Internal vs External Wall Insulation: Which is Better?

Understanding Wall Insulation and Its Importance

Wall insulation is a vital component of a thermally efficient building envelope. Its primary function is to reduce heat transfer through the walls. In winter, it prevents warm air from escaping outwards; in summer, it prevents external heat from penetrating indoors.

Effective wall insulation significantly improves a home’s energy efficiency. It reduces the reliance on heating systems, leading to lower energy consumption and, consequently, lower energy bills. Beyond cost savings, it creates a more comfortable living environment by eliminating cold spots and maintaining a more consistent indoor temperature.

Delving into Internal Wall Insulation (IWI)

Internal Wall Insulation, often referred to as IWI, involves adding insulating material to the inside surface of external walls. This creates a thermal barrier within the existing rooms. It’s typically applied to properties with solid walls, where cavity wall insulation isn’t an option.

The installation process involves fixing insulation boards or creating a stud wall filled with insulation against the interior face of the external wall. A vapour barrier is usually installed to manage moisture, and finally, a new plasterboard layer is fixed and finished. This process effectively moves the thermal boundary inwards.

Installation Process for IWI

Installing IWI is an internal renovation project. It typically involves removing existing skirting boards, architraves, and potentially radiators and other fixtures attached to the external walls. A framework (battens or metal studs) might be erected, or insulation boards can be directly bonded to the wall.

Insulation material is fitted within the framework or as rigid boards. A vapour control layer is often installed to prevent moisture from the interior migrating into the wall structure and condensing. Finally, a plasterboard layer is fixed over the insulation and finished ready for painting or decoration.

Materials Used for IWI

A variety of materials are used for IWI, chosen based on desired thickness, thermal performance, and moisture resistance. Common options include:

• Rigid Insulation Boards: Materials like PIR (Polyisocyanurate) or Phenolic foam offer high thermal resistance with relatively thin profiles, minimising space loss.
• Mineral Wool or Glass Wool: These fibrous materials are used in stud wall constructions. They offer good thermal and acoustic insulation but require a vapour barrier and plasterboard finish.
• Insulated Plasterboard (Dot and Dab): Composite boards with insulation already bonded to plasterboard. These are quick to install by bonding directly to the wall but are less effective at creating a continuous barrier due to the “dot and dab” method potentially leaving air gaps.
• Wood Fibre Boards: A more breathable, natural option, often used in older or historic buildings where moisture management is critical.

Pros of Internal Wall Insulation

IWI offers several advantages that make it a suitable choice for many homeowners:

• Lower Upfront Cost: Generally less expensive to install than EWI, requiring less scaffolding and potentially simpler materials.
• Easier Planning Permission: Less likely to require extensive planning permission, especially if the external appearance of the building is not significantly altered. This is a major plus for properties in conservation areas or those with historical significance.
• Suitable for Listed Buildings: Often the preferred or only option for listed buildings where the external facade must remain untouched.
• No External Changes: Preserves the original external appearance of the building, which is crucial for street appeal or heritage properties.
• Can Be Done Incrementally: Insulation can potentially be installed room by room, spreading the cost and disruption over time.

Cons of Internal Wall Insulation

Despite its benefits, IWI also presents notable drawbacks:

• Reduces Internal Space: Adding insulation layers (even thin ones) reduces the floor area of each room, which can be significant in smaller properties.
• Causes Internal Disruption: Installation is invasive, requiring rooms to be cleared and disrupted for the duration of the work. Skirting boards, radiators, sockets, and light switches on external walls need to be removed and refitted.
• Risk of Thermal Bridging: Areas like internal wall junctions, floor joists, and window reveals can be difficult to insulate effectively, creating cold spots (thermal bridges) where heat can still escape and condensation can form.
• Moisture Risk: Incorrectly installed IWI can trap moisture within the wall structure, potentially leading to damp, mould, and damage to the building fabric. Careful detailing of the vapour barrier is essential.
• Disruption to Fixtures: Relocating or adapting radiators, pipework, electrical sockets, and other fixtures on external walls adds complexity and cost.

Exploring External Wall Insulation (EWI)

External Wall Insulation, or EWI, involves fixing insulating material to the outside surface of the external walls. This method wraps the building in a thermal blanket. Once the insulation is applied, it is typically covered with a reinforced render system or decorative cladding, providing a new, weather-resistant facade. Like IWI, EWI is most commonly applied to solid wall properties.

EWI effectively moves the dew point – the temperature at which water vapour condenses – to outside the original wall structure. This helps protect the existing wall from temperature fluctuations and moisture ingress, potentially improving its longevity.

Installation Process for EWI

The EWI process starts by preparing the external walls, which may involve repairs or cleaning. Insulation boards are then mechanically fixed and/or adhesively bonded to the wall surface. A base coat of render is applied over the insulation, often incorporating a reinforcing mesh to prevent cracking.

This is followed by a top coat of render, which can be coloured and textured, or a different cladding system might be installed. The process requires scaffolding and access to the entire exterior of the building. Details around windows, doors, and eaves must be carefully finished to ensure a continuous, weather-tight seal.

Materials Used for EWI

Common materials for EWI systems include:

• Expanded Polystyrene (EPS): A cost-effective and widely used insulation board.
• Mineral Wool: Offers excellent fire resistance and breathability, making it suitable for certain building types and regulations.
• Phenolic Foam: Provides high thermal performance, allowing for thinner insulation layers.
• Render Systems: The protective outer layer is typically a polymer or silicone-based render, available in numerous colours and finishes.
• Cladding: Timber, composite, or fibre cement cladding can also be used as the external finish over the insulation layer.

Pros of External Wall Insulation

EWI offers significant benefits, particularly regarding performance and external appearance:

• Superior Thermal Performance: By creating a continuous layer around the outside of the building, EWI effectively eliminates thermal bridging through the main wall structure, providing a more consistent and higher level of insulation.
• Protects Existing Structure: The insulation system protects the original wall from weather exposure and temperature extremes, potentially extending the life of the facade.
• Improves Weatherproofing: The new external layer provides excellent protection against rain penetration.
• No Loss of Internal Space: All work is carried out externally, meaning no reduction in room sizes or disruption to internal living areas (apart from accessing window/door reveals).
• Enhances External Appearance: EWI can dramatically transform the look of a property, covering tired or damaged brickwork or render with a fresh, modern finish.
• Less Internal Disruption: Homeowners can typically remain living in their homes with minimal disturbance during the installation process.

Cons of External Wall Insulation

However, EWI also comes with substantial challenges:

• Higher Upfront Cost: EWI is significantly more expensive to install than IWI, largely due to the need for scaffolding and the complexity of finishing around external features.
• Requires Planning Permission: Changes the external appearance of the building, almost always requiring planning permission, especially in conservation areas or for certain property types. This process can add time and uncertainty.
• Alters External Appearance: While some see this as a pro, changing the look of the house significantly might not be desired, particularly for period properties where preserving the original facade is important.
• Disruption to External Features: Integrating features like drainpipes, satellite dishes, external taps, and potentially even meter boxes into the new insulation layer requires careful planning and execution.
• Access Issues: Requires unrestricted access to the entire external perimeter of the building, which can be problematic in terraced properties or those with limited side access.
• Potential for Poor Detailing: If not installed correctly, particularly around windows and doors, EWI can still suffer from thermal bridging or create pathways for water ingress.

Internal vs External Wall Insulation: A Direct Comparison

The question of Internal vs External Wall Insulation: Which is Better? cannot be answered with a simple preference for one over the other. The best choice depends entirely on the specific circumstances of the property and the homeowner’s priorities. Let’s compare the two directly across key factors.

Cost Analysis

Generally speaking, Internal Wall Insulation has a lower material and installation cost than External Wall Insulation. The labour involved is typically less complex than the scaffolding and intricate external finishing required for EWI. However, IWI might incur additional costs for relocating internal fixtures. EWI is a significant investment but can add considerable value to the property and offers higher potential for long-term energy savings due to better overall performance.

Effectiveness and Thermal Performance

External Wall Insulation is generally considered more effective at creating a continuous, high-performance thermal envelope. By wrapping the entire building externally, it significantly reduces or eliminates thermal bridging that can occur at junctions with Internal Wall Insulation. This leads to a more consistent temperature throughout the house and lower heat loss. IWI can be effective but requires meticulous installation to minimise thermal bridging, which is inherently more challenging with internal walls, floors, and ceilings meeting the insulated external wall.

Disruption and Installation Time

The impact of installation disruption is a major differentiating factor. Internal Wall Insulation causes significant disruption inside the home. Rooms become unusable, furniture needs to be moved, and daily life is impacted for the duration of the work. External Wall Insulation, conversely, causes disruption outside the home. While scaffolding and external work can be inconvenient, residents can usually remain living in their homes with relatively less impact on their internal routines compared to IWI. EWI projects often take longer overall due to scaffolding time and weather dependency.

Aesthetics and Planning Permission

This is perhaps the most visible difference. Internal Wall Insulation makes no changes to the external appearance of the building, which is ideal for maintaining original features or complying with aesthetic restrictions. It typically requires little to no planning permission unless the building is listed or in a conservation area where any changes are scrutinised. External Wall Insulation fundamentally changes the look of the property, adding a new rendered or cladded finish. This almost always requires planning permission, which can be a complex and potentially restrictive process, especially for older or heritage properties.

Suitability for Different Properties

The ideal choice also depends heavily on the property’s construction and location.

Suitability for Solid Wall Properties

Both IWI and EWI are primary solutions for insulating solid walls, which lack a cavity for insulation. Choosing between them for a solid-walled house brings the decision back to the trade-offs discussed: cost vs. performance, internal vs. external disruption, and aesthetic preferences vs. planning constraints. A terraced solid-wall house, for example, might find EWI challenging on side walls shared with neighbours, potentially leaning towards IWI (despite the drawbacks) for those specific walls.

Suitability for Cavity Wall Properties

Properties built with a cavity wall (typically post-1930s) usually have the option of cavity wall insulation (CWI) as a first step. CWI is generally cheaper and less disruptive than both IWI and EWI. However, not all cavity walls are suitable for CWI (e.g., if the cavity is narrow, blocked, or the walls are in poor condition). In such cases, or if CWI is insufficient, IWI or EWI become alternative or supplementary options. EWI can be applied over a cavity wall, offering high performance and weather protection, while IWI could be used internally if external changes are prohibited.

Listed Buildings and Conservation Areas

For listed buildings and properties in conservation areas, preserving the external appearance is paramount. This often makes External Wall Insulation extremely difficult or impossible to get planning permission for, as it fundamentally alters the facade. Internal Wall Insulation is often the only viable option in these cases, although even IWI in listed buildings requires careful design and Listed Building Consent to ensure historic features are not damaged and appropriate breathable materials are used to avoid moisture issues.

Factors to Consider When Choosing

Given that there’s no single “better” option, how do you decide between Internal vs External Wall Insulation: Which is Better? Consider the following factors relevant to your specific situation:

Your Budget

Evaluate the initial cost of each option and compare it to the potential long-term energy savings. While IWI is cheaper upfront, EWI might offer greater savings over time due to superior performance. Also, factor in potential grants or funding schemes available for insulation work.

Tolerance for Disruption

Assess how much disruption you can tolerate and where you’d prefer it to occur. Are you prepared for internal rooms to be out of action, or would you prefer scaffolding and external work around your property? Consider the timeline of the project as well.

Property Type and Condition

Consider the age, construction type (solid vs. cavity), and current condition of your walls. Are there existing damp issues? Are external features complex? Is the property listed or in a conservation area? These structural and regulatory factors heavily influence feasibility and suitability.

Aesthetic Goals

Do you want to preserve the existing look of your house, or are you open to, or even desirous of, changing the exterior facade? The choice between invisible internal insulation and a transformative external render is significant.

Local Planning Regulations

Crucially, check with your local authority’s planning department. Understand the requirements for your specific location and property type. This step can quickly determine if EWI is a viable option or if you must focus on IWI.

Conclusion

The debate of Internal vs External Wall Insulation: Which is Better? ultimately concludes that neither is universally superior. Both Internal Wall Insulation (IWI) and External Wall Insulation (EWI) are effective methods for dramatically improving the thermal performance of solid walls, but they come with different price tags, levels of disruption, performance characteristics, and impacts on a property’s appearance and regulatory hurdles.

IWI is often the preferred choice where budget is tighter, external appearance must be preserved (especially for listed buildings or in conservation areas), and internal disruption is manageable. However, it comes with the drawbacks of reduced internal space and a higher risk of thermal bridging if not installed perfectly.

EWI, while a more significant investment and subject to stricter planning controls due to changing the facade, offers superior thermal performance by wrapping the building in a continuous layer. It also protects the original structure and causes less internal disruption. It’s often favoured when maximising energy efficiency is the top priority and external renovation is desired or permissible.

Choosing the right option requires careful consideration of your property’s specific characteristics, your budget, tolerance for disruption, and local planning regulations. It is highly recommended to obtain quotes and advice from experienced insulation professionals who can assess your property and guide you towards the most suitable solution for your needs, ensuring that your investment leads to a warmer, more energy-efficient, and comfortable home.