Most UK mould problems start in November and become visible by February. The data underneath them is predictable: sustained humidity, cold surfaces, reduced ventilation.
The seasonal pattern.
Winter indoor air holds more moisture relative to the air's capacity, because cold air holds less moisture absolutely. Combined with showering, cooking and drying laundry indoors, winter relative humidity in a typical UK home runs 15–25% higher than summer.
The mould-risk arithmetic.
Mould spores germinate when the water-activity at a surface is above ~0.75 — roughly 75–80% relative humidity sustained against that surface. Cold corners locally elevate RH. So even a room at 55% RH can have an 80% RH micro-environment at its coldest wall.
What good winter humidity looks like.
- 30–50% RH room average. Healthy, comfortable, low mould risk.
- 50–60% RH room average. Elevated — investigate why before it sustains.
- 60–70% RH room average sustained. Mould-risk zone — if a cold surface is involved, expect issues within weeks.
- Above 70% RH room average sustained. Active mould risk — ventilate aggressively, dehumidify, identify moisture sources.
The ventilation-vs-heating trade-off.
Opening a window lowers RH but also lowers temperature. Most UK homes can run cross-flow ventilation for 10–15 minutes a day even in winter without measurable heat penalty — and that single intervention typically drops daily-average RH by 5–10 percentage points.
The retrofit answer.
For homes that struggle with winter humidity year after year, the long-term answer is mechanical ventilation with heat recovery (MVHR) — which provides constant fresh air with ~85% of the outgoing heat recovered. Continuous monitoring before and after MVHR retrofit gives you the payback story.
For landlords specifically.
A continuous humidity record across the tenancy is the most useful single piece of evidence in a damp dispute — and Awaab's Law makes the data trail effectively mandatory for social landlords in England. See our landlord mould monitoring guide.