Why walls matter so much for heat loss
External walls are one of the largest continuous surfaces of a home, so even a modest rate of heat loss per square metre adds up to a significant total. In many older London properties — particularly those with solid brick walls — the walls are the single biggest heat-loss element after air leakage. Understanding how yours perform is central to any plan to make the home warmer and cheaper to heat.
But 'insulate the walls' is not a diagnosis. Whether internal wall insulation, external wall insulation or a cavity fill is right — and where it will pay back — depends on your construction, the actual heat-loss pattern and the moisture risk. A wall heat loss investigation establishes all three.
Solid walls, cavity walls and mixed construction
London's housing stock is varied. Victorian and Edwardian homes typically have solid walls; interwar and post-war homes often have cavities; extensions and refurbishments create mixed construction within a single property. Each behaves differently and needs a different insulation approach. We identify your actual construction and how each wall is performing, so recommendations are appropriate and moisture-safe.
Unfilled and failed cavities
A cavity that was never filled loses heat readily; one that was filled poorly can slump, bridge and even channel moisture across the wall. Thermal imaging reveals cold bands and patchy patterns that point to these problems, so you avoid the costly mistake of filling a cavity that should be left alone.
Thermal bridges and cold spots in walls
Walls lose extra heat at junctions and around openings — where they meet the floor, the roof and the windows, and around lintels and structural elements. These thermal bridges create localised cold spots that waste heat and often trigger condensation and mould. Locating them is essential to designing insulation that performs as intended, rather than simply moving the cold spot somewhere else.
How Retrofit IQ investigates wall heat loss
We measure the walls in the context of the whole building. Thermal imaging in valid conditions maps heat loss and thermal bridges. We assess construction and insulation continuity. Where air leakage or moisture are contributing, blower door testing, smoke tracing and moisture readings are used to separate the effects. The interpretation is grounded in building physics — because a thermal image without expertise is just a colourful picture.
You receive a measured picture of how your walls perform and a prioritised, moisture-safe insulation strategy tailored to your construction.
Part of a full Building Performance Investigation
Walls are one element of a connected system. A wall heat loss investigation delivers the most value as part of a full Building Performance Investigation, which weighs wall losses against air leakage, floors, roof and windows to give you one prioritised plan — fabric first, in the right order.
Understanding how much your walls lose
The rate at which a wall loses heat is described by its U-value — the lower the number, the better the wall performs. A solid Victorian brick wall might have a U-value several times higher than a modern insulated wall, meaning it loses heat far faster for every square metre. You do not need to become an expert in the figures, but understanding that your walls have a measurable performance — and that it can be dramatically improved — is the starting point. Our investigation combines thermal imaging with knowledge of your construction to explain, in plain terms, how your walls are performing and what is realistically achievable.
Solid-wall options compared
If you have solid walls, you have two broad routes to a warmer wall: internal wall insulation, applied to the inside face, or external wall insulation, applied to the outside and rendered or clad over. Internal insulation is less disruptive externally and can be done room by room, but reduces internal space and demands careful detailing at junctions and around windows to avoid trapping moisture. External insulation keeps the internal space and thermal mass and wraps the building continuously, but affects the external appearance and costs more. There is no universally right answer — the best choice depends on your home, and our report weighs them for your specific situation.
Cavities: fill, clear or leave alone
For homes with cavity walls, the decision is more nuanced than 'get the cavity filled'. A clean, suitable cavity in a sheltered location may benefit from insulation; an exposed wall, a cavity already partly bridged with debris, or one previously filled and slumped, may be better cleared or left alone, because filling it could drive moisture across to the inner leaf. Thermal imaging often reveals whether a cavity is filled, empty or patchy. We help you avoid the costly and sometimes damaging mistake of filling a cavity that should not be.
Sequencing wall works with everything else
Walls are a large investment, so it pays to sequence them correctly against the rest of the fabric. In many homes, cheaper measures — air sealing, loft insulation, thermal-bridge details — should come first, both because they are cost-effective and because they change how the walls need to be treated. Insulating walls also interacts with ventilation and moisture, so it should never be done in isolation. Our report places wall works in the context of the whole building, so you invest at the right time and in the right order.
Party walls and the party-wall bypass
In terraced and semi-detached homes, the party wall shared with a neighbour can be an unexpected source of heat loss. Where a cavity party wall is open at the top, air can circulate up into the cold loft and draw heat out — a phenomenon known as the party-wall bypass. Because you share this wall, it is easy to assume it stays warm, but thermal imaging sometimes reveals otherwise. Identifying and, where appropriate, sealing this bypass is a lesser-known improvement that our whole-building approach is well placed to spot.
Rendered, clad and brick-faced finishes
External wall insulation changes the outward appearance of a home, because the insulation is finished with render, cladding or a brick-effect system. The choice affects both looks and durability, and in some settings — conservation areas, or where the brickwork is a valued feature — external insulation may not be appropriate, making an internal approach preferable. Part of our role is to weigh these appearance and planning factors alongside the thermal and moisture ones, so any recommendation is realistic for your particular property.
Planning, conservation and appearance
Many London homes sit in conservation areas or have features that limit what can be done externally. This does not mean a solid-walled home cannot be improved — it means the strategy must respect the constraints, often favouring internal insulation and careful detailing. Understanding these limits early prevents wasted effort on approaches that would never gain consent or would spoil the character of the home. We factor them into the plan, so recommendations are ones you can actually carry out.
How wall works change the room
Insulating a wall does more than lower the U-value on paper. Internally, it warms the inside surface, so the room feels more comfortable and is far less prone to condensation and mould on that wall. Externally, it keeps the wall's thermal mass within the heated space, helping the home hold its warmth. Either way, the everyday experience is a wall that no longer feels cold to the touch and a room that heats more evenly and holds its heat longer — the tangible payback for the investment.
What you receive
- A measured picture of how your walls lose heat
- Thermal images and identified thermal bridges
- Confirmation of your wall construction and insulation
- A moisture-safe, prioritised insulation strategy
- Clear next steps any competent contractor can follow
A real-world investigation example
Before committing to external wall insulation on a solid-walled London house, the owner asked us to measure what the walls were actually losing and where. Thermal imaging in valid conditions showed the main walls losing heat fairly uniformly, but with pronounced cold bridges at the floor junctions and around the original window reveals, and one previously-insulated return wall performing poorly because of a gap in the insulation. This changed the specification: the reveals and junctions needed particular attention, and the poorly-insulated wall needed opening up rather than over-boarding. Measuring first meant the insulation investment was designed for the building's real weak points rather than applied as a uniform blanket.
Common homeowner mistakes
The biggest mistake is specifying wall insulation from assumptions about the construction rather than measurement, so the money is spread evenly instead of aimed at the worst-performing areas and the thermal bridges. Another is ignoring the window reveals and junctions, which then become cold spots and mould traps even after the main wall is insulated. Filling a cavity that is unsuitable, bridged or already damp is a recurring and damaging error. And insulating the walls before addressing air leakage often disappoints, because the two losses are different and the leakage was doing much of the damage.