How a Bowl of Salt Helps Reduce Indoor Humidity and Mould

Damp walls, misted windows and a faint musty smell you cannot quite identify: this is the sort of indoor humidity that gradually creates the conditions for mould. According to one scientist, there is a straightforward, inexpensive moisture buffer already sitting in your kitchen: a bowl of salt placed in the corners.

The paint had started to blister near floor level, the wardrobe carried a stale smell, and condensation left the window sills damp and tacky each morning. Beside the cold outside wall was what appeared to be a cereal bowl of rock salt, a little clumped together, with a small puddle beneath it like tears.

He gave a near-apologetic shrug. “The lab trick,” he said. “Salt eats water.” The scientist in me was sceptical, but by the end of the week the bowl was heavier and a shiny layer of brine had appeared. Somehow, the room felt calmer. It was only ordinary table salt.

The quiet physics of a salty bowl

Look at a bowl of coarse salt on a humid morning and it may first form a crust before becoming slippery. Leave it for a day or two, and the crystals can break down into a watery layer at the base. There is no magic involved; it is simple chemistry at work in your kitchen.

Salt attracts water molecules from the air. Once it takes in that moisture, its surface starts dissolving and produces a salty liquid, which can draw in further water. Moisture is persistent, but a humble bowl can help slow its progress.

In a quick home experiment, I weighed a bowl containing 500 g of rock salt and left it in a bathroom that often reached 70% relative humidity. Forty-eight hours later, it weighed 85 g more. Brine was clearly visible, enough to wobble when touched with a fingertip. It was not a flood, but it was certainly measurable.

The same arrangement in a drier bedroom, at about 50% RH, showed almost no change. A neighbour put two bowls in a cupboard that remained around 72% RH; during the day, his inexpensive hygrometer fell to 64% over the course of a week, and the musty smell became less noticeable. It was not completely dry, but it was less welcoming to mould.

The underlying science is straightforward. Salt is hygroscopic, which means it attracts water vapour. At room temperature, ordinary sodium chloride begins to liquefy actively - a process known as deliquescence - once the air reaches roughly 75% relative humidity. The resulting brine continues to pull moisture, changing the microclimate immediately around the bowl.

Even a local reduction in humidity can make a difference. Most household moulds flourish when air remains above 60% RH for extended periods, particularly beside cold surfaces where thin films of water develop. If a small amount of brine keeps the air along that wall nearer the 50–60% range, mould colonies develop more slowly. Less fuzz. Less odour.

Where to place a bowl of salt and how to use it

Keep it uncomplicated. Choose a broad, shallow bowl made from ceramic or glass, then fill it with coarse salt; rock salt or kosher salt are both suitable. A larger surface area allows more air to come into contact with the crystal edges. Use one bowl in a small room or two in a large bedroom, positioning them in the coldest corners or below a window where condensation gathers.

For a neater arrangement, place a colander or mesh sieve inside a larger bowl, with the salt in the upper section and the brine collecting below. Put it on a coaster to protect the floor. When liquid begins to accumulate, tip it away, add more salt and let the bowl air out. Change the brine weekly if it has become soupy.

We all know the moment when a smell makes you wrinkle your nose and wonder, “Something’s growing.” Realistically, no one keeps up this routine every day. Instead, choose one day - Sunday, for example - to inspect the bowls, wipe down cold corners and open a window for ten minutes. Keep salt away from unprotected metal, as brine causes rust. If you have young children or pets, keep bowls out of reach because salty water is not safe to drink.

If a room feels especially humid, use a fan on a low setting to move air across the salt’s surface. Combine this with simple habits: keep lids on pans while boiling, cover laundry baskets and leave doors slightly open after showering. Target under 60% RH with a basic hygrometer, which can cost less than a takeaway. The bowl is an aid, not the answer to everything.

Treat it as triage. A bowl can reduce humidity peaks, but leaks and cold bridges still dictate the conditions. As Dr. Lina Mercer, an indoor air scientist I spoke with, told me:

“Salt is a buffer. It trims the top off daily humidity spikes, especially in small, closed zones. It doesn’t cure water getting in - it buys you time to notice and fix it.”

• Open trickle vents or windows for 5–10 minutes twice daily.
• Dry clothes near an open window or outdoors where possible.
• Put lids on pans when cooking and run the extractor for 10 minutes afterwards.
• Maintain steady heating on cold days to prevent condensation cycles.
• Look for hidden leaks; salt cannot overcome a constant drip.

What a bowl of salt cannot replace - and why it still helps

One point is worth repeating: salt cannot extract moisture from a wall or repair a leaking pipe. It will not prevent rain penetration or solve a thermal bridge behind a wardrobe. What it can do is reduce the humidity spikes that encourage surface moisture and mould, especially in stagnant corners. In small rooms, cupboards and beneath sinks - areas where a dehumidifier may not fit or may seem excessive - salt can be a modest, quiet helper.

There are clear limitations. Standard table salt becomes properly active only above about 75% RH, so in less humid conditions it gives a gentle nudge rather than a strong pull. That is still useful, since mould is opportunistic but slow to establish. If you need greater moisture control, calcium chloride desiccants and plug-in dehumidifiers are available for good reason. Begin with salt, monitor your RH for a week and make a decision based on evidence rather than instinct. Your nose may notice the change first.