That black bloom in the corner of the bedroom, the grey stripe over the window, the mould behind the wardrobe on an outside wall — nine times out of ten that’s cold bridging, not “drying clothes indoors”. Junction detailing is a daily reality of our work: window reveal and sill detailing appears on 33 of our real quotes, and condensation-control measures on 54 — because insulating the flat wall is the easy part. This guide explains what a thermal bridge is, why mould finds it first, and how external and internal wall insulation each have to deal with it.
What a cold bridge actually is
Heat doesn’t leave a house evenly. It takes the path of least resistance, and wherever a material with poor insulating value passes through the building fabric — dense concrete, steel, solid brick — heat streams out faster there than through the surrounding wall. That path is a thermal bridge, or in plain trade language, a cold bridge.
The consequence indoors is a cold patch. The internal surface above a concrete lintel might run several degrees colder than the plaster half a metre away. You can’t feel it by hand most of the year, but your indoor air can: warm air holds more moisture than cold air, and when moist room air touches a surface cold enough — below its dew point — the moisture condenses onto it. Do that every evening through a heating season and you have damp plaster, then mould.
This is why mould patterns are diagnostic. Damp from a leak spreads from a point and stains. Condensation from cold bridging follows the hidden structure — it draws a neat line over the lintel, shadows the mortar joints, fills the corner. The mould is drawing you a map of your wall’s thermal weak points.
Where cold bridges hide in a typical London home
- Lintels. Concrete or steel lintels over windows and doors conduct heat far faster than brick. The classic symptom is a horizontal mould stripe on the plaster just above the window.
- Window and door reveals. The wall wraps in towards the frame and is at its thinnest there — often a single brick’s width, or less where frames were replaced. Reveals are the most common cold bridge we treat, which is why they show up on 33 of our quotes as a separately priced line.
- External corners. Pure geometry: more outside surface shedding heat than inside surface receiving it, plus poor indoor air circulation. Corners behind furniture are worst — the wardrobe blocks the little warmth and airflow the corner was getting.
- Floor and ceiling junctions. Where a suspended timber floor meets an outside wall, or a solid party-wall spandrel runs through, the junction leaks heat along the structure.
- Bay windows and chimney breasts. Bays are almost entirely junctions — we’ve detailed bay windows on 9 quotes and chimney-related works on another 9 — and both concentrate cold surfaces in small, poorly ventilated volumes.
If your home is a Victorian or Edwardian solid-wall terrace, the whole wall is, in a sense, one big bridge — there is no cavity to slow heat down. Our guide to insulating a Victorian terrace covers that wider problem; this page is about the junctions.
Why mould appears at bridges first
Mould spores are everywhere; what they need is sustained surface moisture. The flat, insulated part of a wall stays warm enough that condensation never forms, or dries before mould can establish. A bridge gives mould the two things it needs: a surface that drops below the dew point every evening, and — in corners and behind furniture — still air that never dries it out.
That’s also why the standard advice of “wipe it with mould spray and open the windows” only ever buys time. Ventilation lowers the moisture load, and it genuinely matters — we include ventilation and extract works on 70 of our quotes — but if the surface itself runs cold, winter will bring the mould back. The durable fix is to warm the surface, and the only way to warm the surface permanently is insulation over or around the bridge.
How external wall insulation wraps the bridges
External wall insulation is the more forgiving of the two approaches, and cold bridging is the main reason why. Because the insulation goes on the outside, it runs continuously past lintels, corners, floor junctions and party-wall intersections — the entire masonry mass ends up on the warm side, and the bridges are simply buried under the blanket. The old cold spots warm up, and the wall itself starts storing heat instead of leaking it.
The junctions that remain are the openings. At every window and door the EWI has to stop, and how the installer handles that edge decides whether you get a wrapped house or a house with a cold picture-frame around every window. The detail that matters — and the one to ask any installer about — is the reveal board: a thin slice of insulation (usually 20–30mm, because the frame leaves no room for more) returned into the reveal and sealed to the frame with trims, with the sill extended to throw water clear of the new render. Skipping reveal boards saves a day’s fiddly work and leaves the most condensation-prone junction in the house untreated. It’s a line we price explicitly, every time.
How internal wall insulation must detail them
Internal wall insulation can absolutely deal with cold bridging — but it has to be designed, not just boarded. When you insulate a wall from the inside, the masonry behind the insulation runs colder than before. Any junction you fail to treat is now the warmest escape route left, and condensation concentrates on it harder than it did before the work. Poorly detailed IWI doesn’t just miss bridges — it sharpens them.
Good IWI practice, which you should see itemised on any quote:
- Insulation returned into reveals — even 20mm of insulated board into the window reveal keeps that junction above dew point.
- Lapping at flanking junctions — returning a strip of insulation along the ceiling, floor and party wall where they meet the external wall, so the bridge path is stretched and weakened.
- An airtight vapour layer, taped and sealed, so moist room air can’t slip behind the boards and condense on the cold masonry — we carry airtightness and vapour-membrane items on 73 of our quotes.
- Condensation control and extract ventilation where the room’s moisture load demands it.
If you’re weighing the two approaches, our EWI vs IWI guide compares them properly — but on cold bridging specifically, the summary is: EWI wraps bridges almost automatically; IWI beats them only with careful detailing.
The honest caveat: when bridging isn’t your problem
Not every mould patch is a cold bridge, and insulating a wall that doesn’t need it is money wasted. If mould covers whole walls evenly, or appears in a bathroom with no extractor, the problem is moisture load and ventilation, not fabric — fix the extract fan first. If the damp is at skirting level with a tide mark, suspect rising or penetrating damp, which insulation will hide, not cure; we carry damp and mould remediation works on 32 of our quotes precisely because the underlying cause has to be fixed before boards go anywhere near a wall.
The pattern is the clue: sharp, structure-following patches at corners, lintels and reveals point to bridging; even coverage points to ventilation; tide marks point to damp. A free survey settles it in half an hour — get a free, fixed, itemised quote and we’ll tell you which problem you actually have, including when the answer is an extractor fan rather than an insulation job.
Cold bridging FAQs
What is cold bridging in simple terms?
A cold bridge (thermal bridge) is a spot where heat escapes faster than through the surrounding fabric — a concrete lintel, an uninsulated window reveal, an external corner. The inside surface at that spot runs colder than the rest of the wall, so moisture from indoor air condenses there first, which is why mould appears in patches and stripes rather than evenly across a wall.
Why does mould always appear in the corners of rooms?
Two effects stack up. Geometrically, an external corner has more outside surface losing heat than inside surface collecting it, so it runs colder than the flat wall. On top of that, air circulation in corners is poor — especially behind furniture — so the cold surface never gets warmed or dried by moving room air. Cold surface plus still, moist air is exactly the recipe for condensation and mould.
Does external wall insulation fix cold bridges?
It deals with most of them in one move, which is one of its biggest advantages. Because EWI wraps the outside of the building continuously, lintels, corners, and the wall itself all end up on the warm side of the insulation. The junctions that remain — window reveals, sills, eaves — still need proper detailing with reveal boards and trims, which is where installer quality shows.
Can internal wall insulation make cold bridging worse?
Badly detailed, yes. If you insulate the flat wall but stop dead at reveals, party walls and floor junctions, those spots become relatively even colder than before, and condensation concentrates on them. Good IWI design returns insulation into reveals, laps it at floor and ceiling junctions, and includes condensation control — items we price explicitly on our quotes rather than hoping for the best.
Is the black mould in my corner dangerous?
Persistent mould is a health concern, particularly for respiratory conditions, and it is also a symptom: the fabric there is cold enough for the surface to sit below the dew point of your indoor air. Cleaning it off treats the symptom. Insulating the bridge — or improving ventilation and heating patterns where the fabric is fine — treats the cause. A survey can tell you which case you have.