Insulating solid walls from the inside: drylining, condensation and Part L

Updated 6 November 2026 · By the Drylining team

Short answer: Solid walls are insulated from the inside with insulated plasterboard or a stud frame filled with insulation and a vapour control layer. In England, renovating 25% or more of an external wall normally brings it under building regulations, with a target U-value of 0.30 W/m²K for internal insulation of solid walls where practical. Damp must be fixed first and window reveals and junctions detailed to avoid condensation.

Insulating solid walls from the inside: drylining, condensation and Part L

Why solid walls feel cold

Most UK houses built before the 1920s and 1930s have solid brick or stone walls with no cavity. An uninsulated solid brick wall loses heat far faster than a modern insulated wall, and the inside surface stays cold in winter, which makes rooms feel chilly even with the heating on and can lead to condensation and mould in corners. Internal insulation is one of the most effective upgrades for these homes, though it does reduce room size.

Before choosing a system it helps to know how the wall is built. A 225 mm solid brick wall, a stone wall with rubble fill and a 1950s solid block wall all behave differently. Older walls built with lime mortar and lime plaster were designed to absorb and release moisture, and sealing them in with impermeable materials can cause problems if moisture has nowhere to go. A quick look at the wall thickness, the bonding pattern of the brickwork and the existing plaster usually tells an experienced installer what they are dealing with.

Internal insulation options

Option	Typical thickness	Notes
Foam-backed insulated plasterboard, bonded	About 50 to 90 mm	Quick, thin for its performance, includes a vapour control layer in many products
Insulated plasterboard, mechanically fixed to battens	About 70 to 110 mm	Used where the wall is uneven or bonding is not suitable
Metal or timber stud frame with insulation and board	About 75 to 150 mm	Room for services and thicker insulation, separate vapour control layer
Vapour-open systems such as wood fibre with lime plaster	About 40 to 100 mm	Often preferred on older breathable walls and in historic buildings

Avoiding condensation

Insulating the inside makes the original wall colder. If warm, moist indoor air can reach that cold surface it condenses, behind the lining where you cannot see it. To stop this, foam-backed boards and stud linings normally include a vapour control layer on the warm side, with all joints and edges sealed. The guidance in BS 5250 covers how to manage moisture in buildings, and larger projects benefit from a condensation risk assessment.

Fix any rising damp, penetrating damp, leaking gutters or failed pointing before lining
Do not bond boards to damp or salt-contaminated walls
Seal around sockets, switches and pipes that pass through the lining
Keep the bathroom and kitchen extract fans working

Cold bridges at windows, floors and party walls

Where the insulation stops, heat finds a way out. Window and door reveals, the junction with intermediate floors and the ends of internal walls are the usual weak points. Where space allows, a thin insulated board is returned into window reveals. Leaving reveals uninsulated can concentrate condensation around the window, which is a common cause of mould after an internal insulation job.

Building regulations in England

Under Part L, renovating a thermal element, which includes adding or replacing a layer such as a new lining, normally brings it within building regulations when 25% or more of an external wall is renovated. Approved Document L gives a target U-value of 0.30 W/m²K for solid walls insulated internally, unless that is not technically or functionally feasible, for example where it would reduce a room too much, in which case the best reasonable standard is expected. This kind of work is usually notified to building control. Wales, Scotland and Northern Ireland have their own rules, so check locally.

Listed buildings need listed building consent for most internal alterations, and conservation officers often prefer vapour-open systems on historic walls.

What you will need to move

Radiators, which come off and go back on extended brackets or through fixings into the wall
Sockets and switches, extended to the new surface with deeper boxes
Skirting, architraves and window boards
Kitchen units and wall cupboards on that wall
Coving and picture rails

These knock-on costs often make up a significant part of the total, so include them when comparing quotes.

Is internal insulation worth it?

For a cold room with solid walls, internal insulation usually makes a noticeable difference to comfort straight away, because the wall surface is no longer cold and rooms warm up faster. It can be done room by room, which makes it easier to fit around family life and budgets than external insulation, and it does not change the outside appearance of the house, which matters in conservation areas and on attractive brick or stone frontages.

	Internal insulation	External insulation
Changes outside appearance	No	Yes
Can be done room by room	Yes	Usually whole elevations
Reduces room size	Yes	No
Disruption inside	Significant in the rooms treated	Limited
Planning permission	Not usually needed	Often needed, especially in conservation areas

The trade-offs are lost floor space and the need to deal with radiators, sockets and joinery on every wall that is lined. On narrow rooms, a thinner high-performance board on the worst wall can be a sensible compromise.