Why Mould Is Not Just a Heating Problem

29.07.2026 4 min read

When mould appears on an external wall, the usual advice is the same: heat more, ventilate more. That misses the point. Whether mould grows is not decided by room temperature but by the temperature of the room-side surface of the building element — and that can lie well below the room air temperature without anyone noticing.

The limit value is set out in a standard, and it is surprisingly high

DIN 4108-2 requires a temperature factor of f_Rsi ≥ 0,70 at the least favourable point of the room-side surface in the area of thermal bridges. The factor is dimensionless and describes what proportion of the temperature difference between inside and outside still reaches the surface:

f_Rsi = (θ_si − θ_e) / (θ_i − θ_e)

The boundary conditions of the verification are fixed for residential use: 20 °C indoor air, −5 °C outdoor air, 50 % relative room air humidity, no more than 80 % relative humidity at the surface of the building element. Applying these gives a minimum surface temperature of 12,6 °C.

This 12,6 °C is not a rounded convention. It follows from the Magnus formula: at 20 °C and 50 % room air humidity, the water vapour pressure is 11,66 hPa. The surface temperature at which this vapour pressure amounts to 80 % of the saturation vapour pressure is 12,62 °C.

Why the dew point is the wrong benchmark

In practice, the dew point is often used as a reference — the temperature at which the air is saturated and water condenses out. At 20 °C and 50 % relative humidity, this is 9,3 °C.

But the mould threshold already lies at 12,6 °C, i.e. 3,3 Kelvin above it. Anyone waiting for visible condensation is waiting too long. As the Verbraucherzentrale puts it: “From around 70 to 80 per cent directly at a cool wall, mould can grow. For this, the wall need not feel damp nor show visible condensation.”

The 80 per cent rule is a duration rule

The mould guide issued by the Umweltbundesamt (German Federal Environment Agency) is precise on this point. The moisture threshold below which, under otherwise optimal conditions, no growth occurs lies at around 70 % relative humidity at the surface. Between 70 and 80 %, it is sufficient for the moisture to be “present over a longer period” — materials do not need to be visibly wet for this. Above 80 %, almost all species relevant to indoor environments grow.

The species differ considerably in this respect. This is expressed as the a_w value, which corresponds to the numerical value of the relative surface humidity divided by 100:

Aspergillus restrictus — from a_w 0,70 to 0,75
Aspergillus versicolor — from 0,78
Penicillium chrysogenum — from 0,78 to 0,81
Cladosporium cladosporioides — from 0,86 to 0,88
Stachybotrys chartarum — only from 0,94

The verification deliberately calculates for the unfavourable case

A detail that is easily overlooked: in the mould verification, the room-side heat transfer resistance is set at R_si = 0,25 m²K/W — almost double the 0,13 m²K/W used in the U-value calculation.

This is not an error but a deliberate choice. The higher value represents impeded heat transfer: in room corners, at edges, behind furniture, behind curtains. It is precisely there, where mould first appears, that less heat from the room reaches the surface.

This also explains why mould typically starts in corners and behind cupboards rather than in the middle of an open wall surface.

What this means in practice

Three things can be derived from this:

The surface temperature is the measurement that matters, not the room temperature. An infrared thermometer at the coldest point tells you more than any room thermostat.
The relative room air humidity shifts the limit value. If it rises from 50 to 60 %, the required minimum surface temperature also rises. The verification only applies under its stated boundary conditions.
f_Rsi ≥ 0,70 does not mean “no mould”. It is a minimum requirement under standardised conditions at the least favourable point — not a free pass and not a design target.

And what about usage?

In fairness: the Umweltbundesamt explicitly categorises the causes of moisture damage into structural and usage-related. Anyone who claims mould has nothing to do with heating and ventilation is oversimplifying just as much as the opposite view.

The point is a different one: if the surface temperature is structurally below the criterion, this can hardly be compensated for by user behaviour any more. In that case, no ventilation log will help — only a change to the building element itself.

Sources

DIN 4108-2 — Thermal protection and energy economy in buildings, minimum requirements to thermal insulation. Note: since May 2026, the 2026-05 version applies, replacing 2013-02; Table 3 has been redesigned.
Umweltbundesamt, guide to the prevention, detection and remediation of mould infestation in buildings (2017), Chapter 1.2 and Table 3
Verbraucherzentrale NRW, “Living without mould”, p. 6
Deutscher Wetterdienst, Weather and Climate Glossary, entries “Relative humidity” and “Dew point”
Landeshauptstadt München, FES information sheet on minimum thermal insulation under DIN 4108-2

The numerical values in this article are calculated values under the standard, based on the stated boundary conditions. They are not measured values from a specific building and do not constitute a commitment for any particular building project.