Can you insulate a solid wall internally? (UK guide) | External Wall Insulation Answers

The short answer

Yes, you can insulate a solid wall internally, and it is one of the two standard ways to treat solid walls in the UK (the other being external insulation). Internal wall insulation (IWI) is fixed to the inside face of external walls using either insulated plasterboard bonded to the wall or a studwork system filled with insulation and boarded over. It works well and costs less than external insulation, but because it moves the cold side of the wall closer to the room, it must be detailed carefully — with the right vapour control, treatment of cold bridges at junctions, and adequate ventilation — or it can cause condensation behind the insulation. On older breathable walls it should usually use vapour-open, breathable materials rather than standard foil-faced boards.

Insulating a solid wall internally is entirely feasible, and very common in UK terraces and period homes, but it is a job where the hidden detailing matters more than the visible boards. Here is what makes it work.

Internal insulation at a glance

Feasible?Yes, widely done in the UK
Main systemsInsulated board or studwork build-up
Added thicknessTypically ~60–120mm per wall
Critical detailVapour control + ventilation
Older homesUse breathable, vapour-open systems

The two ways to do it

Insulated plasterboard (laminate board): a board of insulation bonded to plasterboard is fixed directly to the wall with adhesive dabs or mechanical fixings, then taped and skimmed. It is quick and takes up less depth, making it popular where space is tight. Studwork build-up: a timber or metal frame is fixed off the wall, the gap filled with insulation (rigid board or mineral wool), a vapour control layer added where the design calls for it, then plasterboard fixed over. This allows thicker insulation and a service void for cables, but takes more depth. For older, breathable walls a third route is common: wood-fibre insulation board finished with lime plaster, which insulates while letting the wall continue to dry.

Getting moisture detailing right

This is the part that decides whether IWI succeeds. By adding insulation inside, you leave the masonry colder and wetter, and you move the point where water vapour can condense (the dew point) closer to the room. To manage this safely an installer will: control how much warm, moist indoor air can reach the cold wall (using either a vapour control layer with sealed-tight detailing, or a breathable system that lets the wall dry); insulate carefully around window and door reveals and at floor and ceiling junctions to limit cold bridges; and make sure the home has adequate ventilation, often including extract fans in kitchens and bathrooms. Skip these and you risk condensation, damp and mould hidden behind the boards.

The most common mistake: fitting standard foil-faced boards to a pre-1919 breathable wall and sealing it tight. If the wall cannot dry inwards and the vapour control is imperfect, moisture gets trapped. Older walls usually need breathable wood-fibre and lime systems instead.

When internal insulation is the wrong choice

IWI is not always the right answer. It is generally a poor choice where the wall already has unresolved damp (rising, penetrating or from a failed gutter) — that must be fixed first. It is risky on walls heavily exposed to driving rain without addressing the external face, because the masonry stays wetter. It is also disruptive in rooms with extensive fitted joinery, ornate cornicing or limited space you cannot afford to lose. In those cases external insulation, or a hybrid approach, may suit the building better.

Situation	Internal insulation suitability
Attractive brick/stone frontage	Good — keeps exterior look
Conservation area frontage	Often the practical choice
Existing unresolved damp	Fix damp first
Severe driving-rain exposure	Needs care / external may suit better
Very small rooms	Weigh space loss carefully

Indicative guidance, not a substitute for a site assessment. Sources: Historic England; Energy Saving Trust.

Doing it properly under PAS 2035

UK best practice for retrofit insulation now follows PAS 2035, which requires a whole-house assessment before specifying a system — looking at the wall construction, how the building handles moisture, occupancy and ventilation, then designing the insulation to suit. Using a TrustMark-registered retrofit installer working to that standard is the surest way to get IWI that improves comfort without storing up damp. It is more thorough (and sometimes pricier) than a quick board-and-skim, but on a solid wall that diligence is exactly what prevents the failures internal insulation is sometimes blamed for.

What the work involves room by room

Practically, internal insulation is a building job, not just a fix-boards-to-the-wall task, so it helps to know what a room goes through. The room is usually cleared and the external walls stripped back — skirting, architraves, picture rails and sometimes coving removed, radiators taken off and electrical fittings disconnected. The wall is checked for damp and any defects cured first. The insulation system is then fixed and the surface finished, services are repositioned to sit proud of the new face, and finally the joinery is reinstated and the room redecorated. Window reveals become deeper, which changes how curtains and blinds fit, and door frames near treated walls may need adjusting.

Because of all this, most people cannot use a room while it is being done, and doing several rooms means living around the work or moving out briefly. Planning the sequence — worst-affected or coldest rooms first — and combining the insulation with redecoration you intended anyway makes the disruption count for more. The visible result is a wall that is barely different to look at but noticeably warmer to the touch, with the real performance and safety hidden in the vapour control and ventilation detailing behind it.

Choosing a system for your wall

The right internal system is not the same for every wall, and matching it to the building is what separates a lasting job from a risky one. On a modern or less moisture-sensitive solid wall, a high-performance insulated plasterboard with a correctly designed and sealed vapour control layer can work well and keeps the build-up thin. On a pre-1919 lime-built wall that needs to breathe, the safer route is usually a vapour-open wood-fibre and lime system that lets the masonry keep drying, even though it is thicker and needs a specialist finish. Putting a sealed board on a breathable wall is the classic mismatch that causes trapped moisture.

Thickness and target performance also feed into the choice. Where space is tight, a thinner high-performance board buys a reasonable U-value without losing too much room, but it leaves the wall surface cooler, so vapour and ventilation detailing matter more. Where the wall must breathe, the system is chosen for compatibility first and thickness second. This is exactly the kind of judgement a PAS 2035 assessment is designed to make — weighing the wall's age, moisture behaviour, available space and your target together — rather than defaulting to whatever board is lowest-cost or most familiar.

Getting the junctions and reveals right

The places internal insulation most often falls short are not the open wall but the edges — window and door reveals, the line where the wall meets the floor, the ceiling, and any internal partition walls. Insulation usually stops at these points, leaving a strip of uninsulated, colder masonry that acts as a cold bridge. Cold bridges waste heat and, worse, create cool surfaces where room moisture can condense, which is why mould so often appears in corners and around windows after a basic insulation job. Good practice carries a thinner band of insulation a short way around reveals and across junctions to soften these cold spots.

This detailing is fiddly and adds time, which is exactly why cheaper installations skip it, and why the same installations are the ones that later show damp at the edges. When you discuss a job, ask specifically how reveals and junctions will be treated, not just what goes on the main wall. Handling these properly is one of the clearest differences between an internal insulation job that simply looks finished and one that genuinely performs and stays dry over the long term.

Frequently asked questions

Will internal insulation cause damp?

It can if detailed badly — fitting the wrong system, ignoring cold bridges at junctions, or having poor ventilation can trap moisture and cause condensation behind the boards. Done correctly, with the right vapour strategy and adequate ventilation, internal insulation does not cause damp. Any existing damp must be fixed first.

How thick is internal solid wall insulation?

The insulation plus plasterboard usually adds around 60–120mm to each treated wall, depending on the material and the U-value targeted. Higher-performance systems are thicker; thinner systems insulate less, so it is a balance between performance and lost space.

Do I need to insulate every wall in the room?

Only the external (outside-facing) walls need insulating, since internal walls separate heated rooms. In a mid-terrace that often means just the front and back walls; in a detached house it can mean all four external walls.

Sources & further reading

Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.