EWI vs IWI vs CWI: A Technical Comparison

A practical explanation of how different insulation methods interact with solid‑wall and cavity‑wall homes

Homes across Northampton and Northamptonshire include a mix of solid brick, stone, early blockwork and later cavity‑wall construction. Because each wall type behaves differently, external wall insulation (EWI), internal wall insulation (IWI) and cavity wall insulation (CWI) deliver different outcomes in terms of heat flow, moisture behaviour and long‑term stability.

This comparison explains how each method works, where it is typically suitable, and what homeowners usually notice in practice.

1. What each method actually does

External Wall Insulation (EWI)

Insulation is applied to the outside of the building, forming a continuous thermal layer around the masonry.
This keeps the wall warm, stabilises moisture behaviour and reduces heat loss through the structure.

Internal Wall Insulation (IWI)

Insulation is applied to the inside of external walls.
The internal surface becomes warmer, but the masonry behind the insulation remains cold and behaves more like an external element.

Cavity Wall Insulation (CWI)

Insulation is injected into the cavity between two masonry leaves.
It reduces air movement within the cavity and slows heat transfer across the wall.
CWI is only suitable where a true cavity exists and where the cavity is in appropriate condition.

2. How each method affects heat flow

EWI

• moves the thermal gradient outward

• keeps the masonry warm

• reduces heat loss significantly

• creates more stable internal temperatures

IWI

• keeps the masonry cold

• improves internal surface temperature

• increases sensitivity to thermal bridges at floors, ceilings and reveals

• requires careful detailing to avoid cold spots

CWI

• reduces heat transfer across the cavity

• performance depends on cavity width, exposure and condition

• not applicable to solid walls

3. How each method affects moisture behaviour

EWI

• stabilises drying by keeping the wall warmer

• reduces moisture retention

• lowers freeze–thaw stress

• shifts the dew point outward

• supports predictable moisture movement

More detail is in our guide to moisture behaviour in solid‑wall construction.

IWI

• slows drying because the wall stays cold

• moisture remains in the masonry longer

• dew point can move into the structure

• requires careful vapour control and junction design

Because the structural wall remains colder when insulated internally, its ability to dry outward is reduced. In solid masonry construction, long‑term durability is closely linked to the wall’s drying potential, so any reduction in outward drying needs careful consideration.

IWI is not inherently “wrong”, but it behaves differently and requires significantly tighter design control.

CWI

• works well in clean, unobstructed cavities

• can be problematic in cavities with debris, bridging or high exposure

• not suitable for solid masonry

4. A note on older cavity‑wall homes

Many older cavity‑wall properties across Northamptonshire were built with early cavity designs that were never intended to act as insulation spaces.
In these homes, the cavity often served primarily as a drainage channel, allowing moisture to move downwards and exit through weep paths rather than functioning as a sealed air pocket.

Because of this:

• filling the cavity can significantly change how the wall manages moisture

• debris, mortar snots and partial bridging are common in older construction

• uneven cavity widths affect both thermal performance and moisture behaviour

• exposure levels (especially wind‑driven rain) become more critical once the cavity is filled

Although CWI is often marketed as a simple, low‑disruption upgrade, older cavity walls require proper inspection and engineering assessment to confirm whether the cavity is suitable and how the wall will behave once filled.

5. Installation complexity and disruption

EWI

• external work

• minimal disruption inside

• requires scaffolding

• allows full façade renewal (render or brick‑slip)

IWI

• internal work

• reduces room size

• requires moving radiators, sockets and skirting

• more disruption to daily use

CWI

• least disruptive

• drilled from outside

• quick installation

• only suitable for cavity walls in appropriate condition

6. Suitability by wall type

EWI

Suitable for solid brick, stone, blockwork and many cavity walls.

IWI

Possible on most wall types, but requires more careful design on solid masonry.

CWI

Only suitable for cavity walls.
Not applicable to solid brick, stone or blockwork.
Older cavity walls require additional assessment.

More detail on wall types is in our guide to which walls can use external wall insulation.

7. Typical outcomes homeowners notice

EWI

• more stable room temperatures

• fewer cold surfaces

• reduced heating demand

• refreshed external appearance

IWI

• warmer internal surfaces

• improved comfort

• reduced room size

• more sensitive to detailing and moisture behaviour

CWI

• moderate improvement in heat loss

• minimal visual change

• dependent on cavity condition and exposure

8. Summary — different methods for different buildings

Each insulation method can be appropriate depending on wall construction, exposure conditions and specific building constraints.

For solid‑wall masonry, approaches that keep the structural wall warmer generally provide more stable long‑term moisture behaviour.

This is why external insulation often aligns more naturally with the physics of older brick and stone construction, while internal insulation and cavity‑fill approaches require more detailed assessment and design control.

If you’re comparing insulation options

Every property is different.
The most reliable way to understand what will work is to look at the building itself — construction type, exposure, coatings and detailing all matter.

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