£4,000 to £8,000. Cheaper than external wall insulation, but there's a trade-off most people don't think about until the boards go up: you lose space. Every insulated wall moves inward by 75mm to 100mm. In a small Victorian terrace, that can turn a comfortable room into a tight one.
Is it worth it? For most solid-walled homes, yes. The savings are real, the grants can cover the full cost, and the comfort improvement is dramatic. But you need to go in with your eyes open about what changes.
What Is Internal Wall Insulation and Who Needs It?
Internal wall insulation (sometimes called dry lining) means adding insulation to the inside face of your external walls. Rigid insulation boards are fixed to the wall, then plasterboarded over and skimmed to give a smooth finish ready for decorating.
Check if you qualify
Answer a few quick questions to see which government energy grants you're eligible for. Free, instant results.
You need it if your house has solid walls (no cavity) and you can't or don't want to insulate externally. That covers three main groups:
Pre-1930s houses with solid brick or stone walls and no cavity to fill
Listed buildings where external insulation would alter the protected appearance
Properties in conservation areas where planning restrictions prevent external cladding
If your house has cavity walls, you don't need this. Cavity wall insulation is cheaper, faster and doesn't affect room sizes. Check your EPC certificate or measure the wall thickness at a window opening. Solid walls are typically around 220mm thick (one brick length). Cavity walls are 260mm to 270mm.
Internal is roughly half the price. But it delivers less thermal improvement (15 to 30 SAP points versus 20 to 40 for external) and it comes with the space penalty. On a cost-per-SAP-point basis, internal wall insulation delivers roughly 1.0 to 1.5 SAP points per £1,000 spent, compared to 0.8 to 1.2 for external. Internal is better value per pound, but external gives you more total improvement.
Our view: if grants cover the cost, go external. If you're paying out of pocket, internal is the pragmatic choice. If your building is listed, internal is likely your only option anyway.
How Much Space Do You Actually Lose?
This is the question that decides it for many people, so let's put real numbers on it.
A typical internal wall insulation system adds 75mm to 100mm to each insulated wall. That's the insulation board (40mm to 60mm of rigid PIR foam or mineral wool) plus the plasterboard and skim coat (12.5mm plasterboard plus 3mm skim).
Here's what that means in practice for common room sizes:
Room
Original Size
After 2 Walls Insulated
Space Lost
Living room
4.5m x 3.5m (15.75 sq m)
4.3m x 3.4m (14.62 sq m)
1.13 sq m (7%)
Bedroom
3.5m x 3.0m (10.5 sq m)
3.3m x 2.9m (9.57 sq m)
0.93 sq m (9%)
Small bedroom
2.8m x 2.5m (7.0 sq m)
2.6m x 2.4m (6.24 sq m)
0.76 sq m (11%)
In a decent-sized living room, 7% is noticeable but liveable. In a small bedroom that's already tight, 11% can mean the difference between fitting a double bed with bedside tables and not. Measure your rooms before committing.
You only insulate external walls, not internal partition walls. A mid-terrace house might only have two external walls (front and back), so only those rooms lose space. A detached house has four external walls, so every room is affected.
One thing to factor in: window sills, skirting boards, light switches, plug sockets and radiators all need repositioning or extending. Radiator pipes need rerouting. This is where the labour cost and disruption add up. It's not just sticking boards to a wall.
Grants for Internal Wall Insulation
ECO4 covers internal wall insulation as an eligible measure. If you're on qualifying benefits (Universal Credit, Pension Credit, Child Tax Credit, Income Support, JSA or ESA) and your home has an EPC rating of D or below, your energy supplier could fund the full installation, per GOV.UK. Internal wall insulation delivers a solid SAP improvement (15 to 30 points), which counts towards your supplier's ECO4 targets.
ECO4 runs until December 2026.
Warm Homes: Local Grant may cover internal wall insulation for households with income under £36,000 and an EPC of D or below, per GOV.UK. Council priorities vary. Some focus on solid wall properties because they're the hardest to treat and have the worst energy ratings.
GBIS can also fund internal wall insulation for homes in council tax bands A to D.
For listed buildings, grant funding is particularly valuable because the alternative (doing nothing) leaves you with a cold, expensive-to-heat house. Heritage-specific grants from Historic England or local authority heritage funds may also be available, though they're competitive and limited.
Internal wall insulation is done room by room. Each room takes three to five days, depending on complexity. Here's what to expect.
Preparation: The room is cleared. Furniture moved out or to the centre under dust sheets. Skirting boards removed. Radiators taken off the wall (pipes capped temporarily). Light switches and plug sockets disconnected by an electrician.
Insulation: Rigid insulation boards (PIR foam like Kingspan or Celotex, or mineral wool boards) are cut to size and fixed to the wall with adhesive and mechanical fixings. Gaps around windows, doors and junctions are sealed with expanding foam or tape to prevent cold bridges.
Plasterboarding: Plasterboard is fixed over the insulation, either directly bonded or on a separate framework. Joints are taped and filled.
Finishing: The plasterboard is skimmed with a thin coat of plaster to give a smooth surface. Skirting boards are refitted (often new ones, because the wall line has moved). Radiators are remounted on longer brackets. Electrics are reconnected with extended back boxes for the sockets and switches.
Decorating: You'll need to repaint or re-wallpaper the insulated walls. The plaster skim needs to dry fully first, usually a week or two depending on ventilation.
The disruption is real. You're effectively redecorating every room that gets insulated. If you're doing the whole house, expect to live with the work for two to four weeks, moving from room to room. Some people do one or two rooms at a time, spreading the work and cost over months.
Moisture and Condensation
This is the technical risk with internal wall insulation, and it needs honest treatment.
When you insulate the inside of a wall, the wall itself gets colder. The insulation keeps the heat inside the room, which is the point, but the wall behind the insulation drops in temperature. If warm, moist air from inside the room reaches that cold wall surface, condensation can form between the insulation and the wall. Over time, that leads to damp and mould.
The fix is a vapour control layer (VCL), a membrane fitted on the warm side of the insulation that prevents moisture from reaching the cold wall. Most rigid PIR boards have a foil facing that acts as a VCL. Mineral wool systems need a separate membrane.
In older solid-walled buildings, particularly stone or lime-mortared brick, breathability matters. These walls were designed to absorb and release moisture. Sealing them with impermeable insulation can trap moisture in the wall structure. For these properties, breathable insulation systems (wood fibre boards, lime-based insulated plaster) are often recommended by conservation specialists, though they're more expensive.
A good installer will assess your wall construction, check for existing damp, and recommend the right system. If they don't mention moisture management, find someone who does.
Open the eligibility checker. Two minutes. You'll see which grants cover internal wall insulation for your home and whether you could get it fully funded.
For most rooms, yes. You're losing 75mm to 100mm per external wall, which works out to about 7% of floor area in a typical living room. The comfort improvement is significant, your heating bills drop by around £465 a year, and the walls feel warm to the touch instead of cold. In very small rooms (under 7 square metres), measure carefully before committing. The space loss is more noticeable there.
Can I do internal wall insulation myself?
Technically possible but not recommended for most people. The insulation boards themselves are easy enough to cut and fix, but the detailing around windows, doors, sockets and radiators is where problems occur. Poor detailing creates cold bridges and condensation risk. If you're a competent DIYer with plastering skills, you could save on labour, but the vapour control layer and junction details need to be right. Getting it wrong causes damp.
Does internal wall insulation work on stone walls?
Yes, but the system needs to be breathable. Stone and lime-mortared walls manage moisture by absorbing and releasing it. Impermeable insulation (standard PIR boards with foil facing) can trap moisture in the stone, causing damage over time. Wood fibre boards or lime-based insulated render are better choices for stone walls. They cost more but they work with the wall rather than against it.
How long does internal wall insulation last?
The insulation material lasts indefinitely. PIR foam boards and mineral wool don't degrade. The plasterboard and skim finish last as long as any normal plastered wall. The only maintenance consideration is the vapour control layer, which should remain intact for the life of the installation provided nobody punctures it with fixings. If you're hanging heavy items on insulated walls, use appropriate fixings that don't compromise the VCL.
Will internal wall insulation stop condensation on my walls?
It should reduce condensation on the wall surface because the plasterboard face will be warmer. But if the vapour control layer is poorly installed or missing, condensation can form behind the insulation where you can't see it. This is worse than surface condensation because it leads to hidden damp and mould. Proper installation with a continuous VCL is essential. Adequate ventilation in the room (trickle vents, extractor fans) also matters.