Why Mould Comes Back After Treatment

Five reasons

1The moisture cause stayedThe most common one. The treatment removed the fungus, but the wall still condenses, the pipe still leaks or the ventilation still does not work. The fungus comes back where the moisture is — every time.

2The fungus was deeper than it lookedThe surface was treated, but the fungus stayed inside the plasterboard or plaster. After a while it grew back through.

3Too small an area was treatedThe fungus spreads through material further than it shows on the surface. If only the visible patch was treated, it comes back from the edges.

4The structure was damp when it was treatedThere is no point treating a wet wall — it has to be dried first. This is the typical mistake after flooding, when people want to "get it all sorted quickly".

5It was painted overPaint hides the patch for a few months. The fungus underneath keeps growing and eventually comes through.

Only one of the five reasons is about the treatment itself. The other four are about what was not done around it.

Why it comes back specifically in winter

A very common picture: the patch appears every year from October to March, stops in spring and seems to have gone by summer. That is no mystery — it is the condensation timetable.

Why the season decides it

During the heating season the inner face of external walls, window frames and wall corners cool down, while the amount of moisture in the indoor air stays the same or rises: windows are kept shut, airing happens less often, laundry is dried indoors. When warm air meets a cool surface, moisture settles out.

In summer the wall is warm, there is no condensation, and the fungus stops growing — but it has not gone anywhere, it is simply waiting for the next season.

The corner of an external wall is usually several degrees cooler than the middle of the wall, because heat escapes there in two directions. It is the same difference of a few degrees that decides whether condensation forms. That is why the recurrence is so precise in its location — the same corner every year.

Oct–Marcondensation season

40–60%normal indoor humidity

from €40repeat inspection

What recurs most in which building

1Pre-war buildingThe most frequent repeats are cases where the chimney ventilation flue was left without an air supply after the windows were replaced, and, on the lower floors, rising damp from the foundations.

2Panel blockPatches recur along the lines of panel joints and external wall corners. If a cold bridge remains in the structure, treatment alone is never enough.

3New buildIn the first winters recurrence is usually construction moisture plus ventilation that is not switched on. Here the answer is often not treatment but air exchange and time.

How to check which is yours

Sign	Likely reason
It came back in exactly the same place	The moisture cause has not been dealt with
It came back around the previously treated area	Too small an area was treated
It came through the paint	It was painted over untreated fungus
It came back after a few weeks	The structure was damp at the time of treatment
It appears every winter	Condensation — seasonal cooling of the surface

Before calling anyone out again it is worth gathering a few facts. They help you avoid paying twice for the same mistake.

Note how long after the previous treatment the patch came back.
Note whether it came back in exactly the same place or along the edges of the treated area.
Measure the humidity with a hygrometer over several days in a row.
Check the extract with a sheet of paper — if it does not stick, the cause was never dealt with.
Find out whether finishes were removed last time or only the surface treated.

The third and fourth points together usually answer the whole question. If humidity is consistently above 70% and the extract is not working, it does not matter how well the previous treatment was carried out — the conditions for the fungus are exactly as they were.

What we do

We handle a repeat case differently from a first-time one. If mould has already come back, treatment alone is evidently not enough — so we start with a measurement and a diagnosis, and only then talk about treating.

Measurement, not treatment

We start with moisture readings, to understand what was not resolved before.

Assessing the previous treatment

We assess how deep and how wide the previous work went.

Assessing the material

We check whether the fungus has stayed inside the material.

Treatment at the right scope

We treat a sufficient area and depth, drying the structure out first.

Recommendations for the cause

We state clearly what has to change — without that it will recur again.

If we conclude that the cause is structural and requires building work or a repair to the shared ventilation, we say so. That is not always what a client wants to hear, but it is the only answer that solves the problem.

✓ What we do in a repeat case

Start with a measurement, not a treatment.
Check how deep into the material the fungus has stayed.
Treat a substantially wider band than the visible area.
Say so if the fix requires building work that we do not carry out.

✗ What we do not do

Repeat the same treatment at the same scope.
Treat a wet structure.
Promise that treatment alone will solve a structural problem.
Sell work when the readings show the ventilation is what needs fixing.

What we do not do in this situation

A repeat case is the easiest place to sell unnecessary work, so we will state the limits clearly. We do not carry out plumbing, roofing or facade repairs, we do not clean ventilation shafts, and we do not do insulation work. If the readings show that is exactly what is needed, we name it specifically — which spot, which sign, which reading — and say who should deal with it.

We also do not treat a wet structure. If that is precisely what was done last time, there is no sense repeating the same mistake. First drying with checked readings, then treatment.

And there are cases where we say no treatment is needed at all. If it turns out that what came back is not mould but salt efflorescence or an old dry water stain, there is nothing to treat — what is needed is redecoration and, if the readings show it, dealing with the moisture source.

What decides the scope of repeat work

1How wide an area was worked on beforeIf only the visible patch was treated previously, this time the band around it has to be widened substantially.

2Whether finishes come offIf the fungus has stayed in wallpaper, plasterboard or plaster, the material has to be cut out — surface treatment has already failed once.

3Whether drying is neededIf there is still water in the structure, that is a separate job with equipment and several days.

4AccessWhether the affected spot is clear, or behind fitted furniture, under a floor covering or at height.

Prices

Inspection with moisture readings from €40. In repeat cases the inspection matters more than ever — without it there is a strong chance of repeating the same mistake that already failed once.

Frequently asked questions

How long after treatment should mould not come back?

If the cause has been dealt with, it should not come back at all. If the cause is structural and unresolved, a return is a matter of time regardless of how good the treatment was.

Can you treat a place where another company has already worked?

Yes. We start with a measurement to understand why the previous treatment did not hold, and then choose the approach.

Mould appears every winter and disappears in summer — what is that?

That is a classic condensation case: in winter the surface cools and moisture settles on it. The answer is a change in heat and air exchange, not more frequent cleaning.

Why mould comes back