Victorian houses were typically built with solid external walls in facing brick or rendered. Photo by Inigo.
Solid walls and early cavity wall constructions are common in Victorian and post-war homes, which together make up the largest share of the UK’s housing stock. Insulating these buildings is therefore essential for reducing the country’s energy demand and carbon emissions.
There is a lot of misinformation about retrofitting insulation in old homes, leading to preventable issues like damp and mould. This guide aims to provide homeowners with essential knowledge on insulating solid and early cavity walls, helping you minimise risks and choose the right builders.
For a typical house, around 35% of heat loss occurs through external walls, compared to approximately 15% through the floor and 25% through the roof. Insulating external walls, therefore, plays an important role in improving thermal performance.
Solid and uninsulated early cavity walls, in particular, can feel damp and cold during the winter. Installing insulation can significantly improve thermal comfort and help keep them warm and dry.
Whilst insulating external walls is a key consideration, it should be done as part of a whole-house upgrade to experience the full benefits.
Thermal imaging of a semi-detached Victorian House post retrofit. Image by Rajat Gupta.
Before we get into the specifics of how to insulate external walls, first we should look at the types of insulation best suited to the job.
Insulation for solid and early cavity walls should be vapour permeable and, ideally hygroscopic, to maintain the breathability of these structures.
When insulating older buildings, choosing the right material is crucial. The key considerations are maintaining breathability and managing moisture effectively.
Wood Fibre Insulation: The All-Rounder
Mineral Wool: The Budget-Friendly Choice
Blown Mineral Fibre: Cavity Solution
Aerogel: The Space-Saving Option
Insulating Lime-based Plaster: Heritage-Friendly
Plastic foam insulation, such as PIR, is not recommended for old buildings due to its vapour-closed and non-hygroscopic properties, which increase the risk of damp-related issues.
Rigid wood fibre external wall insulation. Photo by Steico.
Insulating a solid wall with external wall insulation (EWI) is the most effective approach. Fixing insulation on the outside eliminates thermal bridges and exposed the masonry’s thermal mass to inside, which helps regulate room temperatures in both winter and summer.
The existing brickwork must be flat before the insulation can be applied, often requiring a levelling layer, such as a clay or lime render.
EWI is typically finished with render, but other options include timber boarding, hanging tiles, and sheet metal.
Solid Wall with External Wall Insulation: 1. External render system; 2. Rigid wood fibre insulation mechanically fixed; 3. Insulating reveal boards; 4. Existing wall; 5. Interior plaster finish; 6. Existing sash window; 7. Sill extension piece.
Internal Wall Insulation (IWI) is less effective than EWI, but it’s the only option when you need to preserve the building’s external appearance.
For best results, remove the existing plaster from the brick wall and apply a levelling base coat to create a flat surface for the insulation. Rigid boards can then be mechanically fixed directly to the masonry and finished with lime render. If you prefer plasterboard instead of lime plaster (a cheaper option), you will need to install an initial layer of rigid insulation, a VCL on top, then timber battens and flexible insulation between them.
Solid Wall with Internal Wall Insulation & Lime Plaster Finish: 1. Existing brick wall stripped of plaster; 2. Levelling layer (lime or clay plaster, if required); 3. Rigid wood fibre insulation; 4. Lime plaster interior finish; 5. Sash window; 6. Reinstated skirting board; 7. Reinstated cornicing.
While modern cavity walls have cavity insulation, early cavity wall construction – common in homes built from the 1930s – is uninsulated. Insulating these structures can be challenging, posing a high risk of damp problems if the work is not properly designed and installed.
Avoid retrofitting cavity wall insulation (CWI) on its own, as it significantly increases the risk of penetrative damp (rainwater migrating through the insulation) and to a lesser extent interstitial condensation (caused by blocking airflow in the cavity).
If you wish to insulate the cavity, the best approach is to close it off at the top and bottom and pair blown mineral fibre cavity insulation with external wall insulation. The EWI offers added protection from the elements, while the mineral fibre insulation is water-resistant but breathable, allowing vapour to pass through without letting rainwater track inwards. If you're considering CWI, a robust ventilation strategy is crucial to reducing condensation-related risks.
Early Cavity Wall with EWI & CWI: 1. External render finish; 2. Rigid wood fibre insulation mechanically fixed; 3. Insulating reveal boards; 4. Existing wall; 5. Full fill mineral fibre insulation; 6. Interior finish; 7. Casement window; 8. Sill extension.
Installing external wall insulation (EWI) on an uninsulated cavity wall is not always desirable – if you wish to preserve your home’s external appearance – or feasible – due to planning restrictions. In these cases, internal wall insulation (IWI) is your only alternative.
It’s important to note that IWI should not be installed alongside CWI. This combination presents similar risks to installing just CWI (as noted in the previous section). In this scenario, keeping the cavity clear prevents rainwater from bridging the gap and maintains proper airflow through the structure.
The illustration below allows for a plasterboard interior lining, which can also be applied to the solid wall build up instead of lime plaster.
Early Cavity Wall with Internal Wall Insulation & Plasterboard Interior Lining: 1. Existing cavity wall stripped of plaster; 2. Levelling layer (lime or clay plaster, if required); 3. Rigid wood fibre insulation; 4. Insulating reveal boards; 5. Vapour control layer; 6. Battens with flexible wood fibre insulation between; 7. Plasterboard and skim finish; 8. Casement window; 9. Reinstated skirtings; 10. Reinstated cornicing.
The 2022 update of Approved Document L adopts a more balanced approach to insulating old buildings, in contrast to the heat-loss-centric focus of previous versions.
It sets out:
U-values measure the rate of heat loss through a material or building element (such as walls, roofs, or windows). The lower the U-value, the better the material is at insulating and reducing heat loss.
Approved Document L sets out minimum (0.7 W/m²K), average (0.5 W/m²K) and target (0.3 W/m²K) U-values for insulating external walls.
The table below shows approximately how much wood fibre or aerogel insulation you’ll need to meet each of these requirements.
When it comes to insulating your property, more isn't always better. Contrary to common belief, thicker insulation doesn't always translate to proportionally higher energy savings. In fact, academic research suggests that there's an optimal point beyond which additional insulation thickness yields diminishing returns.
For example, consider an uninsulated solid brick wall with a U-value of 2 W/m²K. Installing 40 millimetres of wood fibre insulation reduces heat loss by 69%, achieving a U-value of about 0.60 W/m²K. Tripling the thickness to 120 millimetres to reach the target U-value of 0.3 W/m²K only reduces heat loss by an additional 18%. For a typical house, this would result in less than 100 kWh in energy savings per year, or about £10 per week at £0.10/kWh.
Thinner insulation can also help maximise floor area and minimise the impact on original architectural details like decorative plasterwork and mouldings.
Insulating external walls is rarely cost-effective when done in isolation. According to the Energy Saving Trust, the average installation cost for external wall insulation for a three-bedroom semi-detached home is about £10,000, while savings on energy bills is circa £400 per year. This equates to about a 25 year payback period, not accounting for increases in energy prices.
However, when wall insulation is integrated into a broader home improvement project, such as re-rendering or re-plastering, the marginal cost of adding insulation is reduced, improving its cost-effectiveness and shortening the payback period.
With stricter energy efficiency standards coming into force and growing awareness of sustainable living by the general public, well insulated homes are becoming increasingly attractive to buyers, potentially boosting market values.
That said, the main benefits of insulation, currently, are enhanced thermal comfort and reducing your home’s carbon emissions.
Rendered external wall insulation installed to the rear of a Victorian terrace house. Photo by Prewett Bizley Architects.
Insulating old buildings can increase the risk of damp from water penetration (the weather) and interstitial condensation (the build-up of moisture within the building’s fabric).
For solid brick walls, external wall insulation (EWI) offers the best protection from penetrative damp as it shields the masonry from the elements. If you’re installing IWI ensure the existing masonry is in a good state of repair to prevent water ingress.
When insulating early cavity walls, avoid just installing cavity wall insulation (CWI), as it increases the risk of penetrative damp. Always pair CWI with EWI for better weather protection. Or just install IWI to avoid the risk altogether.
With regards to interstitial condensation, using vapour-permeable and hygroscopic insulation, along with vapour-permeable renders (for EWI) will allow the walls to ‘breathe’ while improving airtightness.
Lastly, ensure good building ventilation to keep indoor vapour levels in check – this measure alone will significantly reduce condensation related issues.
Damp-related issues caused by a defective cavity wall insulation installation. Photo by the BBC.
U-values, while an important measure of energy efficiency and a key component in achieving compliance, are not the be all and end all. Other important factors to consider in order to achieve a warm, comfortable home include:
Slimline aerogel internal wall insulation was installed to the solid external walls of this Edwardian retrofit project by Architecture for London. Photo by Lorenzo Zandri.
External wall insulation (EWI) is the most effective method for insulating solid external walls from an energy efficiency perspective as it provides continuous coverage that minimises thermal bridges. However, it changes the building's appearance, making it unsuitable for properties with attractive facades. Here, internal wall insulation (IWI) should be used.
For uninsulated cavity walls, avoid just installing cavity wall insulation (CWI), as it can increase the risk of damp. Always pair CWI with EWI for better protection.
To experience the full benefits of any insulation method, it's vitally important to ensure proper installation and eliminate air leakages through careful attention to detail and high-quality workmanship.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.
About the Author
Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.
