What a thermal bridge is
A thermal bridge is a part of the building fabric that conducts heat out more readily than the construction around it. Heat takes the easy route, the inside surface at that point runs colder than the surrounding wall, and if it drops below the dew point of the room air, it condenses. Mould follows the condensation, which is why growth appears in the same predictable places.
Where they occur
Concrete or steel lintels over windows and doors; window and door reveals; junctions where two external walls meet; the junction of wall and ground floor, and wall and roof; balconies and canopies passing through the envelope; wall ties and structural fixings; and any area where insulation is missing, compressed, displaced or simply stops short. In cavity walls, poorly installed or slumped insulation creates bridges that did not exist on the drawings.
Why the mould is always in the corner
Corners lose heat in two directions at once and have the least air movement in the room. Put a wardrobe against an external wall in an unheated bedroom and you have added a still, unheated pocket to the coldest surface in the house. Nothing about the occupier's behaviour has changed; the temperature of that one surface has.
How it is identified
Surface temperatures measured against the room's dew point tell you whether a surface is at risk. Thermal imaging shows the pattern — where insulation stops, where a lintel runs, where a cavity is bridged — and is much faster than guesswork, though it shows temperature rather than moisture and has to be read alongside the readings.
What can be done about it
Options range from the free to the substantial: move furniture off cold walls and allow air to circulate; raise background heating so surfaces stay above dew point; reduce the moisture load so dew point falls; insulate the reveals, heads and junctions that are bridging; or insulate the elevation properly, with the junctions detailed so you do not simply relocate the cold spot.
The mistake to avoid
Insulating the middle of a wall and ignoring the reveals, the lintel and the floor junction moves the problem rather than solving it: the wall is warm, the remaining bridges are now the coldest surfaces in the room, and the mould reappears in a neat line around the window. Continuity is the whole point of the exercise.
Limitations of general guidance
This article is general guidance. It cannot confirm the cause of dampness in a specific property. Recommendations should follow an inspection of the building, its external environment and its internal conditions.
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