Which walls can use EWI

A practical overview of wall types typically suitable for external wall insulation

External wall insulation can be applied to a wide range of wall constructions. The key principle is simple: EWI works best on walls that benefit from being kept warm, dry and protected from external weathering. Most solid‑wall homes fall into this category, and many cavity‑wall and modern constructions can also be suitable when assessed correctly.

1. Solid walls (brick, stone, blockwork, concrete)

Most solid‑wall homes across Northamptonshire are well‑suited to EWI.
Despite differences in material, these walls behave similarly:

• they lose heat quickly

• internal surfaces become cold in winter

• moisture remains longer in cold masonry

• the structure performs more predictably when kept warm

This group includes:

• solid brick

• solid stone (limestone, ironstone, rubble stone, hybrids)

• solid blockwork (dense or lightweight)

• solid concrete (cast in situ, precast panels, dense concrete block)

EWI provides a continuous external layer that stabilises temperature, improves drying behaviour and reduces cold internal surfaces.

2. Cavity‑wall homes (case‑by‑case suitability)

Some cavity‑wall buildings can use external wall insulation, but suitability depends on how the wall is built and how it manages moisture. Cavity walls vary widely in width, exposure and construction quality, so an assessment is usually needed before specifying EWI.

EWI may be appropriate when:

• the cavity is narrow or partially filled

• the external leaf is cold or highly exposed

• a façade renewal is planned (render or brick‑slip)

The key question is whether the external leaf and cavity can continue to dry safely once insulated. When this is confirmed, EWI can work well on selected cavity‑wall homes.

3. Mixed‑construction homes

Many Northamptonshire properties combine:

• brick

• stone

• blockwork

• concrete

• later extensions

• infill panels

EWI performs well on mixed construction because it creates a continuous thermal envelope across all materials, reducing cold bridges between different wall types.

4. Timber frame walls

Timber frame construction can also use EWI, but only with correct system design. Timber structures rely on safe outward drying, so the insulation system must be vapour‑open and compatible with the existing sheathing and membranes.

General guidance:

• modern timber frame (OSB + breather membrane) → often suitable after assessment

• older or heritage timber frame → requires more detailed evaluation

The principle is simple:
timber frame must always retain a safe outward drying path.

5. Walls that require additional assessment

Some walls can use EWI, but need investigation before specifying a system:

• walls with structural movement

• walls with moisture trapped under dense cement coatings

• walls with failing render or paint layers

• walls with high ground‑moisture loading

• heritage substrates (cob, wattle and daub, lath and plaster)

These are not exclusions — they simply require understanding how the wall will dry once insulated.

6. Walls generally unsuitable for EWI

EWI is typically not used on:

• façades that must remain visually unchanged (heritage restrictions)

• walls with severe structural instability

• walls where external access is not possible

In such cases, internal or hybrid approaches may be considered, but they require more design control.

Summary

EWI is suitable for most solid‑wall homes — brick, stone, blockwork and concrete — and can also be used on selected cavity‑wall and timber‑frame buildings after assessment.
The key question is not “Can EWI be installed?”
but “How will the wall behave once it is kept warm and protected from the outside?”

A brief review of wall type, exposure and existing coatings usually provides a clear answer.