Internal Wall Insulation: The Complete Guide for UK Homeowners

What is internal wall insulation?

Internal wall insulation, or IWI, is an insulating layer fitted to the inside face of your external walls — typically as boards bonded to the wall, sometimes within a battened framework. Unlike external wall insulation, IWI doesn't touch the outside of the building. Nothing visible changes from the street.

A typical IWI build-up runs (from cold side to warm side): your existing solid masonry wall, an air-and-vapour control layer (AVCL), an insulation board with integrated vapour barrier, and a plaster skim or plasterboard finish. Total thickness usually 65-95mm.

The thermal effect is similar to EWI on paper. A 9-inch solid brick wall at U=2.1 W/m²K drops to about 0.30 W/m²K with 65-85mm of IWI. The rooms feel measurably warmer; condensation on cold walls disappears; heating bills drop in the same 25-40% range.

The difference, and the reason IWI is harder than EWI, is that you're now insulating from the warm side. The cold side of the wall stays cold all winter. Any warm, humid internal air that finds its way past the vapour control layer condenses against that cold mass and gets trapped between the insulation and the masonry. Every IWI failure we've ever inspected has been a moisture management failure.

When IWI is the right call

Choose IWI when:

• You can't change the external appearance — conservation areas, listed buildings, terraces with neighbours' opinions, or properties in flat blocks where you don't control the elevation.
• You want to insulate selectively — only the rooms most affected by cold, or one wall at a time across years.
• You're renovating internally anyway and the disruption is already happening.
• The exterior is in good condition and you don't want to disturb it.

IWI is the wrong call when:

• The wall is actively damp. Penetrating damp through pointing or rising damp from below — fix these first, then insulate.
• You can't accept losing room space. 65-95mm off every external wall sounds small but it adds up — a 3m wall loses about 20cm off room width.
• Your room has elaborate cornicing, mouldings or panelling you want to keep. They have to come off and very rarely go back the same.
• You want a quick job. IWI involves stripping rooms back to bare brick, decanting furniture, and full redecoration. It's a serious renovation project.

IWI systems and materials

The four routes installers actually use, in order of how often we recommend them:

SWIP integrated AVCL system

SWIP is what we install on most British solid-wall homes. The boards have an integrated foil-faced air and vapour control layer, eliminating the most common failure mode (gappy site-applied AVCLs). They go up fast, achieve full BBA-certified performance, and the supplier provides the entire system from adhesive to finishing tape — no mixing components.

PIR insulated plasterboard

The cost-effective option — Kingspan K17 and similar laminated boards. Thinner build-up, lower cost per m². The trade-off: slightly trickier vapour detail at junctions, and less forgiving of imperfect substrates.

Wood fibre with lime plaster

The breathable option for older buildings where managing moisture matters more than chasing the lowest possible U-value. STEICO Flex or Pavatex boards friction-fit into a battened frame, finished with lime plaster. The whole build-up stays vapour-open so any moisture that does get in can pass back out. Essential for pre-1850 and listed homes.

Mineral wool batten system

A timber or metal batten frame is fixed to the wall, mineral wool friction-fits between battens, plasterboard goes on the warm face. Used where the wall is too uneven for direct adhesion, or where services need running between studs.

How much IWI costs in 2026

For private (non-grant) jobs in London and the South East:

• £60-£90 per m² for PIR insulated plasterboard, single-room work.
• £75-£110 per m² for SWIP or equivalent BBA-certified integrated systems.
• £110-£160 per m² for wood fibre with lime plaster on listed buildings.

For a 3-bed mid-terrace where you're insulating the front, rear and gable elevations (around 60-80m² internally), expect £5,500-£10,000 all-in including making good and redecoration.

Big cost variables:

• Decant scope. Empty rooms cost less than rooms we have to work around. If you can move out for the duration, the price drops by 15-20%.
• Services. Sockets, light switches, radiators and pipes all need extending forward of the new wall surface. On older homes with lots of surface-mounted cabling, this adds days.
• Cornice and skirting reinstatement. Original cornice rarely re-fits cleanly. Budget for replacement plaster cornice if the original is plaster-of-paris.

The mistakes that wreck IWI installs

We see the same handful of failures over and over:

1. Discontinuous vapour control

Gaps in the AVCL — at sockets, junctions, behind boards, around pipes — let warm humid air through into the cold cavity. Within 2-3 winters you get black mould forming on the cold side of the insulation. Not visible from inside, deeply unhealthy, expensive to remedy.

2. Failing to insulate around openings

Window reveals are thermal weak points. If they're not insulated with at least 25-30mm of board, they'll continue to run cold and condensation will form around the frame. We always insulate reveals as part of the system, not as an afterthought.

3. Boarding straight onto a damp wall

Sometimes the wall has been damp for 100 years and nobody minded because the room was never warm. Insulate the cold mass with closed-cell board on a wall holding moisture and you've sealed the moisture in. We always damp-test walls before specification — if the readings are wrong, we treat first or specify a breathable system.

4. Skirting and floor junctions

The bottom edge of an IWI wall meeting the floor is a classic cold-bridge if the floor void isn't insulated too. Cold air from a suspended timber floor finds its way up behind the new wall and undoes the insulation. Floor insulation should be specified with IWI on suspended-floor properties.

5. Forgetting ventilation

IWI dramatically reduces uncontrolled air leakage. That's good for energy bills but means the home becomes more airtight overall. Without working extract fans in kitchens and bathrooms — and ideally a whole-house ventilation strategy — humidity will build up. Modern IWI specifications always include a ventilation review.

What actually happens — the install process

1. Free survey. Damp readings on every wall, photograph services, agree the room sequence, and discuss decanting.
2. Quote. Itemised: strip-out, system supply, install, services adaptation, decoration, and rubbish removal.
3. Strip-out. Skirting off, radiators off, sockets and switches face-fixed to extension blocks, plaster taken back to brick where required.
4. Substrate prep. Brick re-pointed where loose, surfaces cleaned, any high spots ground down. Dry to 18% MC before insulation goes on.
5. Insulation install. AVCL applied (where separate from board), boards bonded with foam adhesive, mechanical fixings on larger panels, joints taped.
6. Reveals and details. Window reveals, ceiling perimeter, and floor junction all detailed with thinner insulation and continuous AVCL.
7. Plasterboard or skim coat. Finish layer fixed, joints taped and skim-plastered to a paint-ready finish.
8. Reinstatement. Skirting, sockets, switches, radiator pipes extended and refit.
9. Decoration. Mist coat plus two top coats. Most clients use this as an excuse to repaint the room properly.

A typical 3-bed terrace IWI runs 2-3 weeks, working room by room. Most people decant their main bedroom for one week and live around the work in the lounge.

Grants and finance for IWI

IWI is fundable under the same schemes as EWI:

• ECO4 — full or partial funding for income-tested or EPC-tested households.
• Great British Insulation Scheme — broader eligibility, often pays a fixed contribution.
• Local council schemes — especially in boroughs running their own decarbonisation programmes.

Because IWI work is invasive and costly, grant funding is the difference between "this will pay back in 8 years" and "this is essentially free." It's worth checking eligibility before committing.

FAQ

How much room space will I lose?

Around 65-95mm off every insulated wall. On a 4m × 4m room, expect to lose roughly 5-8% of floor area if you insulate all four external walls. Most rooms only have one or two external walls.

Can I install IWI myself?

Technically yes. Practically — please don't. The vapour detail is exacting and the consequences of getting it wrong are hidden until they're catastrophic. If you want grant funding or a manufacturer warranty, professional install is mandatory.

What if I already have damp?

Don't insulate over it. Damp must be diagnosed and fixed first — often a roof, gutter, ground level or pointing issue. We'll tell you straight if your wall isn't ready.

Will IWI affect my home insurance?

No effect on premiums in most cases, but always notify your insurer in writing.

Can I keep my fireplace?

Yes — chimney breasts can be insulated around or left exposed depending on use. Live, in-use fireplaces need clearances respected for safety.

Thinking about IWI?We specialise in solid-wall properties across London and the South East. Book a free survey and we'll damp-test the walls, talk through the room sequence, and quote in writing.