EWI vs IWI: Choosing External or Internal Wall Insulation in 2026

External wall insulation (EWI) costs £6,000 to £14,000 for a Lancashire home and wraps the building in a thermal blanket without affecting your internal living space. Internal wall insulation (IWI) costs £4,000 to £8,000 and is fitted inside the home, reducing room sizes by 50mm to 100mm per wall but avoiding any change to the building’s external appearance. Both save an estimated £300 to £600 per year on heating. The right choice depends on your budget, planning constraints, property type, and personal tolerance for disruption.

Which Lancashire Homes Need Solid Wall Insulation?

Solid wall insulation is for homes with no cavity – where the walls are a single solid layer of brick, stone, or a combination. In Lancashire, this includes most homes built before the 1920s. The county has a huge stock of these properties: the stone-built terraces of the mill towns (Burnley, Blackburn, Accrington, Darwen, Nelson, Colne), the Victorian and Edwardian houses across Preston, Lancaster, and Clitheroe, and many rural cottages and farmhouses throughout the Ribble Valley and Forest of Bowland.

If you are unsure whether your home has solid or cavity walls, check the wall thickness. A solid brick wall is typically 230mm (9 inches) thick. A cavity wall is 270mm to 300mm (10.5 to 12 inches). From outside, solid walls usually have alternating rows of bricks laid lengthwise and crosswise (you can see the short end of some bricks), while cavity walls show only lengthwise bricks. Stone-built Lancashire homes are almost always solid.

Solid walls lose heat about twice as fast as cavity walls. Without insulation, a solid-walled Lancashire terrace can lose 35% to 45% of its heat through the walls, making this the single biggest area for improvement in these homes.

External Wall Insulation: How It Works

EWI involves fixing insulation boards (typically 50mm to 100mm of expanded polystyrene, mineral wool, or phenolic foam) to the outside of your walls with adhesive and mechanical fixings. The boards are then covered with a reinforced render finish that can be smooth, textured, or patterned to your choice.

The finished appearance changes the look of your home. A stone-built Lancashire terrace that receives EWI will look like a rendered building afterwards. For some homeowners, this is a positive change – the new render provides a fresh, modern appearance. For others, particularly those who value the original stone or brickwork, losing the traditional look is a significant drawback.

EWI also requires adjustments to window sills (which need extending to cover the additional wall thickness), rain gutters, downpipes, gas meters, and external lights. Door and window reveals are insulated too, which slightly reduces the size of window and door openings from the outside.

EWI Costs for Lancashire Homes

• Mid-terrace house (Burnley, Blackburn, Accrington) – £6,000 to £9,000. Only the front and back walls need insulating (party walls are shared with neighbours).
• End-terrace or semi-detached – £8,000 to £12,000. An additional side wall increases the area.
• Detached house – £10,000 to £14,000+. All four walls require insulation.

government energy efficiency schemes can fully fund EWI for qualifying Lancashire households. Given the high cost of EWI, this grant makes an enormous difference. If you meet the benefits criteria and have a low EPC rating, the entire £8,000 to £12,000 job could be done at no cost to you.

Internal Wall Insulation: How It Works

IWI is applied to the inside face of external walls. The two main methods are insulated plasterboard (rigid insulation bonded to plasterboard, fixed directly to the wall) and stud frame with infill (a timber frame built against the wall, filled with mineral wool or rigid insulation, then plasterboarded).

Insulated plasterboard is thinner (typically 25mm to 50mm total) and quicker to install. Stud frame systems are thicker (50mm to 100mm) but provide better insulation values. The choice depends on how much room you are willing to sacrifice and the target U-value.

IWI requires removing and refitting radiators, sockets, switches, and any wall-mounted fixtures on the affected walls. Skirting boards, coving, and window reveals all need adjusting. Every room with an external wall needs decorating after the work. The disruption is significant – you effectively lose each treated room for one to three days during installation.

IWI Costs for Lancashire Homes

• Mid-terrace (front and back walls only) – £4,000 to £6,000 including decoration.
• End-terrace or semi – £5,000 to £7,500.
• Detached – £6,000 to £9,000.

These costs include the insulation materials, plasterboard, labour, and basic re-decoration of the affected rooms. If you want a high-quality decorative finish (wallpapering, feature walls), the decoration costs will be higher.

Comparing Performance

Both EWI and IWI achieve similar thermal performance when properly installed. A solid stone wall with a U-value of 1.5 to 2.0 W/m2K can be improved to 0.25 to 0.35 W/m2K with either method, depending on the insulation thickness.

However, EWI has a slight performance advantage because it wraps the entire wall including the thermal mass of the stone or brick. This means the wall itself acts as a heat store, absorbing warmth during the day and releasing it slowly, helping to maintain stable indoor temperatures. IWI separates the insulation from the wall mass, so the thermal storage benefit is lost.

EWI also eliminates cold bridges at wall junctions, floor-to-wall connections, and around window openings more effectively than IWI, because the insulation is continuous across the entire wall surface. Cold bridges with IWI can lead to condensation and mould at corners and junctions if the installation is not carefully detailed.

Planning Permission and Conservation Considerations

EWI changes the external appearance of your home and may need planning permission, particularly in conservation areas around Clitheroe, Whalley, Lancaster city centre, parts of Preston, and various Lancashire villages. If your property is listed, EWI is very unlikely to be permitted. IWI does not change the external appearance and never requires planning permission.

For Lancashire’s mill town terraces, EWI can actually improve the streetscape when applied consistently to a row of houses. Some government-funded EWI programmes in Burnley and Blackburn have transformed entire streets, replacing crumbling render and exposed brick with uniform, clean insulated finishes. In these cases, the visual change is widely seen as positive.

Where stone frontages have heritage value (common in the Ribble Valley and Forest of Bowland villages), IWI is usually the only option for the front wall, while EWI may be acceptable on the rear wall that is not visible from the street. This combined approach offers a practical compromise.

Making Your Decision

Choose EWI if you want no reduction in room sizes, your property is not listed or in a conservation area (or permission is obtainable), you are comfortable with a rendered finish replacing the original external appearance, you want the best thermal performance and fewest cold bridges, and you can secure grant funding to cover the higher cost.

Choose IWI if you want to preserve the original external appearance of your Lancashire home, your property is listed or in a conservation area where EWI is not permitted, your budget is limited and you are paying out of pocket, you can tolerate the internal disruption and loss of room space, or you only want to insulate certain rooms rather than the whole house.

Many Lancashire homeowners end up choosing a hybrid approach: EWI on the rear elevation (less visible) and IWI on the front elevation (preserving the street appearance). This gives a good balance of performance, aesthetics, and cost.

Can I install solid wall insulation myself?

IWI using insulated plasterboard is a feasible DIY project for competent home improvers. It involves cutting boards to size, applying adhesive, fixing to walls, and taping joints. EWI is not a DIY job – it requires scaffolding, specialist render application skills, and detailing around windows and edges that demands professional training. For either type to qualify for grants or 0% VAT, professional installation is required.

Will solid wall insulation cause damp or condensation?

If correctly installed with appropriate vapour management, solid wall insulation should not cause damp. Poor installations that trap moisture between the insulation and the wall can lead to problems. EWI with breathable render allows moisture to escape outwards. IWI needs a vapour control layer on the warm side to prevent condensation forming within the wall. Always use an experienced installer who understands moisture management in solid walls.

How long does solid wall insulation take to install?

EWI takes one to three weeks for a typical Lancashire terrace, depending on size and complexity. Weather affects the timeline because render application needs dry conditions – Lancashire’s climate can add delays. IWI takes one to two weeks for a whole house, with each room taking one to three days. You can continue living in the house during either installation, though expect disruption to daily routines.