Solid walls — the standard construction in any pre-1920 British home — lose heat at roughly twice the rate of modern cavity walls. A solid brick wall typically has a U-value of around 2.0 W/m²K; Building Regulations Part L targets for new build are 0.18 W/m²K. That gap is why solid-wall insulation is usually the single highest-impact upgrade a homeowner can make — and why the ECO4 and Great British Insulation Scheme (GBIS) grant programmes prioritise it.
There are two ways to insulate a solid wall: attach insulation to the outside (External Wall Insulation, EWI) or fit it to the inside (Internal Wall Insulation, IWI). Both can get a solid brick wall down to around 0.30 W/m²K or better. They are not interchangeable. The right choice depends on your building, your planning constraints, your budget, and — critically — whether you understand the failure modes of each.
What Is External Wall Insulation?
EWI involves fixing insulation boards (typically expanded polystyrene, mineral wool, or phenolic foam) to the external face of the wall, then applying a reinforced render finish or — in some systems — a rain-screen cladding. The insulation layer typically ranges from 60mm to 150mm depending on the target U-value.
U-Value Targets and PAS 2035
PAS 2035:2023 is the publicly available specification that governs retrofit work in the UK. Under ECO4 or GBIS-funded projects, compliance with PAS 2035 is mandatory. It requires a Retrofit Coordinator to oversee all medium and large-scale retrofit projects, and a Retrofit Assessor to prepare a Whole House Plan before installation begins. This whole-house approach exists because insulating walls without addressing ventilation can cause condensation and moisture damage — a very real risk in older housing stock.
The target U-value for EWI under PAS 2035 is 0.30 W/m²K or better (or the best achievable if structural constraints prevent this). A 90mm EPS board on a solid brick wall will typically achieve around 0.28–0.30 W/m²K.
Planning Implications for EWI
EWI changes the external appearance of a building, which means it is not automatically permitted development. Under the Town and Country Planning (General Permitted Development) (England) Order 2015 (GPDO), householder PD rights do not extend to cladding that uses a material visibly different from what was there before in a conservation area.
If your home is in a conservation area, Article 4 Direction area, or is a listed building, you will need planning permission or listed building consent before proceeding. Heritage England's guidance strongly discourages EWI on listed buildings where it would obscure original fabric. In many cases, IWI is the only viable route for heritage properties.
For non-designated buildings outside conservation areas, EWI is generally permitted development provided it does not exceed the permitted tolerances — but always confirm with your local planning authority before proceeding.
Common EWI Failure Modes
- Render cracking at board joints if boards are not staggered correctly or movement joints are omitted
- Water ingress behind the system through poorly detailed junctions at windows, sills, and eaves — the most common cause of EWI failure in the UK
- Bridging where existing meter boxes, pipes, or brackets penetrate the insulation layer without thermal break collars
- Inadequate surface preparation — EWI on friable or contaminated masonry will delaminate
What Is Internal Wall Insulation?
IWI involves fixing insulation to the internal face of external walls. There are two principal methods:
Thermal battens (service cavity system): Timber or metal battens are fixed to the wall, creating a cavity that houses services (electrics, pipes), with rigid insulation fitted between the battens and plasterboard on the face. This system allows services to be run without penetrating the insulation and reduces thermal bridging at the battens if insulated correctly.
Dot-and-dab (direct bond): Rigid insulation-backed plasterboard (often PIR foam bonded to plasterboard) is adhered directly to the wall with dabs of adhesive. Faster and cheaper, but leaves a hidden void between board and wall where moisture can accumulate — a significant risk in walls subject to penetrating damp.
Condensation Risk and Vapour Control Layers
IWI moves the cold face of the wall inward. In a traditional solid brick wall, the dew point (where moisture in the air condenses) was within the masonry itself. With IWI added, the dew point can shift to the interface between the insulation and the original wall — exactly where moisture collects if vapour is not managed.
PAS 2035 requires an interstitial condensation risk analysis (typically using WUFI or BR 262 calculation method) for all IWI installations. Where the analysis indicates risk, a continuous vapour control layer (VCL) — typically a foil-faced membrane — must be fitted on the warm side of the insulation. In practice, achieving a continuous VCL around sockets, pipes, and corners is demanding workmanship, and many failures arise from VCL discontinuities.
Room Size Loss
IWI reduces the floor area of every room it is fitted to. A 70mm PIR system on dot-and-dab reduces the room dimension by around 75–80mm per treated wall. In a standard Victorian terrace living room (3.5m wide), IWI on both long walls reduces width to approximately 3.35m — a meaningful loss in a small room. Loft conversions, bay window reveals, and stairwells require careful detailing to avoid thermal bridging at junctions.
EWI vs IWI: Costs Comparison
| System | Typical Cost Range | Notes |
|---|---|---|
| EWI — EPS render system | £100–£150/m² | Most common; requires scaffolding |
| EWI — Mineral wool render | £120–£175/m² | Better fire performance |
| EWI — Rain-screen cladding | £150–£200/m² | Planning-sensitive; premium finish |
| IWI — Dot-and-dab PIR | £40–£65/m² | Fastest; condensation risk if poorly detailed |
| IWI — Thermal batten system | £65–£100/m² | Better moisture management; preferred under PAS 2035 |
For a typical semi-detached property with 70m² of external wall area, EWI costs £7,000–£14,000 installed; IWI typically £2,800–£7,000 — though IWI triggers additional costs for electrical first-fix relocation, skirting boards, window sill extensions, and making good around architraves.
ECO4 and GBIS Grant Eligibility
Both EWI and IWI are eligible for grant funding under ECO4 (Energy Company Obligation 4) and the Great British Insulation Scheme (GBIS), subject to eligibility criteria:
- ECO4 targets fuel-poor households and those receiving means-tested benefits. It can cover the full cost of installation. Homes must have an EPC rating of D or below at the point of application.
- GBIS is broader — targeting homes with EPC D or below. It is not means-tested, though the level of subsidy varies by household income band and local authority area. Grant values for solid wall insulation under GBIS typically range from £1,500 to £10,000 depending on circumstances.
Both schemes require a PAS 2035-compliant Retrofit Coordinator and Assessor. Work must be carried out by a TrustMark-registered contractor.
The Sequencing Rule: Insulate Before You Upgrade Heating
This point cannot be overstated. Upgrading to a heat pump before insulating a solid-wall property is a common and expensive mistake. Heat pumps operate most efficiently at low flow temperatures (45–55°C) — which requires a well-insulated building with low heat loss. A solid-wall home with U-values above 1.0 W/m²K will need radiators running at 65–70°C to compensate, forcing the heat pump into its least efficient operating range and inflating energy bills.
The correct sequence is: draught-proof, insulate the building fabric, then install the low-carbon heating system. This order also maximises your SAP score improvement on the EPC, which can unlock better mortgage rates and improve rental yield.
Which System Is Right for You?
Choose EWI if: the external appearance needs updating anyway; the building is in reasonable structural condition; there is no planning restriction; and room sizes cannot be sacrificed.
Choose IWI if: the building is in a conservation area or is listed; external appearance must be preserved; or you are working room by room rather than whole-house.
In practice, many solid-wall homes use a hybrid — EWI on the rear elevation (where planning is less sensitive) and IWI on the front. A Retrofit Assessor under PAS 2035 will model both options and advise based on your specific building's risk profile.
Related Guides
- Solid Wall Insulation Costs UK — full cost breakdown for EWI and IWI with installer selection tips
- Cavity Wall Insulation Costs UK — how cavity fill differs from solid wall insulation and when it is appropriate
- EPC Improvement: Full Room-by-Room Action Plan — the correct sequencing of insulation and heating measures for maximum SAP points
- Heat Pump Installation: ASHP vs GSHP, the BUS Grant, and Planning Rules — why insulating before installing a heat pump is essential, and how the two upgrades interact
- Energy Efficiency Home Improvements UK — which upgrades give the best EPC uplift and how to prioritise on a limited budget
- EPC Rating: How to Improve Your Energy Performance Certificate — the complete EPC improvement roadmap including ECO4/GBIS grant eligibility and the SAP uplift table for each measure