A dark patch in the corner behind the sofa. Black dots on the bedroom window frame. A grey haze on the wooden skirting board you painted only last year. But why does the same product work in one spot and not in another? In this article you'll find out where mould comes from, how to tackle mould surface by surface and when it's better to stop cleaning.
Short answer
You remove mould with heat, not with bleach: removing the colour isn't the same as killing the organism. On a dense surface such as tiles or glass, the mould sits on top and you simply wipe the spot away. On porous material such as plaster, wood and sealant, the growth sits below the surface. A product that only removes the colour leaves the mould alive and the patch comes back. The cause is almost always moisture that sits for too long.
Where does mould in the home come from?
Mould is an organism that grows where moisture, warmth and something edible come together.
The spores are always in the air, indoors and out. So they're not the problem, and you can't clean them away either. The problem is the combination that lets them grow: a surface that stays damp for a long time.
That surface is almost always the coldest one in the room. Warm air holds more moisture than cold air. When that warm indoor air meets a cold outside wall, a single-glazed window or a poorly insulated corner, the moisture condenses there. That's exactly where the patch starts.
Food is the third part, and that's usually dust, skin cells or the layer of paste under old wallpaper. Mould doesn't eat the tile. Mould eats what's on the tile.
Where in the home does mould appear most often?
Below you'll find the five spots where things most often go wrong in homes, in the order you're most likely to come across them.
1. The cold outside wall
An outside wall on the north or west side stays cold the longest, and if there's a sofa or a cupboard against it, the air can't circulate. Move large furniture a few centimetres away from the wall so air can flow behind it.
2. The window frame and the glass
Condensation forms on the window frame, especially in the bedroom, where you breathe out moisture at night. Wipe the frame dry every day in winter and open the window briefly, even on cold days.
3. The bathroom
The bathroom has the highest humidity in the house every day, plus soap scum as food. The grout and the sealant are the weak spots, because they're porous.
4. Wood and skirting boards
On wood, mould penetrates the fibre, so the patch often leaves a shadow after cleaning. Untreated wood absorbs moisture like a sponge, which makes it more susceptible than painted wood.
5. Behind wallpaper and in corners
Behind wallpaper there's paste, and paste is food. A patch that shows through the paper is usually bigger than it looks, because the growth is on the back.
How to remove mould in 5 steps
Below you'll find five steps that work in any room. You start by ventilating, followed by protecting, killing with heat, wiping away and finally drying.
1. Ventilate before you start
Ventilate beforehand. Open a window so the spores you loosen go outside instead of through the room. Wear gloves and keep your face out of the steam.
2. Cover what you don't want to touch
Covering up takes two minutes and prevents most of the follow-up work. Put an old sheet or plastic sheeting under the spot, because whatever you remove from a wall falls down.
3. Kill the mould with heat
Heat tackles the organism itself. That's the difference with bleaching: a bleach-based product removes the black colour while the mould carries on living in the pores, so the patch comes back. For mould patches, the Achaté Handheld Steam Cleaner PRO is the right fit. It delivers 3.5 bar, heats up in three to five minutes and weighs 1.2 kg, so you can hold it for a long time. Work in short strips and keep the nozzle moving.
4. Wipe the spot with a cloth you throw away
Whatever you loosen has to leave the room. Wipe the spot with a cloth and throw it away afterwards, or wash it separately at a high temperature. A cloth you reuse just moves the problem to the next room.
5. Let the surface dry completely
Drying is the step that decides whether the work lasts. Leave the window open for another hour and feel whether the surface is really dry before you put anything against it. A damp spot starts again within days.
Note: never mix bleach with a cleaner containing ammonia. That combination produces a toxic gas. If you're working in a small room or in a cupboard, heat is the safer route, because no fumes are released.
By surface: which Achaté product should you use?
The surface determines whether you remove the mould or just the colour, and therefore which device and which attachment you use. In the table below you'll find the most common surfaces at a glance.
Surfaces, approach and the right model
| Surface | Model and attachment | What to watch out for |
|---|---|---|
| Tiles and glass | Handheld Steam Cleaner PRO, nozzle with cloth | Dense layer, mould sits on top |
| Grout | Handheld Steam Cleaner PRO with round brush, then the grout head of the cleaning brush | Porous, two rounds for old grout |
| Sanitary sealant | Handheld Steam Cleaner PRO, pointed nozzle without brush | Penetrated sealant won't come clean |
| Painted wall | Handheld Steam Cleaner PRO, short bursts with cloth | Too much water damages the paint layer |
| Window frame and corners | Handheld Steam Cleaner PRO, pointed nozzle | Condensation returns here every day |
| Large wall areas and floors | Steam Cleaner PRO V2, floor head | Work in sections, not the whole wall at once |
| Plaster and gypsum | Work dry, no steam | Absorbs moisture, can bulge |
| Untreated wood | Sand dry, then treat | Mould penetrates the fibre |
| Textiles and mattresses | Spot Cleaner with spray extraction | The moisture has to come back out, not just go in |
| Wallpaper | Remove, don't clean | The growth is on the back |
Not sure whether a surface is porous? A drop of water will tell you: if it soaks in, it's porous and the mould goes deeper than you can see.
The rule of thumb for the two steam models: the handheld steam cleaner is for targeted work on small, tricky spots, the PRO V2 for large surfaces in one go. With mould, it's almost always the first, because mould patches are small and in awkward places.
Does a steam cleaner work against mould?
Yes, a steam cleaner works against mould, because heat kills the organism instead of bleaching it.
That's the difference with most shop-bought products. They remove the colour, so the spot looks clean, while the mould simply lives on in the pores. That's why the same patch is back in exactly the same place two weeks later.
Steam also runs on tap water. In a bedroom, a cupboard or a crawl space, that makes a noticeable difference: no fumes, no residue where you keep your clothes or your food.
What steam can't do is solve a structural problem. If there's a cold bridge, a leak or excessive humidity behind it, you remove the mould and the patch comes back later, until that cause has been dealt with.
When should you stop and have someone take a look?
Stopping is the right choice as soon as the mould is no longer a cleaning problem.
Three signs point to that:
- It keeps coming back in the same spot, even after you've cleaned it and started ventilating better.
- The patch is larger than about half a square metre or appears in several places in the same room.
- It's behind or under a finish: behind wallpaper, under a floor, in the plaster, or somewhere you'd damage the surface by removing it.
In those cases there's moisture in the structure, and cleaning won't help. A damp specialist can measure where it's coming from, and that's cheaper than redoing the same bit of wall every year.
Want to keep mould away? Choose steam from Achaté
Mould disappears with heat and stays away with dry air. A product that bleaches the colour away buys you a few weeks, and then the same patch is back. If you kill the mould and remove the cause of the moisture, the surface stays clean.
The Achaté Handheld Steam Cleaner PRO is the handy choice for mould patches and comes with a brush attachment for frames, grout, sealant and corners. For larger surfaces and floors, there's the Achaté Steam Cleaner PRO V2. If the grout is stubborn, the Achaté Electric Cleaning Brush with the narrow grout head scrubs it afterwards. If the mould is in textiles, a spot cleaner pulls the moisture back out of the fibre along with the dirt.
The bathroom has its own step-by-step guide, because soap scum and daily condensation come together there. And for mould in a mattress, there's a separate guide: read how to get mould out of a mattress.
Want to tackle mould without bringing bleach into your home? Take a look at the Achaté steam cleaners and choose the model that suits your job.
Frequently asked questions about removing mould
What's the best product for removing mould?
The best product depends on the surface. On dense surfaces such as tiles and glass, heat works most thoroughly, because it kills the mould instead of just bleaching the colour. On porous material such as plaster, wood and sealant, the mould sits below the surface and cleaning often isn't enough. Never mix bleach with a cleaner containing ammonia.
How do you remove black mould?
You remove black mould by treating the spot with heat first and then wiping it with a cloth you throw away. Ventilate while you work and wear gloves. If a dark shadow remains after drying, the damage is in the material and cleaning is no longer enough.
Does mould come back after cleaning?
Mould comes back if the cause remains or if only the colour has been removed. Bleaching lightens the spot while the organism lives on in the pores. If the patch keeps coming back despite heat and better ventilation, there's moisture in the structure and measuring it is the next step.
Is mould in the home dangerous?
A small patch is mostly a nuisance and easy to tackle yourself. With larger areas, more spores get into the air, which can irritate the airways and is especially noticeable in people with allergies or asthma. So ventilate while you work, and have large or recurring patches looked at by a specialist.
