for the room, with a clean grille
• Leaving the door open after showering
• A washable, moisture-tolerant paint finish

If the fan is noisy but a tissue held to the grille does not pull toward it, it is not moving air.

Corners and edges on the top floor

Mold in a line along the edge of a top-floor ceiling, or in a corner, often points to missing or compressed insulation above. That strip of ceiling is colder than the rest, so moisture condenses there. It can also be a sign of an ice dam in winter. The fix is in the attic: insulation, air sealing and ventilation.

Under a bathroom or a roof

Look for the source directly above and uphill from it.

• A leaking shower, tub drain, toilet seal or supply line
• Failed flashing around a vent, chimney or skylight
• Missing shingles
• A bathroom fan duct that is dripping condensation back down

Until the leak is fixed, cleaning is pointless.

How to clean condensation mold

Health Canada treats three or fewer patches with a total area under one square metre, as a small amount a homeowner can generally clean.

1. Wear gloves, safety glasses and a well-fitting N95 respirator. Working overhead, eye protection matters.
2. Cover the floor and anything below.
3. Lightly dampen the area so that spores are not released.
4. Wash with water and dish detergent on a sponge or cloth. Do not soak the drywall.
5. Wipe with a clean damp cloth.
6. Dry the room with the fan or a dehumidifier.

Textured or stippled ceilings cannot be scrubbed. In an older house a textured ceiling may contain asbestos and should be tested before anyone scrapes or sands it.

When the ceiling has to come down

• The drywall is soft, sagging or crumbling
• The stain keeps returning
• The insulation above is wet
• Mold is visible on the attic side or on the framing

In that case the affected section is removed under containment, the cavity is dried, the cause is fixed and the ceiling is rebuilt.

Repainting

Clean first, let the ceiling dry fully, then use a stain-blocking primer before the finish coat. Painting over active mold only hides it for a few weeks.

What the pattern tells you

The shape of the growth is a useful clue before anyone opens anything.

• Fine, scattered speckling over a wide area points to condensation. The whole surface has been damp, lightly and often.
• A defined patch with a brown or yellow edge points to a leak. Water arrived at one place and spread outwards.
• Lines or stripes that follow the framing are usually ghosting, where dust and moisture settle on the colder strips of ceiling under the joists. It shows where insulation is thin.
• Growth around a pot light, a fan or an attic hatch suggests warm, damp air escaping into the attic at that opening.
• A patch that grows after rain or a thaw is a roof problem until shown otherwise.

Looking above the ceiling

Where there is an attic over the affected room, a look from above settles most questions. Take a flashlight and check, from the hatch if you can:

• Whether the insulation above the patch is wet, flattened or stained
• Whether the underside of the roof boards is dark or frosted in winter
• Whether the bathroom fan duct is connected and runs to the outside, not into the attic
• Whether insulation is missing or pushed aside at the edges

Do not walk on the ceiling drywall, and do not disturb loose grey or gold granular insulation, which may be vermiculite.

Textured and older ceilings

Sprayed “popcorn” texture is difficult to clean. It holds moisture, and it comes off when scrubbed. Texture applied before the 1990s can contain asbestos, so it should be tested before it is scraped, sanded or cut. Mold on a textured ceiling usually ends in removal of the affected area, done with the right precautions.

Is it urgent?

Small patches of condensation mold are a maintenance job, not an emergency. Treat it as urgent when the ceiling is wet, sagging or dripping, when the patch is growing quickly, or when someone in the home has asthma, allergies or a weakened immune system.

More: mold removal and remediation.