Cavity Wall vs External Wall Insulation: Complete Comparison Guide 2025

Wall insulation is one of the most effective ways to reduce heat loss in UK homes, but choosing between cavity wall and external wall insulation can be confusing. Each method suits different property types, budgets, and circumstances. This comprehensive comparison helps you understand which option is right for your home and how to access government grants that could cover the full cost.

Understanding Your Wall Type

Before comparing insulation methods, you need to determine your wall construction type. This dictates which insulation options are available to you.

Cavity Walls

Most UK homes built after 1920 have cavity walls, consisting of two layers of brick or block with a gap (cavity) between them. The cavity is typically 50-100mm wide and was originally designed to prevent moisture penetration. These walls are ideal candidates for cavity wall insulation.

How to identify cavity walls:

• Built after 1920 (most likely)
• Bricks appear in a regular pattern, all facing the same direction
• Wall thickness is typically 250-300mm (10-12 inches)
• The wall feels slightly hollow when tapped

Solid Walls

Homes built before 1920 typically have solid walls with no cavity. These walls are usually just one brick thick (220mm) and lose heat far more readily than cavity walls. Solid walls require external or internal wall insulation.

How to identify solid walls:

• Built before 1920 (most likely)
• Bricks show an alternating pattern (headers and stretchers)
• Wall thickness is typically 220mm (9 inches)
• The wall feels solid when tapped

Approximately 8 million UK homes have uninsulated solid walls, representing the single largest source of domestic heat loss in the country.

Cavity Wall Insulation: The Overview

Cavity wall insulation involves filling the gap between your external and internal walls with insulating material. It's a quick, minimally disruptive solution for homes with suitable cavities.

How Cavity Wall Insulation Works

Installers drill small holes (approximately 22mm diameter) through the external wall at regular intervals, then inject insulation material through these holes using specialist equipment. The holes are filled and sealed afterwards, with the work typically completed in just one day.

Cavity Wall Insulation Materials

• Mineral wool (rock or glass wool): Most common, excellent thermal and acoustic properties
• Polystyrene beads with adhesive: Good for irregular cavities
• Polyurethane foam: Highest insulation value, more expensive
• EPS beads: Budget-friendly option with decent performance

Cavity Wall Insulation Costs

However, if you qualify for government grants like ECO4 or GBIS, cavity wall insulation may be completely free. These schemes prioritise low-income households and those on qualifying benefits.

Energy Savings from Cavity Wall Insulation

Cavity wall insulation can reduce heat loss through walls by up to 65%, delivering substantial savings:

• Semi-detached house: £390 per year saving
• Detached house: £475 per year saving
• Mid-terrace house: £270 per year saving

Based on these savings, a self-funded cavity wall insulation installation typically pays for itself within 2-5 years, making it one of the fastest payback home improvements available.

External Wall Insulation: The Overview

External wall insulation (EWI) involves fixing insulation boards to your external walls and covering them with a protective render or cladding. It's the preferred solution for solid wall properties and delivers exceptional thermal performance.

How External Wall Insulation Works

Installers fix insulation boards to your exterior walls using mechanical fixings or adhesive, then apply a base coat, reinforcement mesh, and weather-resistant render or cladding finish. The process transforms your home's appearance while dramatically improving thermal efficiency.

External Wall Insulation Materials

• EPS (Expanded Polystyrene): Most affordable, good insulation value
• Mineral wool: Better fire resistance, higher cost
• Phenolic foam: Thinnest profile for best insulation
• XPS (Extruded Polystyrene): High performance, moisture resistant

Insulation boards are typically 100-150mm thick, with thicker boards providing better performance but adding more depth to your walls (which affects windowsills, guttering, and roof overhangs).

External Wall Insulation Costs

External wall insulation is significantly more expensive than cavity wall insulation due to the materials, labour, and scaffolding required. However, ECO4 funding can cover the entire cost for eligible households, making this high-value improvement accessible to those who need it most.

Energy Savings from External Wall Insulation

External wall insulation delivers the highest thermal performance, reducing wall heat loss by up to 75%:

• Semi-detached solid wall house: £555 per year saving
• Detached solid wall house: £650 per year saving
• Mid-terrace solid wall house: £400 per year saving

For self-funded installations, external wall insulation typically pays for itself within 18-30 years. However, when considering property value increases, improved comfort, and protection against rising energy prices, the true payback period is much shorter.

Head-to-Head Comparison

Detailed Pros and Cons

Cavity Wall Insulation Advantages

• Quick installation: Completed in just one day with minimal disruption
• Affordable: Lowest cost wall insulation option
• Fast payback: Recoups investment within 2-5 years
• Invisible: No change to your home's appearance
• No planning permission: Simple, straightforward installation
• Widely available grants: Often fully funded through government schemes
• Proven technology: Decades of successful installations in UK

Cavity Wall Insulation Disadvantages

• Not suitable for solid walls: Only works with cavity wall construction
• Potential damp issues: Poorly installed insulation can bridge the cavity and cause damp (rare with proper installation)
• Not suitable for exposed locations: High wind-driven rain areas may not be suitable
• Lower thermal performance: Limited by cavity width (typically 50-100mm)
• Difficult to remove: Once installed, removal is challenging and expensive

External Wall Insulation Advantages

• Superior thermal performance: Can achieve U-values below 0.3 W/m²K
• Weatherproofing: Protects and extends the life of your exterior walls
• No interior space lost: Work done externally preserves room sizes
• Modernises appearance: Can dramatically improve kerb appeal
• Eliminates cold bridging: Continuous insulation eliminates thermal bridges
• Increases property value: Significant value addition from improved EPC and appearance
• Reduces condensation: Keeps walls warmer, preventing internal condensation
• Sound insulation: Provides acoustic benefits alongside thermal improvement

External Wall Insulation Disadvantages

• High cost: 10-15 times more expensive than cavity wall insulation
• Long installation: Takes 2-4 weeks with scaffolding in place
• Planning permission: May be required, especially in conservation areas
• Changes appearance: Can be positive or negative depending on preferences
• Requires modifications: Windowsills, gutters, and roof overhangs need adjustment
• Long payback: Takes 18-30 years to recoup costs through savings alone
• Weather dependent: Installation requires dry conditions

When to Choose Cavity Wall Insulation

Cavity wall insulation is the right choice when:

• Your home was built after 1920 with cavity walls
• Your walls are uninsulated (check for existing insulation via a borescope survey)
• You want a quick, minimally disruptive installation
• Budget is limited and you're self-funding
• You want the fastest payback period
• Your property isn't in a high-exposure location (coastal, elevated)
• You prefer to maintain your home's current appearance

When to Choose External Wall Insulation

External wall insulation is the right choice when:

• Your home has solid walls (typically pre-1920 construction)
• You qualify for ECO4 funding covering the full cost
• Your home needs external weatherproofing or render repair
• You're planning to update your home's appearance anyway
• Preserving interior room sizes is important
• You want the highest possible thermal performance
• You're undertaking a comprehensive renovation
• Your property has significant damp or weathering issues

What About Internal Wall Insulation?

There's a third option for solid wall properties: internal wall insulation (IWI). This involves fixing insulation boards to your interior walls, then finishing with plasterboard.

When Internal Wall Insulation Makes Sense

• Your property is in a conservation area where external changes aren't permitted
• You have distinctive exterior brickwork you want to preserve
• You're renovating rooms anyway and can combine the work
• Your property is mid-terrace with only one or two external walls
• External wall insulation is prohibited by planning restrictions

Internal Wall Insulation Costs and Savings

• Cost: £4,000 - £9,000 (depending on property size)
• Annual savings: £350 - £550
• Room size reduction: Typically lose 100mm (4 inches) per wall
• Disruption: High - requires emptying rooms and redecoration

Internal wall insulation is typically the least preferred option due to room size reduction and disruption, but it's valuable when external solutions aren't possible. ECO4 can also fund internal wall insulation for eligible households.

Accessing Government Grants for Wall Insulation

The ECO4 and GBIS schemes can fund wall insulation completely free for eligible households. Here's how to access this support:

ECO4 Eligibility

You qualify for free wall insulation under ECO4 if:

• You receive means-tested benefits (Universal Credit, Pension Credit, ESA, JSA, etc.)
• Your household income is below £31,000-£36,000 (via ECO4 Flex)
• Your property has an EPC rating of D, E, F, or G

ECO4 covers cavity wall insulation, external wall insulation, and internal wall insulation, often combining multiple measures for maximum improvement.

GBIS Eligibility

The Great British Insulation Scheme specifically focuses on insulation measures:

• Available to households with income below £36,000
• Properties must have EPC rating D, E, F, or G
• Covers cavity wall insulation, loft insulation, and room-in-roof insulation
• Can be combined with ECO4 for comprehensive upgrades

How to Apply

1. Check your eligibility using our grant finder tool
2. Contact an ECO4-registered installer for a free home survey
3. Receive a comprehensive improvement plan
4. Confirm eligibility and schedule installation
5. Work completed by certified installers with 25-year guarantees

Installation Quality: What to Look For

Whether choosing cavity wall or external wall insulation, installation quality is critical. Poor installation can reduce performance or even cause problems like damp.

Installer Credentials to Verify

• TrustMark registration: Government-endorsed quality scheme
• CIGA registration: Cavity Insulation Guarantee Agency for cavity work
• SWIGA registration: Solid Wall Insulation Guarantee Agency for EWI
• Insurance-backed guarantees: 25-year protection for your installation
• BBA certification: British Board of Agrément approval for materials used

Pre-Installation Surveys

Reputable installers will always conduct thorough surveys before installation:

• Cavity wall survey: Borescope inspection to check cavity condition and existing insulation
• Damp survey: Checking for existing moisture issues that need resolving first
• Wall construction assessment: Confirming wall type and suitability
• Exposure assessment: Checking for high wind-driven rain risk

Combining Wall Insulation with Other Improvements

Wall insulation delivers best results when part of a comprehensive energy efficiency strategy. Consider combining with:

• Loft insulation: Address roof heat loss for maximum savings
• Double glazing: Complete the thermal envelope
• Heating system upgrade: New efficient boiler or heat pump after insulation improves
• Solar panels: Generate renewable electricity once energy demand is reduced

ECO4's "whole house" approach often combines multiple measures in a single project, typically including wall insulation, loft insulation, and heating upgrades together.

Making Your Decision

The choice between cavity wall and external wall insulation is usually dictated by your property's construction:

• Cavity walls (post-1920): Choose cavity wall insulation
• Solid walls (pre-1920): Choose external wall insulation (or internal if required)

If you qualify for government grants, both options can be 100% funded, removing cost as a barrier. For self-funding households, cavity wall insulation offers unbeatable value with the fastest payback, while external wall insulation provides superior performance and aesthetic transformation at premium cost.

Regardless of which option suits your property, wall insulation is one of the most impactful improvements you can make, delivering comfort, savings, and environmental benefits for decades to come.