Cold spots and thermal bridges
If mould appears in the same corners every winter while the rest of the wall stays clean, you're probably looking at a thermal bridge — a spot where heat escapes faster than through the surrounding wall, leaving the inside surface colder.
What is a thermal bridge?
Heat flows out of a home through walls, roofs, floors and windows. Most of the wall loses heat at a fairly even rate, but some places let it out faster:
- Geometric bridges — external corners, where there's more outside surface than inside surface, so the corner loses heat from two sides.
- Material bridges — where something more conductive passes through the insulation: concrete or steel lintels above windows, concrete floor slabs running out to balconies, metal wall ties or fixings.
- Junctions — where walls meet ceilings, floors or roofs, and the insulation in one doesn't join up with the next. Window reveals are a classic: the wall may be insulated, but the return around the window often isn't.
- Gaps in insulation — missing or slumped loft insulation at the eaves, a cavity wall where the fill didn't reach, or pipes and cables boxed through insulation.
Why they cause mould
The inside surface of a thermal bridge can be several degrees colder than the wall next to it. Colder surfaces push the humidity of the air touching them higher. A room at a comfortable 60% can easily put a cold corner above 80% — the level at which mould can grow if it lasts — without any visible water.
Building standards measure this with the temperature factor (fRsi): how close the inside surface stays to room temperature, from 0 (as cold as outside) to 1 (as warm as the room). UK guidance from BRE recommends a factor of at least 0.75 at junctions in homes to keep mould risk low. Older, uninsulated corners often fall well below that.
How to find your cold spots
- Look for the pattern: mould in top corners of outside walls, along the wall–ceiling line, around window reveals, above lintels, or low down where walls meet solid floors.
- Measure: an infrared thermometer pointed at the corner and then at the middle of the wall, early on a cold morning, shows the difference in seconds.
- Thermal camera: some councils and libraries lend thermal imaging cameras, and some phone attachments are inexpensive. They make bridges obvious.
- Compare with dew point: enter your room reading in our dew point calculator and compare the corner temperature with the "80%" and dew point figures.
What you can do
Quick, low-cost changes
- Move furniture off outside walls and out of corners, ideally 5–10 cm, so room air can warm the surface.
- Don't block radiators with sofas or long curtains, and keep curtains from trapping cold air against reveals overnight.
- Keep problem rooms heated steadily rather than letting them go cold all day.
- Reduce humidity in the room (see ideal humidity), so cold spots stay below the danger level.
Longer-term fixes
- Top up loft insulation right to the eaves, without blocking eaves ventilation.
- Insulate window reveals when other work is done. Even thin insulated board makes a difference.
- When insulating walls, design the junctions — where the insulation stops is where new cold spots form.
Insulating an old wall? The Energy Saving Trust and Historic England both advise fixing existing damp first and choosing a system that suits the wall. Solid brick and stone walls handle moisture differently from modern cavity walls, and badly detailed internal insulation can move condensation into the junctions around it.
Our full assessment estimates corner and behind-furniture temperatures for your wall type, and shows how much moving furniture or lowering humidity helps.