Why External Wall Insulation (EWI)

A practical explanation for homeowners with older solid-wall properties

When we work on older homes around Northamptonshire, we often see the same pattern — cold internal walls, rooms that never quite feel even in temperature, and masonry that stays damp for longer than people expect. Most of these houses were built long before insulation became part of standard construction, so the walls behave very differently from modern cavity-wall buildings.

External wall insulation is one of the approaches that tends to work most naturally with these older structures. It doesn’t change the character of the building — it changes how the wall manages heat and moisture over time.

What external insulation actually does

When insulation is placed on the outside of a wall, the masonry stays warmer and more stable throughout the year. That single shift affects several things at once:

• the wall no longer acts as a cold surface drawing heat out of the room
• internal temperatures become more consistent
• cold corners and patchy condensation are reduced
• the masonry is less stressed by freeze–thaw cycles
• the building fabric dries more predictably

If you want to understand how the full system build-up works and why each layer matters, we explain this in more detail in our guide on how external wall insulation systems work.

Why this approach suits solid-wall homes

Solid brick and stone walls behave very differently from cavity construction. They absorb moisture, release it slowly, and cool down quickly in winter. When we open up older walls in Northampton and nearby villages, we often find the masonry itself is still structurally sound — but thermally unstable, with warm internal air meeting cold external surfaces and moisture sitting in the wall longer than intended.

By insulating externally, the wall stays within a more stable temperature range. This supports natural drying and reduces the amount of moisture the masonry carries through colder months.

If you want a deeper explanation of how moisture behaves inside solid masonry walls, we cover this in our solid wall moisture behaviour guide.

How EWI compares to other insulation methods

Different insulation methods interact with solid masonry in very different ways.

External insulation keeps the structural wall warm.
Internal insulation keeps the structural wall cold.
Cavity insulation only works where a true cavity exists and is in suitable condition.

Each method has its place, but they behave very differently in older buildings. If you want a technical comparison of how these approaches perform in practice, we break this down in our EWI vs IWI vs CWI comparison.

Choosing the right insulation material

Most EWI systems use either EPS or Mineral Wool.

We choose between them based on how the building actually behaves — not just cost or brochure performance.

EPS is often used where thermal performance is the main priority.
Mineral Wool is typically preferred where fire performance, breathability or acoustic improvement are more important.

If you want to understand how these materials differ in real installations, we explain this in more detail in our EPS vs Mineral Wool comparison.

Where EWI can be used

We install EWI on a wide range of wall types — solid brick, stone, blockwork and some cavity wall constructions, depending on their condition. Suitability depends on exposure, existing coatings, moisture levels and the overall stability of the substrate.

If you’re unsure whether your home is suitable, we explain the main construction types in our guide to which walls can use external wall insulation.

What the installation involves

A typical EWI project includes:

• preparing the existing surface
• mechanically fixing insulation boards
• applying a reinforced basecoat
• detailed work around windows, sills and junctions
• finishing with a silicone render or brick-slip system

The aim is not just to add insulation, but to create a continuous, durable external layer that protects the building fabric and improves long-term performance.

Why homeowners choose EWI

Most people don’t come to us asking specifically for external wall insulation.

They usually contact us because their home:

• feels cold even when the heating is on
• has uneven temperatures between rooms
• shows signs of condensation or cold corners
• has ageing render or brickwork that needs attention
• costs more to heat than it should

EWI often helps address several of these issues at once by stabilising the wall and improving the building envelope.

Our approach

Before recommending anything, we look at:

• wall construction
• exposure and shading
• existing moisture behaviour
• previous coatings or render
• detailing around openings
• long-term maintenance considerations

The goal is always the same — choose a solution that works with the building, not against it.

If you’re considering EWI for your home

Every property is different, and the only reliable way to understand what will work is to assess the building itself. A survey allows us to check wall type, exposure, moisture levels and any detailing that may need attention.

If you’d like to discuss your property, we’re always happy to take a look and talk through the options in a straightforward, practical way.