Surface condensation forms when a surface is colder than the dew point of the air touching it, so vapour condenses into visible liquid water on it. Mould, importantly, starts earlier than that — it only needs the surface relative humidity to stay high (around 80%) for sustained periods. So the visible black patches are the end of a process that began with a cold surface and humid air, long before any water was visible.
The mould growth mechanism
Mould spores are present in essentially all indoor air; they are harmless until they find the conditions to germinate. Three things must coincide for sustained growth: a high enough surface relative humidity, a tolerable temperature, and enough time. Building scientists describe this with 'isopleths' — curves showing the combination of surface RH and temperature (and duration) at which a given mould will germinate and grow:
- Surface RH around 80% sustained is the practical lower threshold for the most moisture-tolerant moulds — well below the 100% of visible condensation.
- Temperature in the typical indoor range (say 5–30 °C) is comfortable for mould; growth is fastest in warm, humid conditions.
- Time matters — brief humidity spikes (a shower) don't grow mould; sustained high surface RH (a cold corner all winter) does. This is the 'time of wetness' concept.
| Surface RH (sustained) | Condition | Risk |
|---|---|---|
| Below ~75% | Surface comfortably above dew point | Safe |
| ~80% | Germination threshold for tolerant moulds | Mould growth begins |
| 80–100% | Cold surface, no visible water yet | Active mould growth |
| 100% | Surface at/below dew point | Visible condensation + mould |
The fRsi temperature factor
Building physics quantifies the risk of a cold surface with the temperature factor fRsi — the ratio of (internal surface temperature minus external temperature) to (internal air temperature minus external temperature). A higher fRsi means a warmer internal surface relative to the conditions. UK guidance (informed by BS EN ISO 13788 and BRE work) sets a minimum fRsi of 0.75 for dwellings to avoid mould at junctions. Well-detailed, Passive-House-grade junctions comfortably exceed this; many older or poorly-detailed junctions fail it — which is exactly where mould appears.
Common indoor moulds and health
The usual culprits on damp internal surfaces include Cladosporium, Aspergillus, Penicillium and, in persistently wet conditions, Stachybotrys chartarum (the 'black mould' associated with the worst cases). Beyond staining and odour, mould and the damp conditions that cause it are linked to respiratory irritation, allergic responses and aggravation of asthma — and following high-profile cases, UK guidance now treats persistent damp and mould in homes as a health risk to be addressed at its cause, not just cleaned.
Why bleaching doesn't work
Wiping mould off with bleach or a fungicidal wash removes the visible growth but changes none of the conditions that created it. The surface is still cold; the air is still humid; the surface RH still crosses 80% every winter night. So the mould returns, usually within weeks to months. Treating the mould without treating the building physics is the single most common — and most futile — response to the problem.
How we diagnose it
- Log room temperature and RH over a representative period to capture the real daily humidity cycle.
- Thermal-image the affected surfaces to map the cold spots and read their surface temperatures.
- Calculate dew point and surface RH at the mould locations — confirming the mechanism with numbers.
- Rule penetrating/rising damp in or out (external survey, moisture measurement on multiple substrates).
- Specify a remediation that raises the cold surfaces and controls the humidity — not a coat of fungicide.