Internal Wall Insulation: Costs & Methods

Internal wall insulation (IWI) is the method most UK homeowners use to thermally upgrade solid external walls, older cavity walls with failed ties, or any wall where external insulation isn’t viable. It avoids planning permission and external scaffold in most cases, but it does reduce room size, requires careful vapour management, and involves moving every socket, radiator, and window reveal on the treated walls.

Done properly it is effective and durable. Done wrong, particularly with vapour control omitted, it creates interstitial condensation that rots timber, grows mould, and can cause serious structural damage. This guide covers the main methods, realistic UK costs, and the condensation risk each carries.

Why Internal Insulation Rather Than External?

External wall insulation (EWI) is generally the preferred technical approach: it keeps the thermal mass of the wall warm and on the heated side, avoids all internal disruption, and has no interstitial condensation risk. But EWI is often impractical:

• Planning restrictions in conservation areas and on listed buildings
• Terraced or semi-detached houses where the neighbour won’t co-ordinate
• Aesthetically sensitive facades (brick, render patterns, stone)
• Close site boundaries with insufficient clearance

In all these situations, IWI is the default option.

Methods

Dry-Lining with PIR (Polyisocyanurate) Board

The most common residential method. A PIR rigid foam board (such as Celotex, Kingspan, or Recticel) is bonded directly to the internal face of the external wall using dot-and-dab adhesive or full-bed bonding, then overboarded with a foil-faced vapour control layer (often integral to the PIR board) and a plasterboard finish.

Thickness and performance:

A 75 mm PIR board on a solid 225 mm brick wall brings it to approximately 0.18 W/m²K, which is exactly the Building Regulations Part L notional dwelling figure for external walls in a new build, and a long way from an original value of around 1.8–2.0 W/m²K on uninsulated solid brick.¹

Total depth lost per wall (board + adhesive bedding + plasterboard): typically 75–130 mm depending on specified thickness.

Independent Stud-Wall System

A metal or timber stud frame is built 15–25 mm clear of the inner face of the external wall, then mineral wool or PIR batt is fitted between studs. A continuous vapour control layer (VCL) is taped to the warm face of the studs before plasterboarding.

This method is more expensive and loses more floor area (typically 100–150 mm per treated wall) but allows greater insulation thickness, accommodates services within the stud cavity, and achieves excellent airtightness if the VCL is properly detailed.

Mineral wool batts at 100 mm achieve approximately R 2.5–2.8 m²K/W, less than PIR at the same thickness. At 150 mm, mineral wool reaches R 3.75–4.1 m²K/W, comparable to 75 mm PIR.

Insulated Plasterboard (Dot-and-Dab)

Insulated plasterboard is a composite board with PIR already bonded to the back of the plasterboard. Applied in one operation using dot-and-dab adhesive. Quicker to install, but limited to thinner boards and the adhesive dots create cold bridges if the pattern is irregular. The plasterboard face is 12.5 mm on every mainstream laminate, so the overall thickness is simply the insulation plus 12.5.²³

Sopratherm PL4000 (the former Celotex PL4000) runs 37.5 mm to 77.5 mm overall, meaning 25 mm to 65 mm of PIR at a declared 0.022 W/mK.² EcoTherm’s Eco-Liner covers 37.5 mm to 82.5 mm on the same 12.5 mm board.³ Suitable for modest upgrades where floor space loss must be minimised.

Spray Foam Applied Internally

Low-expansion open-cell or closed-cell spray polyurethane foam (SPF) applied directly to the internal face of the wall. Rarely used on main living spaces but sometimes specified in awkward voids (roof spaces, below-floor voids, irregular masonry). Not recommended as a primary IWI method on habitable rooms: it’s difficult to control thickness precisely, and some spray foam products on roofs have caused mortgage valuation issues. The government’s own 2024 modelling of sprayed foam on timber roofs found the moisture risk is low only where an open-cell, moisture-permeable foam is used with an air and vapour control layer on the warm side and a ventilated space left between the foam and the underlay, which is the same discipline this note asks for on walls.⁴

Cost

Costs vary substantially with method, wall condition, and whether the rooms need to be fully emptied and redecorated afterwards. The figures below assume the client vacates the room, existing plaster remains (not hacked off), and standard socket/radiator repositioning.

Add £30–£60 per socket or switch relocated. Radiator repositioning or new pipe runs: £120–£250 per radiator. Plastering and redecoration are sometimes included but often quoted separately, budget an additional £10–£20/m² for skimming and painting.

Whole-house costs for a typical 3-bed semi with 60–80 m² of treatable external wall: £4,000–£9,000 for mid-range IWI, or up to £12,000+ for a premium stud system with full redecoration.

Condensation Risk

This is the single most important technical issue with IWI and the area where poorly specified jobs fail.

When you insulate an external wall from the inside, you push the thermal gradient into the wall, the point at which the temperature drops to the dew point (where condensation forms) moves into the masonry. If water vapour from inside the building can pass through the plasterboard and insulation, it will condense on the cold masonry face behind the insulation.

Managing condensation risk:

1. Vapour control layer (VCL): A foil VCL on the warm (room) face of the insulation is essential in nearly all IWI installations. In a stud system, the VCL must be a continuous membrane taped at all joints and sealed around penetrations. For PIR board systems, the foil facing on the board acts as the VCL if all joints are taped with foil tape.

2. Thermal modelling: For any wall where there is a risk, solid single-skin brick, rubble stone, or any wall that has had dampness historically, a dew-point calculation (per BS EN ISO 13788:2012, the current edition, or using WUFI software) should be carried out before specifying the insulation.⁵

3. Pre-treatment of the wall: Existing damp in the wall must be diagnosed and treated before insulating. IWI over a wall with active rising damp or penetrating damp will trap moisture and worsen the damp.

4. Ventilation: The room’s ventilation strategy needs to account for the improved airtightness IWI creates. Mechanical extract ventilation or MVHR (mechanical ventilation with heat recovery) becomes more important as the building envelope tightens.

When to Use Internal Wall Insulation

IWI is appropriate when:

• The property has solid external walls (pre-1920s brick, stone, concrete) and external insulation is not viable
• Cavity walls with failed or absent cavity fill where re-injection is not possible
• The building is within a conservation area where EWI would require listed building or planning consent
• Budget permits the full package including redecoration, partial IWI without decoration results in step changes in wall depth at untreated sections

IWI is not appropriate (or needs careful detailing) when:

• There is active rising damp or penetrating damp, fix the moisture first
• The wall has a structural role that internal boarding could obscure (original lime plaster, historic interiors)
• The room is very small, losing 100+ mm on every external wall can make a 3 m × 3 m room unacceptably narrow

Under Building Regulations Part L (Conservation of Fuel and Power), IWI installed as part of a notifiable renovation work package must achieve U ≤ 0.30 W/m²K. That is the improved value in Approved Document L Table 4.3 for a wall taking internal or external insulation, against a threshold of 0.70 W/m²K that triggers the upgrade in the first place. Where 0.30 is not technically or functionally feasible, or would not pay back within 15 years, the wall goes to the lowest U-value that is both, and a lesser standard again may be accepted where meeting it would breach Part C on interstitial and surface condensation.¹

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.