When considering the best insulation for masonry walls, the choice often comes down to External Wall Insulation (EWI) and Cavity Wall Insulation (CWI). Both options can significantly improve your home’s thermal performance, but they serve different purposes and are suitable for different types of properties. Once the decision to opt for EWI is made, the choice of materials becomes crucial, with options like mineral wool, expanded polystyrene (EPS), and phenolic foam offering distinct benefits and drawbacks.

External Wall Insulation vs. Cavity Wall Insulation

External Wall Insulation

EWI is a comprehensive system in which insulation material is applied to the exterior of the building, followed by protective render or cladding. This is particularly useful for solid walls, which are common in older homes and lack the cavity present in modern wall construction.

Benefits of EWI:

• Suitable for solid walls: Ideal for older properties that do not have a cavity to fill.
• Significant energy savings: External insulation offers excellent thermal efficiency, reducing heat loss and energy bills.
• Protects the façade: EWI improves the building’s weather resistance, offering a second layer of protection against the elements.
• Enhanced aesthetics: Applying a render or cladding system can also refresh the property’s appearance, adding curb appeal.

Drawbacks of EWI:

• Higher initial cost: EWI is more expensive upfront than CWI, but the long-term energy savings can offset this cost.
• Disruption: EWI requires professional installation and can be disruptive because it requires working on the property’s exterior.

Cavity Wall Insulation (CWI)

CWI involves filling the gap between two layers of masonry (the cavity) with insulating material. This method is widely used in homes built after the 1920s, with common cavity walls.

Benefits of CWI:

• Lower cost: CWI is generally more affordable than EWI and can be installed quickly.
• Energy efficiency improvement: Filling the cavity reduces heat loss, making it an effective way to improve thermal efficiency.
• Minimal disruption: The installation is less disruptive than EWI, as it usually requires drilling holes in the outer wall and injecting the insulation.

Drawbacks of CWI:

• Limited to cavity walls: It cannot be used in solid-wall homes, making it unsuitable for many older properties.
• Damp issues: If improperly installed, CWI can cause moisture to build up within the cavity, leading to damp problems.

Comparing Insulation Materials for EWI

Once EWI is selected as the best solution for your masonry walls, the next decision is which insulation material to use. The three most common choices are mineral wool, expanded polystyrene (EPS), and phenolic foam. Each has its own advantages and is suited to different property needs.

Mineral Wool

Mineral wool, made from spun stone or glass fibres, is a popular choice for EWI systems due to its superior fire resistance and breathability. At EWI Pro, we often recommend mineral wool for projects where fire safety and sound insulation are priorities.

Pros of Mineral Wool:

• Fire-resistant: Mineral wool is non-combustible, offering high safety in the event of a fire.
• Breathability: Its permeable nature allows moisture to escape from the building, reducing the risk of condensation build-up.
• Acoustic insulation: Mineral wool provides excellent soundproofing, making it a great choice for properties in noisy areas.

Cons of Mineral Wool:

• Higher cost: Mineral wool is typically more expensive than EPS, which may not suit every budget.
• Heavier material: Mineral wool’s weight requires more robust fixing systems and can be more challenging to install than lighter alternatives like EPS.

Expanded Polystyrene (EPS)

EPS is one of the most commonly used insulation materials for EWI. It is lightweight, easy to handle, and offers good thermal insulation at an affordable price.

Pros of EPS:

• Cost-effective: EPS is the most budget-friendly option, providing solid thermal performance without breaking the bank.
• Lightweight and easy to install: The ease of installation makes it a popular choice for many external wall insulation systems.
• Good thermal performance: EPS offers excellent insulation, helping to reduce heat loss and save on energy bills.

Cons of EPS:

• Lower fire resistance: While EPS is treated with flame retardants, it is not as fire-resistant as mineral wool, making it less ideal in areas where fire safety concerns.
• Less breathable: EPS is less permeable than mineral wool, which can lead to issues with condensation in some properties.

Phenolic Foam

Phenolic foam is the most thermally efficient insulation material available for EWI. It has a much lower thermal conductivity than EPS or mineral wool, meaning you can achieve the same level of insulation with a thinner board.

Pros of Phenolic Foam:

• Superior thermal performance: Phenolic foam offers the best thermal efficiency, making it ideal for projects where space-saving is a priority.
• Thin profile: Its slim profile means that you can achieve high levels of insulation without significantly altering the property’s external appearance.
• Good moisture resistance: Phenolic foam has excellent moisture resistance, reducing the risk of damp issues.

Cons of Phenolic Foam:

• Higher cost: Phenolic foam is more expensive than both EPS and mineral wool, which may make it less attractive for budget-conscious projects.
• Lower fire resistance: Although phenolic foam has a low fire spread rate, it is still not as fire-resistant as mineral wool, so it might not be the best choice for properties where fire safety is a top concern.