Internal vs External Wall Insulation for London Period Homes
Are you trying to understand the difference between internal and external wall insulation in a Victorian or Edwardian home.
Are you wondering which approach will make your terrace warmer without damaging the character that makes it special.
If so, you are in the right place.
London’s older homes are full of charm, but their solid brick walls are cold in winter, hot in summer and often vulnerable to condensation. Choosing the right insulation approach can feel overwhelming, especially when you also need to think about planning restrictions, moisture movement and the details that turn a house into a long-term home.
This guide will walk you through both options so you can make a well-considered decision that suits your home, your budget and the way you live.
If you want a broader overview before diving into the details, you may find our complete guide helpful.
Read: How to Insulate a Period London Home
Why Solid Wall Insulation Matters in London
Victorian and Edwardian homes were built with solid brick walls. These walls do not contain the air space that modern cavity walls have. Without that air gap, heat moves through the wall quickly. As a result, rooms feel cold to the touch and often stay colder than the air inside.
When the wall remains cold, the surface can attract moisture. This is one of the reasons that older homes sometimes show patches of condensation or black mould.
Insulation helps slow down this heat transfer, and when done with the right materials, it also helps moisture move safely through the wall.
A well-insulated wall can transform everyday comfort. The home feels warmer, drier, quieter and easier to heat. Energy bills fall. A future heat pump becomes far more efficient.
Internal Wall Insulation (IWI)
What Internal Wall Insulation Is
Internal wall insulation is added to the room side of the external wall.
This is usually done with wood fibre boards, cork boards, calcium silicate panels or insulated plasterboard. These materials slow heat loss. Many of them are vapour open, which means they allow moisture to pass through safely.
The insulation is usually paired with airtightness measures. Airtightness means reducing unwanted draughts while still allowing controlled ventilation that keeps air fresh and healthy.
When Internal Insulation Makes Sense
Internal insulation is common in many London homes, especially when external changes are restricted.
You may consider internal insulation when:
Your home sits within a conservation area and external alterations are not permitted.
You want to keep the original front elevation intact in a Victorian terrace.
Your property is a flat within a larger building where external wrapping is not possible.
You are retrofitting room by room and want a phased approach.
Your side boundary is tight and does not allow the thickness of an external layer.
You may have seen this approach in some of our work, where a two-storey flat with a loft extension required breathable internal insulation to solid brick walls.
Benefits of Internal Wall Insulation
Internal insulation offers several real-life advantages.
It leaves the external appearance unchanged, which is essential for heritage character.
It improves comfort immediately in bedrooms and living spaces.
It allows you to upgrade a home step by step when a full house retrofit is not possible.
Natural materials improve indoor air quality and help regulate humidity.
It works well in spaces already being refurbished or redesigned.
Homeowners often tell us that this upgrade transforms the way they use a room. A cold front reception can become a favourite reading space. A drafty bedroom becomes warm and restful.
Challenges and Risks of Internal Insulation
No insulation strategy is perfect. Internal insulation needs careful detailing to avoid problems.
It reduces room size slightly, which can matter in narrow Victorian rooms.
Junctions with floors and ceilings must be considered to avoid cold bridges.
A cold bridge is an area where heat flows rapidly through the construction.
Incorrect materials can trap moisture inside the wall.
Detailed airtightness work is required to avoid hidden condensation.
The work is disruptive because it takes place inside the home
If you want a deeper look into moisture risks, you may find this helpful.
Wood fibre boards being fixed to a Victorian solid brick wall in Islington. This breathable system supports moisture control and improves comfort in older London homes.
A calm, crafted loft bedroom in Hackney. The new timber joinery and linings were designed to integrate a warm roof build up and natural internal wall insulation, while preserving the clean interior lines of the space.
External Wall Insulation (EWI)
What External Wall Insulation Is
External wall insulation is applied to the outside of the building. Insulation boards are fixed to the brickwork and then finished with a protective render. Mineral-based renders or lime renders are often used because they allow the wall to remain vapour open.
In effect, the home receives a warm protective jacket.
When External Wall Insulation Makes Sense
External insulation is often the most thermally effective option, especially in London terraces where heat escapes at floor, ceiling and corner junctions.
You may consider external insulation when:
The home is not in a conservation area.
You want to improve rooms facing the rear garden or side alley.
You are carrying out major refurbishment work or building an extension.
You want to reduce cold bridges in a clear and robust way.
You want to future-proof the home for a heat pump.
It is a strong choice for the rear of a Victorian property where character constraints are less strict.
Benefits of External Insulation
No loss of internal space.
Fewer thermal bridges and a more consistent internal temperature.
Protection for the external brickwork from rain and frost.
Improved comfort in winter and reduced overheating in summer when paired with shading.
Usually a higher level of performance for a similar thickness.
Homes with external insulation often feel noticeably quieter because the continuous layer also improves acoustic comfort.
Challenges and Planning Considerations
London adds layers of complexity.
External insulation often needs planning permission.
It is rarely permitted on the front elevation of a terrace.
Junctions with party walls require careful detail to avoid moisture or gaps.
The quality of workmanship affects both performance and appearance.
Older brickwork may need preparation before insulation is added.
If you want to understand the risks of poor installation, you may find this relevant.
Read: External Wall Insulation Scandal: What Went Wrong and How to Avoid It
Applying the first coat of breathable external wall insulation at The Orchard in Haringey. This system protects the original brickwork, improves thermal performance and supports moisture balance in London’s solid wall homes.
A completed external wall insulation upgrade to an interwar semi-detached home in North London. The new render finish improves energy performance while maintaining a clean and familiar streetscape.
Choosing the Right Approach for a Victorian Terrace
So Which Option Is Better
There is no single answer because Victorian terraces vary so much.
The right solution depends on planning context, heritage value, room layouts and future plans.
A simple summary:
Internal Insulation:
Best when the exterior cannot change. Works well in conservation areas and at the front of terraces.
External Insulation:
Best for performance. Works well at the rear of terraces and in less-sensitive boroughs.
In many London homes, the ideal approach is a hybrid strategy. Internal at the front. External at the rear. Breathable. Carefully detailed. Considered as one whole system.
For a more detailed step-by-step on Victorian terraces, you might like this.
Read: Insulating a Victorian Terrace in London: A Practical Guide
The rear of an interwar home in North London following a full external wall insulation upgrade. New timber cladding, improved glazing and roof mounted solar panels create a warmer, more efficient family home.
Why a Whole-House Strategy Matters
Insulation does not exist on its own. It affects:
ventilation
airtightness
moisture movement
heating performance
future retrofit stages
long-term durability
A home feels completely different when all these pieces are working together.
It becomes quieter. Warmer. More even in temperature. Far easier to run.
It supports your comfort and wellbeing every day.
If you want to understand how ventilation fits in, you may enjoy this.
Read: Airtightness and Ventilation for Old London Houses in Plain English
Costs, Disruption and Phasing
Costs vary depending on material choice, existing defects, room layouts and the scope of refurbishment.
Internal insulation can often be phased room by room.
External insulation is usually carried out in one stage.
There is no need to rush. The key is to plan early so the home evolves in a controlled and thoughtful way.
This avoids rework and ensures each stage supports the next.
Conclusion: A London Home That Feels Warm, Dry and Comfortable
Insulating the walls of a Victorian home can make an extraordinary difference to the way a house feels. The right approach will depend on your planning context, the character of your building and the way you live from day to day. If you want clarity before starting work, you may find our Retrofit Strategy Service helpful, and you can also explore what is involved through our Architect’s Home Visit and Appraisal. Both are designed to give you confidence, direction and a well-considered plan.
