How to tell if mould is hiding inside your walls
Yes, it can grow out of sight, and it leaves clues
Mould needs moisture, not light, so it can grow behind plaster, inside plasterboard partitions, under skirting and beneath flooring without showing on the room side. When water gets into a wall from a leak, from the ground or from condensation forming within the structure, the inside can stay damp long after the surface looks dry. The good news is that hidden damp rarely stays completely hidden. It changes how a wall looks, smells and feels, and those changes are what you look for.
What you might see
- Paint that bubbles, blisters or flakes in one area, or plaster that sounds hollow and crumbles.
- Yellow or brown staining, or a faint grey shadow, that spreads slowly or comes and goes with the weather.
- A tide mark low on the wall, or salty white crystals on the surface.
- Wallpaper lifting at the seams, or skirting boards that have warped or swollen.
- Small dark spots appearing along a corner, a skirting line or the edge of a window reveal.
What you might smell or feel
A musty, earthy smell that you can't trace is often the first sign. It tends to be strongest in the morning, after a room has been closed up, or when you open a fitted wardrobe on an outside wall. Run the back of your hand along the wall: a patch that feels clammy or noticeably colder than the plaster around it is worth noting. Some people find the smell is stronger close to a particular socket or skirting board, which can help narrow the spot down.
Where hidden damp usually starts
Walls with a water source on the other side are the first suspects: behind a shower, alongside a kitchen sink, under a window with tired seals, and below a bathroom or roof terrace. Older ground-floor flats can draw moisture up from the ground, which shows as the tide mark described above; our guide to rising damp explains why. In Cyprus, the walls behind and below wall-mounted air-conditioners are another place to check, because a blocked or badly routed condensate pipe can soak the plaster quietly, as covered in mould around air-conditioning units. If you aren't sure which kind of damp you're looking at, this comparison of the three types helps.
What to check yourself, without cutting holes
Keep a simple record. Take dated photos of the stain or the bubbling paint so you can see whether it grows. Note whether the smell or the marks get worse after rain, after showers or when the air-conditioner is running. Look at the other side of the wall if you can, including the outside face, for cracks, broken render or a dripping overflow. Check under the sink and behind the washing machine for slow drips. Avoid opening up the wall or pulling off plaster yourself: it can release spores into the room and often damages more than it reveals.
Confirming it properly
Hidden damp is where testing earns its keep, because the source decides the fix. A moisture-mapped survey of the affected rooms takes moisture readings in every affected room and ends with a room-by-room moisture map sent to your phone, plus a written report with the quote. The work begins with the cause rather than the stain, so the result lasts. Where mould has taken hold inside the wall, contained removal and repair of the damaged wall covers containment, removal, repair and an anti-mould finish.
Smelling mould but can't find it? book an inspection from €75 and we'll locate it, or call +357 99 759144.