What is a thermal bridge?
A thermal bridge is a path through the building fabric where heat escapes more easily than through the surrounding structure — a 'short circuit' for heat. Typical examples include the junction where a floor meets an external wall, the reveals around windows and doors, concrete lintels and balconies, steel beams, and the corners where two external walls meet. At these points the internal surface is colder than the rest of the wall, which both wastes heat and creates the perfect conditions for condensation and mould.
Thermal bridges are one of the most misunderstood causes of cold spots and corner mould. Homeowners often treat the resulting mould as a damp problem and never address the cold junction causing it — so it always returns.
Where thermal bridges occur
In London homes we find thermal bridges most often at floor-to-wall and roof-to-wall junctions, around window and door openings, at concrete lintels above windows, at exposed corners, and where structural elements such as steel beams or balcony slabs penetrate the insulation line. Extensions and refurbishments frequently introduce new bridges where old and new construction meet. Each has a different fix, which is why locating and understanding them precisely matters.
Thermal bridges, condensation and mould
There is a direct chain: a thermal bridge keeps a small area of internal surface colder than the rest of the room; warm, moist indoor air condenses there first; and mould grows on the resulting damp surface. This is exactly why mould so often appears in corners, around windows and at junctions rather than in the middle of a wall. Treating the mould alone never works — you have to warm the cold junction and manage the moisture.
How Retrofit IQ investigates thermal bridges
Thermal imaging in valid conditions locates thermal bridges precisely, showing the cold pattern at junctions, openings and structural elements. We interpret the results through building physics and, where a lasting solution is needed, can model the junction to compare existing and improved details, predict surface temperatures and confirm the condensation risk has been removed. Where air leakage is contributing, blower door testing and smoke tracing separate the effects.
You receive a clear map of your thermal bridges and a strategy to address them safely — detailing that warms the junction without creating a new moisture problem.
Part of a full Building Performance Investigation
Thermal bridges rarely act alone; they interact with insulation, air leakage and ventilation. A thermal bridge investigation is most valuable as part of a full Building Performance Investigation, which sets the bridges in the context of the whole fabric and gives you one prioritised plan. It is particularly important before adding insulation, so bridges are detailed rather than made worse.
How thermal bridges are measured and modelled
Locating a thermal bridge with a thermal camera is the first step; understanding how serious it is, and how to fix it, is the second. In valid winter conditions, thermal imaging shows the cold pattern at a junction and gives us the internal surface temperature — the number that determines whether condensation and mould will form. Where a lasting solution is needed, we can model the junction using recognised building-physics methods, comparing the existing detail with proposed improvements and predicting the surface temperature and heat loss of each. This turns a subjective 'that corner looks cold' into a designed, evidence-based fix.
This level of rigour matters most when you are investing in insulation. A modelled junction tells you, before any work begins, whether your proposed detail will actually raise the surface above the dew point and eliminate the mould risk — or simply move the cold spot a few centimetres. It is the difference between fixing the problem and paying to relocate it.
Thermal bridges in extensions and refurbishments
Some of the worst thermal bridges we find are created by well-intentioned building work. Where a new extension meets the original house, where a loft conversion's insulation stops short, or where a steel beam was installed without a thermal break, heat finds an easy path out. These junctions are frequently the coldest spots in an otherwise improved home, and the first to show condensation. If you have extended or refurbished and now have a cold or mouldy patch nearby, a thermal bridge is a likely culprit — and one that measurement will confirm.
The Passive House approach to thermal bridging
Our investigations are led by a Certified Passive House Designer, and thermal-bridge control is central to the Passive House method. The standard treats junctions as seriously as the main insulation, because a chain is only as strong as its weakest link — and in a building, that link is usually a junction. Applying this discipline to an ordinary home means we look beyond the obvious wall and roof areas to the details that quietly undermine comfort and cause damp. You benefit from the same rigour used on the most demanding low-energy buildings in the world.
Fixing bridges without creating new problems
The right fix for a thermal bridge continues the insulation across the junction so the cold path is broken, without introducing a moisture trap. Depending on the junction, that might mean returning insulation into a window reveal, insulating a lintel, detailing an extension junction correctly, or adding internal insulation with careful attention to the edges. Done poorly, insulating around a bridge can concentrate the cold and make corner mould worse; done properly, it warms the surface and removes the condensation risk for good. This is why detailing — not just adding material — is the heart of the solution.
Interstitial condensation: the hidden risk
Not all condensation forms on surfaces you can see. Interstitial condensation occurs within the build-up of a wall, floor or roof, where warm moist air reaches a layer cold enough to condense inside the construction itself. Thermal bridges and poorly detailed insulation are common triggers, and the danger is that the resulting damp is hidden — quietly wetting insulation, plaster or timber until damage appears. This is exactly why we interpret thermal-bridge findings through building physics rather than surface appearance, and why any insulation strategy we recommend is designed to keep the whole construction dry.
How thermal bridges affect comfort, health and value
A significant thermal bridge is a triple problem: it wastes heat, it lowers a surface below the dew point and grows mould, and — if that mould persists — it can affect both indoor air quality and the perceived condition of the home. Corner mould and cold spots are among the first things a prospective buyer or tenant notices. Addressing the bridges that cause them protects your comfort and health now and the standing of your home later, particularly as expectations around healthy, efficient homes continue to rise.
A typical example: the cold corner
Consider a common scenario we investigate: a bedroom corner on an external wall that is always cold and grows black mould each winter. Thermal imaging shows the corner is several degrees colder than the surrounding wall — a classic geometric thermal bridge, made worse if insulation is missing there. The owner has repeatedly cleaned and repainted, to no avail. The lasting fix is to warm that corner, through correctly detailed insulation, while managing the room's humidity and ventilation. Once the surface stays above the dew point, the condensation — and the mould — simply stop forming.
The best time to investigate thermal bridges
The single most valuable moment to investigate thermal bridges is before you insulate or refurbish. Identifying the junctions in advance means they can be detailed correctly as part of the works, rather than discovered as cold, mouldy spots afterwards. If you have already improved your home and now have a stubborn cold patch or recurring corner mould, an investigation will pinpoint the bridge responsible and set out how to correct it — turning a frustrating, recurring problem into a solved one.
What you receive
- A precise map of your thermal bridges
- Thermal images showing cold junctions and spots
- An explanation of the condensation and mould risk
- A strategy to detail and warm the junctions safely
- Optional building-physics modelling where a lasting fix is needed
A real-world investigation example
A homeowner who had recently had cavity walls insulated was puzzled that mould still appeared in the corners of two rooms and around a window. Thermal imaging revealed the answer clearly: the insulation had improved the main walls, but the corners, the concrete lintels over the windows and the floor-to-wall junctions remained as cold bridges, several degrees below the surrounding surfaces. These localised cold spots were exactly where the room's moist air condensed and mould grew. Because the bridges were now the coldest points, insulating the general wall had actually concentrated the problem there — a pattern only precise thermal imaging reveals, and one we set out how to detail out correctly.
Common homeowner mistakes
The most common mistake is not knowing thermal bridges exist, so insulation is added to the main wall while the corners, lintels and junctions — which then become the coldest, mould-prone spots — are left untreated. Another is trying to cure the resulting corner mould with paint or a dehumidifier, treating the symptom of a cold junction. Poorly detailed insulation can even create new thermal bridges where materials meet. Locating the bridges precisely before designing the insulation is what stops the problem simply migrating to the coldest remaining point.