Why are my walls cold?
A cold wall is a wall that is losing heat faster than it should. Every external wall loses some heat, but when a wall feels noticeably cold — or when condensation and mould begin to appear on it — something more specific is happening. In our investigations across London homes, the cause is almost always one of a handful of things: missing or under-performing insulation, a solid wall with no cavity to fill, a thermal bridge at a junction, or moisture within the wall itself.
The important point is that these have very different fixes. Insulating the wrong element, or treating a condensation problem as if it were penetrating damp, wastes money and leaves you cold. A cold walls investigation identifies which mechanism is actually at work in your home, so the remedy is targeted.
Solid walls, cavity walls and why it matters
Many older London properties have solid brick walls with no cavity, which lose heat far more readily than a modern insulated wall. Others have cavity walls that were never filled, or were filled poorly and have since slumped or bridged. Each situation calls for a different approach, and getting it wrong can trap moisture and cause damage. Establishing your actual wall construction is the first step, and part of what our investigation determines.
Missing or failed insulation
Thermal imaging reveals cold zones where insulation is thin, has slumped, or was never installed. On a solid wall, it shows how much heat the bare masonry is losing. This tells us whether internal or external wall insulation is worth considering — and where it will make the biggest difference.
Thermal bridges at junctions
Cold walls are often coldest at the corners, at floor and ceiling junctions, and around windows. These thermal bridges create localised cold spots that not only waste heat but frequently become the first place mould appears. Locating them precisely is essential to a lasting fix.
Cold walls, condensation and mould
There is a direct link between cold walls and mould. Warm, moist indoor air condenses on the coldest surfaces first — and a cold wall is exactly that. The result is damp patches and black mould, particularly in corners and behind furniture where air cannot circulate. This is why painting over the mould never works: the wall is still cold and the air is still moist. Warm the surface and manage the moisture, and the mould has nowhere to form.
How Retrofit IQ investigates cold walls
We use a measured, whole-building approach rather than a single test. Thermal imaging in valid conditions maps the heat loss across every wall. Moisture and humidity readings, plus dew-point analysis, show whether condensation is a factor. Where air leakage is contributing, blower door testing and smoke tracing locate it. We also assess the wall construction and insulation continuity so the thermal findings are interpreted correctly.
The result is a diagnosis, not just an image: which walls are losing heat, why, and in what order to address them — internal wall insulation, external wall insulation, thermal-bridge control or moisture management.
Part of a full Building Performance Investigation
Cold walls rarely act alone — they interact with air leakage, ventilation and the rest of the building fabric. That is why a cold walls investigation is most valuable as part of a full Building Performance Investigation, which ties everything into one prioritised plan. If you also have cold floors, draughts or high bills, the whole-building service gives you a single coherent answer.
The comfort cost of a cold wall surface
A cold wall does more than waste heat — it changes how the room feels. Because your body radiates warmth towards colder surfaces, standing or sitting beside a cold external wall makes you feel cold even when the air temperature is comfortable. This is why a room with cold walls never feels cosy however high you turn the heating, and why people end up overheating the air to compensate for the cold surfaces. Warming the wall surface, through insulation and thermal-bridge control, lets the room feel comfortable at a lower and cheaper air temperature.
Internal or external wall insulation?
If your walls need insulating, the two main routes are internal and external. Internal wall insulation is applied to the inside face; it can be done room by room and does not alter the outside of the house, but it reduces internal space slightly and requires careful detailing at floors, ceilings and window reveals to avoid trapping moisture. External wall insulation wraps the outside of the building, keeping internal space and thermal mass and treating thermal bridges continuously, but it costs more and changes the external appearance. The right choice depends on your construction, exposure, budget and the moisture assessment — which the investigation provides.
The moisture risk of getting it wrong
Insulating a wall changes how it handles moisture, and this is where poorly executed jobs cause damage. Add internal insulation without attention to the junctions and you can create hidden cold spots where moisture condenses within the wall build-up. Fill an unsuitable cavity and you can bridge moisture across to the inner leaf. This is precisely why we insist on measurement and building-physics interpretation before any recommendation: a warmer wall must also be a drier, healthier wall, not a hidden problem for the future.
How walls fit the wider plan
Because walls are a significant investment, their treatment should be sequenced against the rest of the fabric and coordinated with ventilation. In many homes, sealing air leakage and improving the loft come first, both for cost-effectiveness and because they influence how the walls should be handled. Our report never treats the walls in isolation — it sets them within a prioritised, whole-building plan, so you spend on wall insulation at the right point and in a way that works with everything else you do.
Chimney breasts, party walls and hidden cold
Some cold walls have particular causes worth understanding. A redundant chimney breast can channel cold air down an open flue and chill the surrounding masonry. A party wall shared with an unheated neighbouring space, or one open to a cold loft at the top, can lose more heat than expected. Solid external walls behind fitted furniture stay coldest of all, because trapped air cannot circulate. Our whole-building investigation looks for these less obvious causes rather than assuming a cold wall is simply uninsulated masonry.
Furniture, décor and living with cold walls
Until a cold wall is properly addressed, small changes can reduce the mould and discomfort it causes. Pulling wardrobes and sofas a few centimetres away from cold external walls lets air circulate and keeps the surface warmer and drier. Avoiding sealing a room up completely, and maintaining gentle background heat and ventilation, all help. These are stopgaps rather than solutions — but our report explains both the immediate measures that ease the symptoms and the fabric works that remove the cause for good.
The order we recommend works
For a cold wall, we generally establish the cause first, then sequence the remedy. If air leakage or a thermal bridge is contributing, those are often addressed before or alongside insulation, because they change how the wall should be treated and can be cost-effective early wins. Insulation — internal or external — follows, detailed to keep the wall dry, with ventilation considered so the room does not become prone to condensation elsewhere. This measured sequence is what turns a cold, mould-prone wall into a warm, healthy one that stays that way.
What a warmer wall actually feels like
The change once a cold wall is properly treated is tangible. The surface no longer feels cold to the touch, so you can place furniture against it and use the full room. The mould that kept returning stops, because the surface now stays above the dew point. The room heats more evenly and holds its warmth, often letting you lower the thermostat a little. For a wall that had made a room uncomfortable and prone to damp, the difference in daily living is one of the clearest returns on a fabric investment.
What you receive
- A clear diagnosis of why your walls are cold
- Thermal images showing the heat-loss pattern
- Moisture and dew-point findings where condensation is involved
- A prioritised plan — insulation, thermal-bridge control or moisture management
- A practical strategy any competent contractor can follow
A real-world investigation example
A homeowner in a solid-wall Edwardian flat had repainted a north-facing bedroom wall with anti-mould paint three times, yet black mould kept returning in the corner behind the bed. Thermal imaging showed the solid brick wall losing heat across its whole face, with the external corner several degrees colder still — a classic thermal bridge — and dew-point readings confirmed the surface was dropping below the point at which the room's moist air condenses. The wall was not wet from outside; it was simply too cold. A moisture-safe internal insulation scheme, detailed carefully at the corner and combined with better ventilation, warmed the surface and ended the mould.
Common homeowner mistakes
The most frequent mistake is repainting over mould on a cold wall, which never lasts because the surface is still cold and the air still moist. Another is filling an unsuitable or already-bridged cavity, which can carry moisture across to the inner leaf and make matters worse. Homeowners also insulate a wall without detailing the corners, floor and window junctions, leaving hidden cold spots where moisture then condenses within the build-up. Insulating before establishing the actual wall construction and moisture risk is how well-meaning improvements end up trapping damp.