White mold or efflorescence on basement walls?

What efflorescence is

Concrete, block, brick and mortar contain soluble salts. When water moves through a foundation wall or floor slab, it dissolves some of them. At the inside surface the water evaporates and the salts are left behind as a white deposit.

It can look like:

• A fine white powder
• Fluffy, hair-like crystals
• A hard white crust
• White streaks running down from a crack or mortar joint
• A white tide line a short distance above the floor

Efflorescence is a mineral. It is not alive, does not spread to other materials and is not a health hazard. It is, however, proof that water is moving through the masonry.

What white mold is

Mold comes in many colours, including white. White growth is common on:

• Wooden joists, beams and subfloors above a damp basement
• The paper face of drywall
• Stored cardboard, leather, fabric and furniture
• Wooden shelving and pallets on a basement floor
• Soil and dust on the surface of a concrete wall

It looks cottony, fuzzy or like a thin film, sometimes with a slightly raised texture. Many molds also start out white and darken as they mature.

Telling them apart

What is it growing on? Efflorescence forms only on masonry: concrete, block, brick, stone, mortar and sometimes tile grout. White growth on wood, drywall, fabric or cardboard is mold.

The water test. Spray a little water on it. Efflorescence dissolves and seems to disappear, returning as the surface dries. Mold does not dissolve. It mats down or smears.

The touch test. Wearing a glove, rub some between your fingers. Efflorescence is gritty or crumbles to a fine powder, like salt. Mold feels soft and smears.

Smell. Efflorescence has no smell. Mold is musty.

Pattern. Efflorescence follows the water: along cracks, at mortar joints, at the base of walls. Mold follows damp organic material and still air.

Both can be present together. A damp foundation wall with efflorescence often has mold on the wood, drywall or belongings nearby. On a painted or dusty concrete wall, mold can grow on the paint and dirt even though it cannot feed on the concrete itself.

Dealing with efflorescence

Cleaning

• Brush it off dry with a stiff brush, and vacuum up the dust
• For crusted deposits, scrub with water, or use a masonry efflorescence cleaner according to its label
• Rinse sparingly. Soaking the wall dissolves more salt and brings it back.

Cleaning is cosmetic. The deposit returns as long as water keeps coming through.

Stopping the water

• Extend downspouts and keep eavestroughs clear
• Slope the ground away from the foundation
• Repair cracks
• Check window wells
• Make sure the weeping tile and sump pump are working
• For persistent seepage, exterior waterproofing or an interior drainage system

What not to do. Do not paint or seal over active efflorescence and expect it to hold. Salts forming behind a coating push it off, and the paint blisters and peels. Waterproofing paints applied on the inside can help with dampness in a wall that is basically sound, but they do not hold back real water pressure.

Before finishing a basement. Efflorescence is a warning. Framing, insulation and drywall placed against a wall that passes moisture will get damp and grow mold out of sight. Solve the water first.

Dealing with white mold

• On hard surfaces and sound wood, over a small area: wear gloves, eye protection and a respirator rated for fine particles. Scrub with detergent and water, wipe clean, and dry thoroughly.
• On drywall, insulation, carpet and cardboard: remove and discard. Porous materials cannot be cleaned reliably.
• On joists and subfloor over a large area: this is professional work, done under containment with HEPA vacuuming and physical cleaning. See mold on wood.
• On stored belongings: clean hard items, launder fabrics in hot water, and discard paper and cardboard that are moldy.

Then deal with the humidity, or it comes back.

The common cause: a damp basement

Basements are cool and below ground. Moisture arrives in three ways:

1. Liquid water through cracks, joints and the floor
2. Vapour passing through the concrete from the damp soil outside
3. Condensation, when warm, humid summer air meets cool walls, floors and pipes

Efflorescence comes from the first two. Mold thrives on all three.

Practical steps:

• Run a dehumidifier through the warm months, sized for the space and drained to a floor drain or sump so that it does not shut off when full
• Aim for relative humidity at or below about 50 percent
• Keep basement windows closed in humid weather
• Store things in plastic bins on shelves, not in cardboard on the floor
• Leave a gap between furniture and outside walls
• Insulate cold water pipes

When to get it looked at

• You cannot tell which it is and the area is large
• White growth covers joists or subfloor
• There is a musty smell with no visible source
• Water is coming in regularly
• You are about to finish the basement
• Someone in the home has asthma or a weakened immune system

Laboratory testing is seldom needed to tell efflorescence from mold. The water test and a look at what it is growing on settle most cases.

More: mold removal and remediation.