London is a city of flats. Converted Victorian terraces, interwar mansion blocks, 1960s and 1970s council estates and more recent mid-rise developments house a huge share of the capital’s residents. Many of these buildings share the same weakness: walls that lose heat quickly, feel cold to the touch in winter and are prone to condensation and mould on the inside.
- Older London flats suffer from severe heat loss and mould due to uninsulated solid brick and concrete walls.
- External wall insulation improves thermal efficiency without reducing internal living space or forcing residents to move out during installation.
- Non-combustible mineral wool insulation is widely chosen to satisfy strict fire safety rules on residential buildings.
- Mineral wool offers superior fire protection, vapor breathability, and acoustic insulation compared to lighter polystyrene alternatives.
- Successful insulation projects require proper planning permission, leaseholder consultations, and certified system components.
With energy bills still a major household cost and landlords facing tighter efficiency rules, more freeholders, residents’ associations and managing agents are looking at the outside of their buildings for an answer. External wall insulation is one of the most effective upgrades available, and the choice of insulation material has become one of the first questions on the table.
Why Older London Blocks Lose So Much Heat
Most homes built before the 1930s have solid brick walls with no cavity to fill. Many post-war blocks were built with concrete frames or precast panels, where the junctions between floor slabs and outer walls act as cold bridges. In both cases, heat escapes straight through the structure.
The result is familiar to anyone who has lived in an older London flat: rooms that never quite warm up, black mould in the corners of bedrooms and heating that runs for hours to little effect.
Why Insulating From The Outside Suits Flats
Internal insulation takes space away from rooms that are often small to begin with, and it means disrupting every flat in turn. External wall insulation works differently. Insulation boards are fixed to the outside of the building, reinforced with mesh and finished with a protective render.
For a block of flats this has clear advantages:
- Residents can usually stay in their homes while the work is carried out
- No floor area is lost inside any of the flats
- The whole building is wrapped in one continuous layer, which deals with many cold bridges at floor slabs and junctions
- The new render finish can refresh a tired or stained facade at the same time
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Fire Performance: The Question Every Freeholder Now Asks
Since 2017, the materials used on the outside of residential buildings have been under close scrutiny, and rightly so. In England, the rules on external walls for taller residential buildings now restrict the use of combustible materials, and many freeholders, insurers and lenders take a cautious approach even on lower buildings.
This is where mineral wool comes in. Stone wool insulation is made from volcanic rock that is melted and spun into dense fibres. It is non-combustible, does not add fuel to a fire and is widely specified on projects where fire performance is the priority. For many blocks, it is the natural starting point for any conversation about external insulation.
Every building is different, so the right specification should always be confirmed with Building Control and, on larger or taller blocks, with a fire engineer before any work begins.
Mineral Wool Or EPS: An Honest Comparison
Mineral wool is not the only option, and it is not always the right one. Expanded polystyrene (EPS) boards are lighter, easier to cut and usually cheaper, which is why they remain a common choice for houses and low-rise projects.
Mineral wool has its own strengths beyond fire performance:
- It is vapour-permeable, which helps solid masonry walls dry out naturally rather than trapping moisture
- It offers useful acoustic benefits, a real plus for flats near busy roads, railways or flight paths
- It performs well over the long term with little change in its properties
The trade-offs are weight and handling. Mineral wool boards are heavier, generally need more mechanical fixings and take more care to install. For that reason, they should always be used as part of a complete, tested system rather than mixed with components from different manufacturers.
External Wall Insulation (EWI) Material Performance Comparison
| Insulation Material | Euroclass Fire Rating | Thermal Conductivity (λ W/mK) | Vapour Permeability (Breathability) | Acoustic Reduction | Primary Use Case |
| Mineral / Rock Wool | A1 (Non-Combustible) | 0.035 – 0.038 | High (Vapour Open) | Excellent | Medium & high-rise residential blocks, urban noise zones, high fire-risk facades |
| Grey / Graphite EPS | E (Combustible) | 0.030 – 0.032 | Low to Moderate | Moderate | Low-rise residential buildings (<11m), budget-sensitive retrofit projects |
| Phenolic Foam | B-s1, d0 | 0.018 – 0.021 | Low (Vapour Resistant) | Moderate | Space-restricted facades, narrow alleys, tight roof overhangs where depth is limited |
Source: BS EN 13501-1 Fire Classification Standards, UK Building Regulations (Approved Document B), and Manufacturer Technical Datasheets.
What To Plan Before Work Starts
A successful external insulation project on a block of flats starts well before any boards arrive on site. A few points worth checking early. A well-planned scheme is also one of the clearer examples of home improvements that add value to a building, so it’s worth getting the basics right before work begins:
- Planning permission: unlike houses, flats do not benefit from permitted development rights for external changes, so planning consent is usually needed, and conservation areas bring extra requirements.
- Leaseholder consultation: where major works will cost any leaseholder more than £250, the Section 20 consultation process applies.
- A proper survey of the walls, including their condition, any damp and the details around windows, balconies and pipework. This kind of survey often feeds into the wider asset valuation freeholders rely on when planning major works.
- A certified system with compatible adhesives, fixings, mesh, basecoats and renders.
- An experienced installer who can handle the details, as these decide how well the finished job performs.
Sourcing Materials For London Projects
Choosing a supplier that understands external wall insulation makes a real difference, especially when a project needs a full system rather than a few individual products. Penguin BM, a specialist builders’ merchant based in Amersham, Buckinghamshire, supplies complete EWI systems to contractors, developers and homeowners, with delivery across London and the rest of the UK. Its range of mineral wool insulation boards sits alongside the adhesives, fixings, mesh and render finishes needed to complete a system, and its team can advise on which products suit a particular building.
A Long-Term Investment In The Building
Insulating a block of flats is a significant decision, but it is one of the few upgrades that improves comfort, running costs and the appearance of a building at the same time. For London’s older blocks, where cold walls and condensation have been part of daily life for decades, a well-specified external wall insulation system, with the right attention to fire performance, can change how a building feels to live in for years to come.
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FAQs
Q1. Why Do Older London Flats Lose So Much Heat?
Older London flats lose heat rapidly because solid brick walls and post-war concrete frames lack insulation cavities, creating thermal bridges where warmth escapes.
Q2. Why Is External Wall Insulation Better Than Internal Insulation For Flats?
External wall insulation avoids taking away internal floor space, allows residents to stay inside during work, and creates a continuous insulating layer around the whole building.
Q3. Is Mineral Wool Insulation Non-Combustible And Safe For Flats?
Yes, mineral wool is spun from volcanic rock and is completely non-combustible, making it the safest choice for residential blocks where fire safety is paramount.
Q4. Do You Need Planning Permission For External Wall Insulation On London Flats?
Yes, unlike houses, flats do not have permitted development rights for external changes, so formal planning permission and Section 20 leaseholder consultations are usually required.
Q5. What Is The Difference Between Mineral Wool And EPS Insulation?
Mineral wool provides superior fire performance, breathability, and acoustic insulation, whereas EPS boards are lighter, easier to cut, and generally less expensive.
Sources & References
- Wikipedia contributors. (2026). Mineral wool. In Wikipedia.
- Wikipedia contributors. (2026). External wall insulation. In Wikipedia.
- Ministry of Housing, Communities and Local Government. (2025). Approved Document B: Fire safety (Volume 1 – Dwellings and Volume 2 – Buildings other than dwellings). GOV.UK.
- Planning Portal. (2025). Permitted development rights and planning permission guidance. UK Planning Portal.
- National Energy Foundation. (2025). External wall insulation: Improving thermal performance in solid wall homes. National Energy Foundation.
- Building Research Establishment. (2024). External wall insulation systems and retrofit best practice for residential buildings. BRE Group.
- British Standards Institution. (2024). BS EN 13501‑1: Fire classification of construction products and building elements. BSI.
- Energy Saving Trust. (2024). Solid wall insulation: Costs, benefits and suitability for UK homes. Energy Saving Trust.
- Royal Institution of Chartered Surveyors. (2024). Residential retrofit and energy efficiency guidance. RICS.
- Historic England. (2023). Energy efficiency and traditional homes: Insulating solid walls. Historic England.
Disclaimer: This article belongs to the Real Estate and property maintenance category and is provided solely for general informational and educational purposes. It does not constitute legal, technical, financial, or professional engineering advice, nor is it intended for promotional purposes or brand endorsement. Readers should independently verify all facts, structural requirements, and local building regulations, and consult qualified building professionals or certified engineers before undertaking any renovations or structural insulation works.
