Cold Walls and Thermal Bridges: Why Corners Grow Mould
A thermal bridge is a place where heat escapes faster than through the surrounding wall — a concrete lintel, a balcony slab, a wall/floor junction, an uninsulated corner. BRE notes that the higher heat flows at such junctions "cause localised reduction in the internal surface temperatures, which in turn can lead to surface condensation and mould problems"[1].
The temperature factor (fRsi)
Designers use the temperature factor fRsi = (Tsurface − Tout) / (Tin − Tout). BRE Information Paper IP 1/06 gives critical values; for dwellings, residential buildings and schools it is 0.75[2][1]. With 20 °C inside and 0 °C outside, 0.75 means the coldest inside surface should stay at or above 15 °C.
Why 15 °C? Our own calculation: air at 20 °C and 60% RH next to a 15 °C surface reaches about 80% RH at that surface — the threshold commonly used in building physics for mould risk (it is the criterion in ISO 13788)[3]. The calculator shows this "surface RH".
Test a wall yourself
On a cold day, measure indoor air temperature and RH.
Measure the coldest surface spot (corner, behind a cupboard) and the outdoor temperature.
Compute fRsi with the formula above; below 0.75 in a home means elevated risk under typical conditions.
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