Solid Wall Insulation: Internal vs External Explained

The short answer

Internal wall insulation is generally cheaper per square metre and doesn't change the outside appearance of the property, but reduces internal room space and is disruptive to live through room-by-room. External wall insulation preserves internal space and can refresh a tired exterior, but costs more overall, changes the building's external appearance (which can matter for planning in a conservation area), and needs enough space around the property to add the extra external thickness.

Why solid walls need a different approach

Homes built before roughly the 1920s typically have solid walls — a single thickness of brick or stone with no cavity to fill, unlike the cavity walls in most newer construction. Since there's no gap to inject insulation into, the wall has to be insulated by adding a layer either to the inside face or the outside face of the existing wall.

Internal wall insulation

This involves fixing insulation boards (or a stud framework filled with insulation) to the inside face of external walls, then finishing with plasterboard and decoration as normal. It's typically done room by room, which means living through disruption in whichever room is being worked on, but allows a phased approach to cost and disruption rather than needing the whole house done at once.

Trade-offs: it reduces room size slightly (the insulation layer typically adds 50-100mm+ to wall thickness, depending on the system used), can complicate details like skirting boards, window reveals, electrical sockets and radiators (all need careful detailing to avoid cold bridging and to look tidy), and doesn't change the building's external appearance — a genuine advantage for a listed building or one in a conservation area where external changes would need permission.

External wall insulation

This involves fixing insulation boards to the outside of the existing walls, then applying a protective and decorative render (or a cladding system) over the top. It doesn't reduce any internal space, can be done without disrupting life inside the home room by room, and gives the opportunity to refresh a tired-looking exterior at the same time.

Trade-offs: it's generally more expensive than internal insulation for an equivalent wall area, changes the building's external appearance (render texture and colour, and the loss of original brick/stone detailing underneath, which matters for a period property's character), needs enough clearance around the property (boundary proximity, window/door reveals, roof overhang) to accommodate the extra external thickness, and — in a conservation area or on a listed building — is more likely to need planning permission, since it changes the visible street-facing appearance.

Cost comparison

Internal wall insulation is typically cheaper per square metre, but both options' total project cost depends heavily on the property's total wall area and the specific system used. External wall insulation for a full semi-detached or terraced house commonly runs from roughly £8,000 upwards, sometimes considerably more depending on size, wall condition and finish quality; internal insulation costs vary more by how many rooms are done and to what standard of finish, but per-square-metre is usually the less expensive route if disruption and lost space aren't a concern.

Which suits which property?

• Listed buildings and conservation areas: internal insulation is often the only realistic option, since external appearance typically can't be changed without specific permission, and even then may not be granted.
• Properties with limited external clearance (close to a boundary, narrow passageways): internal insulation avoids the space problem external insulation would create.
• Properties with a tired, worn exterior that needs attention anyway: external insulation can combine a genuine efficiency upgrade with a full exterior refresh in one project.
• Occupied properties where minimising disruption matters: external insulation avoids disturbing life inside the home room by room.

Common mistakes

• Not checking planning requirements before committing to external insulation. Conservation areas and listed buildings have real restrictions here — check with your local planning authority before assuming external insulation is straightforward.
• Underestimating the disruption of internal insulation. It's genuinely a room-by-room upheaval, involving moving furniture, redecorating, and living without full use of each room while it's done.
• Not detailing around windows, sockets and skirting properly. Poor detailing at these junctions is a common source of cold bridging and a less tidy finish — use an experienced, reputable installer.

Which is right for you?

If your property is listed, in a conservation area, or has limited external space, internal insulation is usually the realistic option despite the disruption. If you have a tired exterior needing attention anyway and reasonable external clearance, external insulation can be excellent value combined with that refresh. Either way, this tends to be a longer-payback, higher-cost measure than loft or cavity wall insulation — see our full comparison of insulation costs and payback times for how it fits into a wider priority order, and check whether your home might actually have cavity walls rather than solid walls before assuming this is the right route at all.