Internal vs External Wall Insulation

Choosing between internal vs external wall insulation is one of the biggest decisions in a solid-wall retrofit. Both can reduce heat loss significantly, improve comfort and cut heating demand, but they do it in very different ways. The best answer depends on the age of the house, the wall construction, the available budget, planning constraints and how much disruption you can tolerate.

For many older UK homes, wall insulation matters because solid walls lose a large share of heat. The Energy Saving Trust has long used the rule of thumb that around 35% of heat loss in an uninsulated home can be through the walls. Pre-1920 homes are the most affected because they are more likely to have solid brick or stone walls rather than cavity walls that can be filled more easily.

The challenge is that older walls need careful detailing. Get the moisture strategy wrong and you can trap damp, create mould risk or damage historic fabric. That is why this decision is not just about cost per square metre. It is about choosing the approach that improves performance without causing building pathology later.

When you need wall insulation

If your home has uninsulated solid walls, feels cold even when the heating is on, or costs a lot to keep warm, wall insulation is worth investigating. Homes built before about 1920 are the usual candidates, though some later properties also have solid walls. You can often tell from the brick pattern, wall thickness or EPC notes.

Solid walls are thermally weak compared with modern insulated constructions. They also tend to create cold internal surfaces in winter, which can worsen condensation around furniture, window reveals and external corners. Insulation helps by raising surface temperatures as well as reducing heat loss.

Not every home should jump straight to wall insulation. Loft insulation, draught proofing and heating controls are often easier first steps. But if you want deep retrofit performance, especially before a heat pump, improving the walls can be transformational.

Internal wall insulation (IWI)

Internal wall insulation means insulating the inside face of the external walls. This is usually done using insulated plasterboard, composite boards or more breathable systems such as wood fibre with lime-based finishes. Typical installed costs in the UK are often around £80 to £120 per m², although awkward details, joinery changes and high-spec moisture-safe systems can cost more.

Thickness is commonly in the 50 to 100mm range once insulation and finishes are considered. That can make a meaningful difference to room dimensions, especially in smaller terraces. Skirting boards, sockets, radiators and window reveals also need adjustment, so disruption is real even if the outside of the property stays unchanged.

The main advantages are that IWI is often cheaper than EWI, can be done room by room, and may be the only realistic route on front elevations where external appearance cannot be altered. It is often the practical answer for mid-terrace homes that open directly onto a pavement or where external access is constrained.

The drawbacks are space loss, internal disruption and a greater risk of thermal bridging if details are not done carefully. Junctions at internal floors, partition walls and reveals are harder to perfect from the inside. Poorly designed IWI can also create interstitial condensation risk, particularly in traditional solid-wall buildings where moisture needs to move and dry safely.

External wall insulation (EWI)

External wall insulation wraps the outside of the building with an insulation layer, then finishes it with render, brick slips or cladding. In the UK, typical installed costs are often around £100 to £180 per m², with detached homes, scaffold needs, decorative detailing and premium finishes pushing the price upward.

EWI is generally the more thermally robust solution because it creates a more continuous insulated envelope. It reduces thermal bridging more effectively, keeps the original wall warmer and usually preserves internal room sizes. It also causes less internal disruption because most of the work happens outside.

There are several finish options. Silicone and mineral renders are common, while brick-slip systems can preserve a brick-like look at higher cost. Good EWI projects also include careful detailing around sills, eaves, downpipes, vents and service penetrations.

The disadvantages are cost, scaffolding, appearance changes and planning complexity. EWI changes the outward dimensions and look of the property. That is fine on some detached and semi-detached homes, but far harder on terraces, conservation-area streets and architecturally sensitive buildings.

Head-to-head comparison

Which is better for different homes?

In a terraced house, internal wall insulation is often the only practical route, especially at the front where the wall sits on the pavement line or where changing the façade would look out of place. Rear elevations may still allow EWI in some cases, but a full external wrap is often difficult.

In a detached home, external wall insulation is usually the better technical solution if budget and planning allow. Access is easier, the continuous envelope works better, and you keep all your internal floor area.

In a listed building or a highly sensitive traditional property, neither route is straightforward. Consent issues, heritage concerns and moisture behaviour all need specialist advice. Sometimes targeted secondary measures and careful conservation-led upgrades are more appropriate than full wall insulation.

Semi-detached homes can go either way. If the appearance change is acceptable and budget allows, EWI often wins. If not, IWI may be the compromise route, especially if works are already planned internally.

Breathability and moisture

This is the section most homeowners should take seriously. Older solid walls do not behave like modern cavity walls. Brick, lime mortar, stone and traditional plasters can absorb and release moisture. If you add the wrong insulation layer or vapour barrier in the wrong place, moisture can accumulate where you cannot see it.

For many solid-wall homes, vapour-permeable materials are essential or at least highly desirable. Wood fibre boards, calcium silicate, cork, lime plasters and vapour-open renders are commonly used in more moisture-aware retrofit strategies. In stone buildings especially, avoid assuming that foil-backed insulated plasterboard is automatically acceptable. Used carelessly, impermeable systems can trap moisture and increase decay risk.

That does not mean every breathable system is perfect or every foil-backed product is wrong. It means the build-up must match the wall type, exposure, existing damp condition and ventilation strategy. A proper survey matters far more than choosing the cheapest merchant-board solution.

Planning permission and building regulations

External wall insulation often requires a planning check because it changes the appearance of the building. Some councils may allow it under permitted development in certain situations, but many homeowners should assume that at least a pre-application enquiry is sensible. Conservation areas, article 4 areas and street-facing elevations deserve extra caution.

Internal wall insulation is less visible externally, so planning is usually simpler, but building regulations still apply. In England and Wales, Part L sets energy efficiency requirements for thermal elements when they are renovated. There can be exemptions or reasonableness tests for traditional and heritage buildings, but the work still needs to be approached properly.

Fire detailing, ventilation and condensation risk should also be considered as part of the design, not treated as afterthoughts.

Grants available

Solid wall insulation can be expensive enough that grants make the difference between action and delay. In the UK, ECO4 can fund or part-fund insulation measures for eligible low-income households, particularly where homes are inefficient and occupants meet benefit or vulnerability criteria.

The Home Upgrade Grant, often referred to as HUG2 in local authority delivery, has also supported off-gas homes through area-based programmes. Availability is local and scheme-led rather than universal, so eligibility depends on postcode, tenure, EPC level and household circumstances.

If you are grant-eligible, it is worth checking before paying privately. Wall insulation is one of the higher-cost retrofit measures, and support can materially change the economics.

Real costs for a typical semi

For a typical UK semi-detached house, a whole-house internal wall insulation project often lands around £5,000 to £8,000 for the principal external walls in a fairly simple scope. If you include extensive redecorating, joinery changes, premium breathable systems or tricky reveals, the cost can move higher.

A whole-house external wall insulation project for the same type of property is more often around £8,000 to £15,000. Scaffolding, render specification, access complexity and detailing at rooflines or boundaries are what usually widen the range.

These are useful ballpark figures, not fixed prices. The best quotes are detailed enough to show insulation thickness, finish, reveal treatment, moisture strategy and exactly what happens at junctions rather than just offering a headline number.

Frequently asked questions