TL;DR:
- Internal wall insulation is the primary solid-wall option when external insulation isn’t viable — common in conservation areas, flats, and period properties.
- Costs typically run £4,000 to £15,000 depending on property size and the insulation system used, but ECO4 and the Great British Insulation Scheme can cover all or most of this for eligible households.
- Done wrong, IWI creates condensation and mould problems — vapour control is not optional.
Around 7 million UK homes have solid walls, built before the era of cavity construction. Most of these predate the 1920s. They lose heat at roughly twice the rate of a modern cavity-wall or filled-cavity property, and they are among the hardest to insulate cost-effectively. External wall insulation (EWI) is often the preferred option when it’s viable, but for many solid-wall homes, it isn’t: listed buildings, conservation area restrictions, terraced rows where planning consent is complex, top-floor flats, and homes where the external appearance matters all point toward internal wall insulation instead.
IWI reduces your wall U-value and cuts heating bills substantially, but it comes with real trade-offs that you need to understand before committing.
How Internal Wall Insulation Works
IWI is installed on the inside face of your exterior walls. There are two main system types:
Rigid board systems use insulated plasterboard (PIR or phenolic foam laminated to a plasterboard face) fixed directly to the wall with adhesive or mechanical fixings. This is the most common system for retrofit. Typical board thickness runs from 52mm (insulation plus plasterboard) to 100mm or more, depending on the U-value target. The finished surface is ready to skim and decorate.
Stud wall systems build a new internal timber or metal stud frame set away from the external wall, with mineral wool or other insulation packed in the stud cavity. This creates a larger cavity and can achieve better U-values, but loses more floor space and involves more disruption. It is often combined with service routing (electrical cables, plumbing) in the new cavity.
Both systems require a vapour control layer (VCL) on the warm side of the insulation. This is not optional. Without it, water vapour from the interior migrates into and through the insulation, hits the cold external wall behind it, and condenses. Over time this causes mould, structural damage, and insulation failure. The VCL must be installed carefully — lapped and taped at all joints, sealed around penetrations. This is where poor IWI installations fail.
Heat Loss Reduction and EPC Impact
A solid wall with no insulation typically has a U-value around 2.0 W/m²K. Adding IWI can bring this down to 0.30 W/m²K or better, depending on thickness — similar to a modern cavity wall. For a typical 3-bedroom Victorian terraced house, this can cut heat loss through walls by around 85%, which translates to a meaningful reduction in heating bills and a significant EPC rating improvement.
For EPC-E or F-rated solid-wall homes, IWI commonly moves the rating to C or D, which matters for landlords under current and upcoming minimum EPC standards, and for homeowners whose lenders require improvements before remortgaging.
What It Costs
IWI costs vary significantly based on:
- Property size: a small 2-bed flat might be £4,000-6,000; a larger detached house can reach £12,000-15,000
- Number of external walls being treated: treating all walls maximises benefit but increases cost
- System type: rigid board is generally cheaper than stud wall systems
- Installer region: London and Southeast carry a premium
A reasonable budget estimate for a typical 3-bed semi with solid walls is £6,000-10,000 for the IWI work, excluding any redecoration. This includes all skirting boards, architraves, and electrical socket extensions that need moving after the wall thickness increases.
Grants and Funding in 2026
ECO4 (Energy Company Obligation) is the main grant scheme that covers solid wall insulation. If your household receives qualifying benefits, or if your home is EPC-rated D, E, F, or G, you may be eligible for ECO4 funding that covers the full cost of IWI. Installers work with energy companies to deliver ECO4-funded work. The ECO4 scheme runs until March 2026, with the successor ECO5 scheme expected to continue similar obligations.
Great British Insulation Scheme (GBIS) covers solid wall insulation for households in lower EPC bands and lower council tax bands, as well as those receiving certain means-tested benefits. GBIS delivers either partial or full cost coverage depending on household circumstances.
Warm Homes Local Grant targets fuel poverty households in England and can cover solid wall insulation as part of a whole-house package. Awards are made through local authorities.
To find out what you are eligible for, use the Energy Saving Trust’s grants calculator or contact a Trustmark-registered IWI installer who will carry out a free eligibility check and arrange funding on your behalf.
Disruption and Planning
IWI is not a quick install. Expect two to four days of work per external wall, with the full job taking one to two weeks in a typical house. The disruption is real:
- Furniture must be moved away from all external walls being treated
- Skirting boards, architraves, and window boards are removed and replaced
- Electrical sockets and light switches on external walls need to be extended forward by the thickness of the insulation
- Radiators may need repositioning or pipe extensions
- Redecoration of affected rooms is required afterwards
This is a bigger disruption than EWI (which happens outside and leaves interior finishes largely untouched), which is why EWI is often preferred when viable.
Watch Points Before Commissioning
Get a hygrothermal assessment. A solid wall that has been painted with impermeable masonry paint, or has internal damp issues, needs assessment before IWI. Trapping moisture behind the new insulation is the worst outcome. A surveyor experienced in solid-wall properties should check for existing damp issues before work begins.
Use a Trustmark-registered installer. IWI is a PAS 2030/2035 controlled measure, meaning ECO4-funded work must be designed and installed according to these standards, with independent inspection. Using a registered installer is required for grant funding and provides a route to complaint resolution if work is defective.
Budget for redecoration. The installation cost typically does not include repainting walls, replacing floor coverings that may have been disturbed, or other finishing work. Get a complete project cost including redecoration before committing.
Consider phasing. Not every external wall needs to be treated at once. Some homeowners start with the north-facing wall (the coldest) and phase subsequent walls as budget allows. The benefit is not strictly proportional to coverage — even treating the worst wall delivers meaningful improvement.
IWI vs. EWI: When to Choose Each
External wall insulation is generally preferred when:
- There are no planning restrictions
- The external render or cladding needs replacing anyway
- Interior space is at a premium
- The property is a standard terrace or semi where neighbours are doing the same
Internal wall insulation is typically the choice when:
- The property is listed or in a conservation area
- External appearance cannot change
- It is a top-floor flat or the building’s external envelope is managed collectively
- The external walls have decorative features worth preserving
For many solid-wall homeowners, IWI is not a second choice — it is the only viable option. Done properly, with attention to vapour control and installed by a competent tradesperson, it delivers substantial and lasting energy savings.