White mold in a basement is often not mold at all. White powder on concrete or block is usually efflorescence, a mineral salt left behind when water moves through the wall and dries. Real white mold is living growth, and it is soft, fuzzy or patchy. Spray the spot with water. If it dissolves, it is likely salt. If it smears or stays, it may be mold. Both mean moisture.
White mold vs efflorescence at a glance
The two look alike from across the room. Up close they behave very differently.
| Check | Efflorescence | White mold |
|---|---|---|
| Texture | Dry, powdery or crystalline, like chalk or frost | Fuzzy, cottony, slimy or patchy |
| Surface | Concrete, block, brick, mortar | Anything with organic matter: wood, drywall, paper, dust, stored boxes |
| Water spray | Dissolves or darkens and vanishes as it dries | Beads, smears or stays in place |
| Smell | None | Often musty or earthy |
| Dry brushing | Brushes off as dust, returns later | Smears or leaves a stain |
| Spreads to wood or drywall | No, salt does not grow | Yes, if the material stays damp |
One rule settles many cases. Efflorescence needs masonry. If you see white fuzz on a wood stud, a cardboard box or drywall paper, you can rule out efflorescence and treat it as possible mold.
The water and vinegar test, step by step
This is a screening check. It is common home advice, not a lab result.
- Put on gloves and a disposable N95 mask. Open a window if you can.
- Brush a small spot with a dry stiff brush. Note whether it lifts as dust or smears.
- Spray the spot with plain water. Wait a minute or two.
- Watch the result. Salt tends to dissolve, darken and then dry back to white crystals. Mold tends to bead, hold its shape or smear.
- For a second look, dab a little white vinegar on a hidden patch of bare concrete. Many efflorescence deposits are alkaline and may fizz or soften a little. Mold usually does not react.
- Take a photo before and after, with a coin for scale. A contractor or inspector can use the photos.
Where the test fails
The test is weak on painted walls. Salts can push the paint off in flakes, and the flakes can look like mold. Mold can also grow on top of efflorescence, so a wall can have both at once. A damp, dirty film on salt is a common case.
If the result is unclear, do not guess. A tape-lift sample or a lab culture from an inspector gives a firm answer. Visible mold also does not need a species name before you act on it.
What each one means for the wall
Efflorescence: water is passing through
Efflorescence forms in three steps. Water enters concrete or block, dissolves salts as it travels, and carries them to the surface. The water evaporates and the salt stays. The white powder is harmless to touch in small amounts, but it proves that moisture is moving through the wall.
That does not always mean a leak you can see. It can mean:
- Soil next to the foundation that stays wet, or grading that slopes toward the house.
- Downspouts that empty close to the wall.
- Hairline cracks or tie-rod holes that let water in.
- A failed or clogged drain tile at the footing.
- High indoor humidity that condenses on a cold wall.
Many parts of southern Ontario, including Toronto and Ottawa, have clay-rich soils in places. Clay drains slowly, so water can sit against a foundation after rain and spring melt. Soils vary lot by lot, so a site check matters more than a general rule.
Salt on its own rarely harms the concrete in a short time. Repeated wetting and freezing can add wear over many winters, and the damp wall can start to support mould. So efflorescence is a warning, not an emergency.
White mold: a damp surface with something to eat
Mold needs moisture, a food source and time. Basements supply all three. Drywall paper, wood framing, carpet, cardboard and even household dust can feed it. White mold often appears early, before it darkens to green, grey or black. Colour does not tell you whether it is harmful.
Health Canada says that unless the cause of the moisture is found and fixed, mould will return. See its guide to addressing moisture and mould indoors. Cleaning alone does not cure a wet wall.
Clean up or call a professional
Health Canada’s guidance says homeowners can generally clean small and medium areas themselves. It defines small as one to three patches, each under 1 square metre (10 square feet). Medium means more than three patches, or patches between 1 and 3 square metres. Larger or repeated growth should go to a professional.
For a small area, Health Canada recommends:
- Safety glasses or goggles, a disposable N95 respirator and disposable gloves.
- Plastic sheeting taped over nearby openings and a HEPA vacuum for dust.
- Washing hard surfaces with water and detergent. Bleach is not required.
- Removing and discarding porous materials such as wet drywall or insulation that are growing mold.
The agency also advises keeping indoor relative humidity between 30% and 50%. A small hygrometer shows the number. Read more in its page on reducing humidity, moisture and mould.
Call a professional if the growth is large, if it returns after cleaning, if it is inside a wall cavity, or if anyone in the home has asthma, allergies or a weak immune system. Sewage or flood water changes the job and needs a specialist.
Decision guide: what to do next
| What you found | Likely cause | First step | Who to call |
|---|---|---|---|
| Dry white powder on bare concrete, dissolves in water | Efflorescence | Check grading, gutters, downspouts, cracks | Often no one at first, then a waterproofer if it keeps coming back |
| White powder with damp patches and a musty smell | Efflorescence plus possible mold | Dry the area, run a dehumidifier, test again in 1 to 2 weeks | Mold professional if smell stays |
| White fuzz on wood, drywall or boxes | Likely mold | Fix the moisture, clean or remove material | Mold removal company if over the small-area limit |
| White growth after a leak or flood | Mold, possibly widespread | Stop the water, dry fast | Mold removal or restoration company |
| Powder returns on the same wall each spring | Ongoing water movement | Find the entry point | Waterproofing or foundation company |
The order matters. Mold removal clears the growth. Waterproofing stops the water. If you do only the first, mold usually comes back.
What our boards show for Toronto and Ottawa
The directory ranks companies by Google rating and review count. These figures are data from our boards for October 2026. They are not endorsements, and a rating does not guarantee a result on your wall.
Mold removal. The mold removal companies in Toronto board ranks 19 companies. The median rating is 4.9, the median review count is 76, and the board holds 2,044 reviews in total. Four companies have fewer than 20 reviews. The mold removal companies in Ottawa board ranks 20 companies with a median rating of 4.75, a median of 43 reviews and 1,295 reviews in total. Four of them have fewer than 20 reviews.
Waterproofing. If the wall keeps getting wet, the water source is the real job. The basement waterproofing companies in Toronto board ranks 20 companies, with a median of 81.5 reviews and 3,934 reviews in total. One company has fewer than 20 reviews. The basement waterproofing companies in Ottawa board also ranks 20, with a median of 61 reviews and 1,482 in total. Five have fewer than 20 reviews.
What this means for choosing:
- Ratings cluster near the top, with medians from 4.75 to 5.0. The review count separates companies more than the rating does.
- Ottawa boards have fewer reviews per company than Toronto boards. A smaller sample makes one bad or good review count for more.
- Companies with fewer than 20 reviews may be new or small. Ask them for references and a written scope.
- Compare at least three quotes. Ask each company what it will do about the moisture source, not only the visible growth.
- Open the company profile and read the recent reviews, not just the star average.
For other towns, the mold removal hub lists every board, including Hamilton, Mississauga and Barrie.
Questions to ask before you hire
- Will you test or sample the growth, and who reads the lab result?
- How will you contain the work area and keep spores out of the rest of the house?
- Which materials will you remove, and which will you clean?
- What will you do about the moisture source? If the answer is “nothing”, the problem will likely return.
- Will you provide a written scope, a price and a warranty in plain terms?
Costs vary with the size of the area, the materials involved, access and whether testing is needed. We did not find a reliable Canadian price range, so we leave out a dollar figure. Ask each company to price the same scope in writing so the quotes can be compared.
Prevent it from coming back
- Point downspouts at least a couple of metres away from the foundation.
- Keep soil sloped away from the house.
- Run a dehumidifier in summer and keep humidity between 30% and 50%.
- Keep stored items off the floor and a few centimetres from the walls.
- Vent bathrooms and dryers to the outside.
- Check the basement after heavy rain and during spring melt.
Frequently asked questions
How do I tell white mold in a basement from efflorescence?
Efflorescence is a dry, crystalline mineral salt that sits on concrete or block and dissolves in water. Mold is living growth that smears, stays put when wet and often smells musty. A water spray and a small vinegar test help, but a lab sample is the only firm answer.
Is white mold in a basement dangerous?
Health Canada links damp indoor spaces and mould to respiratory symptoms, so treat it seriously. Small patches can often be cleaned by the homeowner with protection. Large or repeated growth needs a professional, and the moisture source must be fixed.
Does efflorescence mean my basement leaks?
It means water is moving through the masonry and carrying salts to the surface. The wall may be damp rather than flooding. It is a moisture warning, so look at grading, gutters, cracks and drainage.
Can I just paint over white powder on a basement wall?
Paint over efflorescence usually flakes off, because salts keep pushing out from behind it. Paint over mold hides the growth but does not stop it. Find and stop the moisture first, then clean, then finish the wall.