How to Identify Mould

Signs

It grows and changes. If the patch is bigger or a different shape after a couple of weeks, it is live fungus.
Velvety or fluffy texture. Dirt is flat; fungus often has a slight relief.
Irregular, diffuse edges. Fungus does not grow in straight lines.
A characteristic smell. Musty, damp, "basement"-like.
It sits in a damp spot. An external wall, a corner, behind furniture, by a window, in the bathroom.
It does not wash away. Or it washes away and reappears in the same place later.

The most reliable sign is not appearance but behaviour: fungus grows and returns, dirt does not.

Location is almost as informative as appearance. Fungus grows where a surface stays damp for a long time, and in practice that always means an external wall, a corner, the area around a window or a room with a poor extract. If a dark patch is on an internal partition in a warm, dry room, it is usually not fungus — most likely it is an old stain, a dirt deposit or a damaged paint layer.

What is not mould

Appearance	What it most likely is	How to check
White, crystalline, powdery	Salt efflorescence from masonry	Rubs off easily when dry, but returns
Grey, flat, even	Dust deposits	Wipes off and does not return
Yellowish-brown patch with a clean edge	An old water stain	Does not grow, the edges do not change
Black along a silicone edge	Fungus in the silicone	Does not wash away
Dark, velvety, growing	Mould	Increases over time

A practical check: carefully wipe a small area with a damp cloth. Dirt and dust wipe off and do not return. Salt efflorescence wipes off but reappears after a while. Fungus usually leaves a stain in the material, and it comes back in the same place.

✓ Points to fungus

The patch grows and changes shape within weeks.
The surface has a velvety or fluffy texture.
Moisture in the material at the patch is higher than beside it.
There is a characteristic musty smell.

✗ Points to something else

White crystalline powder with no smell — salt efflorescence.
A yellow-brown patch with a clean edge that does not grow — an old water stain.
An even grey film along the ceiling line — dust or soot deposits.
A film that wipes off and does not return for months — dirt.

Salt efflorescence is the thing most often mistaken for mould, especially in basements and on brick walls in pre-war buildings. It is not a fungus and an anti-mould product will not remove it. But it is a sign of moisture: salts are washed out of masonry only where water moves through the structure regularly. So it is a reason to check the structure, not to treat the surface.

Where to check

If you have your suspicions, check these places — mould appears there first and gets noticed there last:

external wall corners, especially near the floor and ceiling;
behind large furniture standing against an external wall;
the back panel of a wardrobe, if it stands against an external wall;
the bottom edges of window frames and the ends of sills;
silicone joints in the bathroom;
under the sink and behind the toilet;
the basement, the loft, the external wall of a loggia;
skirting boards along external walls and the bottom of door frames.

Checking has a season of its own. Condensation-driven mould appears during the heating season, from October to March, when external wall surfaces cool while the moisture in the indoor air stays the same. So mid-winter is the best moment to look at the wall behind a wardrobe — in summer there may be nothing visible even if the problem exists.

40–60%normal indoor humidity

Oct–Marwhen to check

from €40inspection with readings

What to look for before the patch is visible

The signs of mould appear in a sequence, and the visible patch is not the first of them. If you know what to look for earlier, the problem is found at a stage when it is still small.

1SmellA musty, damp, "basement" smell often appears months before a patch. It is more noticeable when you come back after a longer absence.

2Misted windowsIf the windows mist every morning all winter, humidity in the room is systematically too high. Mould in such a room is a matter of time.

3A cold spot on the wallWith your palm you can feel that an external wall corner or a strip along the window reveal is noticeably cooler. Condensation settles there first.

A hygrometer costs a few euros and is the most useful of all the tools. In a living room, normal relative humidity is roughly 40–60%. If the reading is consistently above 70%, the question is no longer whether mould will appear, but where and when.

Where to look in each building type

The type of building tells you which places to check first — it saves time and often answers the question straight away.

✓ Pre-war building

The lower part of walls on the lower floors — rising damp.
Deep window reveals where the air does not move.
The top floor at the timber floor structure, if there has been a roof leak.
A basement with masonry or fieldstone foundations.

✓ Panel block and new build

External wall corners and panel joint lines in the bedroom.
A windowless bathroom with a weak shared extract.
The wall behind a wardrobe against an external wall.
In a new build — every room at once in the first winters, if the ventilation is not switched on.

How we identify it

Visual inspection

We look at the suspicious areas and assess the character of the patch.

Moisture reading

We measure moisture in the material — fungus only grows where there is moisture.

Air humidity assessment

We determine whether the problem is local or on the scale of the whole room.

Checking hidden places

If there is a smell but no visible patch, we look for the source behind finishes and furniture.

Conclusion

We tell you whether it is mould, what the cause is and whether treatment is needed at all.

A measurement separates two things the eye cannot: air humidity and material moisture. If moisture in the material at the centre of the patch is sharply higher than a metre away, the water is coming from the structure — we look for a leak. If the material is dry at depth but air humidity in the room is consistently high, it is condensation, and the answer is air exchange and surface temperature.

We also say when treatment is not needed. If it turns out to be salt efflorescence, an old dry stain or a dust deposit, we name it and treat nothing. That is a more common outcome of an inspection than people expect.

Prices

Inspection with moisture readings from €40. If we go on to carry out treatment after the inspection, we credit the inspection fee against the price of the work.

Frequently asked questions

There is a white fluffy film on the wall — is that mould?

Often it is salt efflorescence from masonry, especially in basements and on brick walls. It is not a fungus, but it points to moisture in the structure that is worth checking.

Will you come and look if I am not sure?

Yes, that is exactly why the inspection is a separate service. If it turns out no treatment is needed, we say so — that is the most honest possible outcome.

Can mould be identified from a photograph?

A photo helps us get our bearings and we are happy to look at one, but the cause cannot be determined from an image — moisture does not show in a photograph. A measurement is needed in any case.