Why it happens
Black mould grows where warm, moist indoor air meets cold wall surfaces. Solid-wall Yorkshire terraces and stone cottages are especially prone.
Short-term fixes don't last
Mould sprays and anti-mould paint treat the symptom. Unless the wall surface is warmed, the mould returns every winter.
The permanent fix
Spray cork insulation raises wall surface temperature above dew point, and external wall insulation does the same from the outside.
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