Is internal wall insulation worth it?

Internal wall insulation (IWI) is the cheaper alternative to external wall insulation for solid-wall homes. It's most common in conservation areas, listed buildings, or where homeowners want to keep the original external brickwork visible. This guide walks through the costs, room size impact, and technical risks.

When does internal wall insulation make sense?

Internal wall insulation is the right choice when:

• External insulation would be refused planning permission. Listed buildings, conservation areas, and some heritage properties cannot change their external appearance. Internal insulation doesn't need planning permission (unless you're listed, in which case you need listed building consent).⁴
• You want to keep the original brickwork visible. Some period homes have attractive brickwork or stone that would be covered by external insulation.
• You can't afford external insulation (£8,000 to £15,000) but can afford internal (£5,000 to £10,000).
• You want to do it room by room. Internal insulation can be done incrementally (one room per year, spreading the cost). External insulation has to be done all at once.

Internal wall insulation does not make sense if:

• You can get planning permission for external insulation (external is easier, lower condensation risk, no room size loss).
• Your rooms are already small (losing 100mm to 150mm would make them unusable).
• You have solid walls but they're already insulated (check your EPC).

How much does internal wall insulation cost in 2026?

• Semi-detached house: £5,000 to £10,000 for the whole house (all external walls on ground and first floor). Per room: £800 to £1,500 depending on wall area.
• Detached house: £7,000 to £14,000 (more external wall area).
• Terraced house: £3,000 to £6,000 (only front and rear walls are external).

The cost includes:

• 60mm to 100mm rigid insulation boards (PIR, phenolic, or wood fibre).
• Vapor control layer (airtight membrane to prevent condensation).
• 12.5mm plasterboard finish.
• Skim coat and paint.
• Moving sockets, switches, and radiators out from the wall.
• Cutting back skirting and architraves, then refitting or replacing them.

The work takes 1 to 2 weeks per floor (ground floor, then first floor). You have to empty the rooms, so it's disruptive. Some homeowners do one room per year to spread the cost and disruption.

How much room size do you lose?

Internal wall insulation adds 100mm to 150mm to each external wall. The exact thickness depends on:

• Insulation thickness: 60mm to 100mm.
• Air gap (if needed): 25mm to 50mm if your wall is at risk of damp.
• Plasterboard and skim: 15mm.

So a typical build-up is:

• 100mm insulation board + 12.5mm plasterboard = 112.5mm total.
• Or 60mm insulation + 25mm air gap + 12.5mm plasterboard = 97.5mm total.

For a 4m x 4m room with two external walls, you lose:

• 100mm on each external wall.
• New internal dimensions: 3.9m x 3.9m.
• Floor area lost: 0.64m² (about 4% of the room).

This is most noticeable in:

• Small rooms (box rooms, bathrooms).
• Alcoves next to chimneys (can become too narrow for furniture).
• Around windows (window reveals become very deep, which can block light).

Ceiling height is not affected.

Can I get a grant?

Yes. Internal wall insulation is covered by the same grants as external wall insulation:

• ECO4: Covers solid wall insulation (internal or external) if you receive certain benefits or your home has an EPC rating of D, E, F, or G. The grant can cover up to £10,000. If your quote is higher, you may need to contribute.³
• Home Upgrade Grant (HUG): For low-income households with off-gas heating. Can cover up to £10,000 for internal wall insulation. Apply through your local authority.⁵
• Great British Insulation Scheme (GBIS): Similar to ECO4. Check with an installer.⁶

To apply, contact an ECO4-registered installer. They check your eligibility and handle the paperwork. The gov.uk Simple Energy Advice service lists approved installers.⁷

How much will I save?

The Energy Saving Trust estimates solid wall insulation (internal or external) saves:

• £260 per year for a semi-detached house
• £370 per year for a detached house
• £160 per year for a mid-terrace house

These figures assume gas central heating and 2026 energy prices (6.24p/kWh for gas).^2,8

Your actual savings depend on how you heat the house. If you heat the whole house to 21°C all day, savings are higher. If you only heat occupied rooms, savings are lower.

What are the risks with internal wall insulation?

Internal wall insulation is technically harder than external insulation because you're making the wall colder on the outside (behind the insulation). This increases the risk of condensation and mould if not done correctly.¹

Condensation risk

When you insulate internally, the original brick wall stays cold (it's now on the outside of the insulation). Warm moist air from the room can pass through the insulation and hit the cold brick surface, where it condenses into water. This water can:

• Cause mould on the back of the insulation.
• Rot timber joist ends embedded in the wall.
• Cause frost damage to the brickwork (if the water freezes).

To prevent this, you need a vapor control layer (airtight membrane) on the warm side of the insulation, plus ventilation in the room (trickle vents in windows, extractor fans in kitchens and bathrooms).¹

Thermal bridging

Floor joists and ceiling joists often sit in pockets in the external wall. If you don't insulate around the joist ends, you get cold spots (thermal bridges) where heat leaks out. This is tricky to fix without major carpentry work, so many internal insulation installs accept some thermal bridging.

Existing damp

If your wall has existing damp (rising damp, penetrating damp, condensation), internal insulation will make it worse. You must fix the damp problem before insulating. Signs of damp:

• Water stains on the wall.
• Peeling paint or wallpaper.
• Mould or musty smell.
• Crumbling plaster.

A proper survey before install checks for damp using a moisture meter. Any reputable installer will walk away if the wall is damp.

What materials should I use?

The most common materials for internal wall insulation are:

• PIR boards (60mm to 100mm): Most common. Good thermal performance, easy to cut and fit. Rigid, so can be fixed direct to the wall with adhesive or battens. Cost: £30 to £50 per m² installed.
• Phenolic foam boards: Slightly better thermal performance than PIR, so you can use thinner boards (50mm to 80mm). More expensive. Cost: £40 to £60 per m² installed.
• Wood fibre boards: Breathable, natural, lower embodied carbon. Thicker (100mm to 140mm for the same U-value). Better for heritage buildings because they let moisture escape. Cost: £50 to £70 per m² installed.⁴
• Insulated plasterboard (thermal laminate): Plasterboard with insulation bonded to the back (40mm to 60mm total thickness). Quick to install, but thinner insulation so thermal performance is lower. Cost: £25 to £40 per m² installed.

For heritage buildings and conservation areas, wood fibre is often preferred because it's breathable and reversible (you can remove it in future without damaging the original wall).⁴

Can I do it room by room?

Yes. Unlike external insulation (which has to be done all at once), internal insulation can be done incrementally. This has two advantages:

• Spread the cost: Do one room per year (£800 to £1,500 per room).
• Lower disruption: You only have to empty one room at a time.

The disadvantage is that you get thermal bridging where the insulated wall meets an uninsulated wall. This creates a cold spot. To minimize this, start with the rooms you heat most (living room, bedrooms) and leave unheated rooms (utility, garage) until last.

Internal vs external: which should I choose?

Choose external wall insulation if:

• You can get planning permission (not listed, not in a conservation area).
• You can afford it (£8,000 to £15,000).
• You want the lowest condensation risk and best thermal performance.
• You don't want to lose room size.

Choose internal wall insulation if:

• External would be refused planning permission (listed, conservation area).
• You want to keep the original brickwork visible.
• You can't afford external and need to do it room by room.
• You can accept the room size loss and the higher condensation risk.

Summary

Internal wall insulation is worth it if you have solid walls, external insulation isn't possible, and you can accept losing 100mm to 150mm of room width. It costs £5,000 to £10,000 for a semi-detached house and saves £160 to £260 per year. Payback is 20 to 40 years without a grant, 8 to 15 years with ECO4 funding.

Condensation risk is higher than external insulation, so you need a proper vapor control layer and good ventilation. Fix any existing damp before insulating. If external insulation is possible, choose that instead.