Tech guide: Is solid wall insulation right for your home?

Solid wall insulation is a key retrofit measure for improving the energy efficiency of older homes, helping to reduce heat loss, cut energy bills, lower carbon emissions, and enhance comfort. Here’s what you need to know about insulating solid walls, both internally and externally.

What is solid wall insulation?

In uninsulated homes, around a third of heat loss occurs through the walls. Solid walls, which lack a cavity, cannot be insulated using cavity wall insulation. Instead, they can be insulated by adding material to the inside or outside of the wall to retain heat more effectively.

How does solid wall insulation work?

There are two main approaches to insulating solid walls:

Internal wall insulation

• How it works: Rigid insulation boards are fitted directly to the inside wall, or a stud wall can be built and filled with insulation material like mineral wool fibre.
• Impact: Slightly reduces the floor area of the room.
• Preparation: Any existing damp issues must be addressed before installation.

External wall insulation

• How it works: Insulation material is attached to the outside wall, covered with plasterwork or cladding, and then decorated.
• Benefits: Improves weatherproofing, sound resistance, and protects brickwork, extending the lifespan of walls.
• Considerations: May require planning permission, depending on local regulations.

Ventilation and solid wall insulation

Solid walls naturally allow water vapour to move through bricks and stones. Adding insulation can change this dynamic, potentially leading to condensation on the inside of walls. To prevent this, professional installers use techniques such as:

• Breathable insulation materials
• Vapour barriers to control moisture movement

Working with trusted professionals ensures ventilation is effectively managed, avoiding future damp issues. If you choose Cosy Homes Oxfordshire, you’ll gain access to our network of vetted contractors who will handle these details for you.

Is solid wall insulation right for your home?

Solid wall insulation is particularly suited to older properties, such as those built before 1920, which likely have solid walls. To identify your wall type:

• Measure the thickness: Walls thinner than 260mm are likely solid.
• Check the construction date: Homes built before cavity walls became standard often have solid walls.

Not sure about your walls? A whole house assessment will confirm this and provide a tailored Whole House Plan to guide your retrofit.

Costs and savings

Solid wall insulation costs more than cavity wall insulation, but it delivers greater energy and carbon savings. According to the Energy Saving Trust (April 2019 data):

• Internal wall insulation: Approx. £7,500 for a semi-detached home
• External wall insulation: Approx. £11,000 for a semi-detached home

These figures are based on 2024 estimates and may vary depending on factors such as property size, location, and installation requirements. For the most up-to-date information, visit the Energy Saving Trust’s advice on solid wall insulation.

Annual energy savings can be up to £540, meaning many homeowners recover the cost within 10–15 years.

By combining solid wall insulation with other energy-saving measures in a Whole House Plan, you can reduce costs and maximise savings.

Why insulation should never be done in isolation

Insulating solid walls without considering ventilation, heating systems, and moisture control can cause serious unintended problems, including:

• Damp and mould due to trapped moisture
• Poor ventilation leading to condensation
• Overheating in summer if shading and airflow aren’t considered

A whole-house approach ensures every upgrade works together, avoiding expensive mistakes.

Real-life example

A few years ago, we worked on an end-terrace cottage in Hook Norton, where both internal and external wall insulation were installed alongside an Air Source Heat Pump. Read the full case study here to see how these measures transformed the property.

Plan your insulation properly

At Cosy Homes Oxfordshire, we take a whole house approach to insulation, ensuring every improvement works with your home’s unique needs for maximum comfort, energy savings, and long-term value.

Get in touch with our team on  0330 223 2742 or by email to book a home energy efficiency assessment and receive a tailored Whole House Plan – helping you make the right refurbishment decisions for a warmer, healthier, and more efficient home.