Solid brick or stone walls built without a cavity have no straightforward insulation route, unlike a modern cavity wall, so the choice between internal and external systems is a real technical decision rather than a simple upgrade.
Both routes can achieve a similar thermal improvement. What differs is the impact on the building, the risk profile if the detailing is wrong, and what planning constraints apply.
Internal wall insulation: the case for it
Internal insulation — rigid board or a insulated plasterboard system fixed to the inside face of the wall — is generally cheaper than external systems and leaves the outside of the building completely unaltered, which matters on a listed building or in a conservation area where external changes face scrutiny.
It can also be done room by room, which suits a phased renovation where the whole house isn't being tackled at once.
The risk with internal insulation
Moving the insulation layer to the inside of the wall pushes the dew point — the point where moisture in the wall condenses — closer to the original masonry, and if the vapour control layer and detailing aren't specified correctly, interstitial condensation can build up inside the wall unseen, leading to timber decay at joist ends or hidden dampness.
This is a genuine risk, not a theoretical one, and it's the main reason internal wall insulation on solid walls needs a specification that accounts for the whole wall build-up, not just a board fixed up without a vapour strategy.
External wall insulation: the case for it
External insulation wraps the whole building in a continuous layer of insulation, finished in render or cladding, which avoids the internal condensation risk because the original wall stays warm and dry on its inner face and the dew point moves outward into the new insulation layer.
It also preserves internal room dimensions entirely and improves thermal mass performance, since the masonry itself becomes part of the warm side of the building.
The trade-offs with external insulation
It changes the building's appearance completely, covering brick or stone detailing, window reveals and existing render, which is often unacceptable on a listed building, in a conservation area, or simply aesthetically to an owner who values the original elevation.
It's also generally more expensive to install across a whole building than doing individual rooms internally, and access, scaffolding and window reveal detailing add to both cost and time on site.
Common questions
Can I insulate just one wall internally and leave the rest?+
Yes, room-by-room internal insulation is common and doesn't require doing the whole house at once, though the vapour control detailing at each junction with an uninsulated wall needs care to avoid creating a cold bridge or condensation point.
Will external insulation work on a listed building?+
Rarely without objection, because it alters the historic external appearance that listing exists to protect. Internal insulation, applied carefully with the right vapour strategy, is the far more common route on listed and conservation-area properties.
Does either type of insulation stop damp that's already present?+
No — existing damp from a specific source, such as a failed damp-proof course or blocked ventilation, needs diagnosing and fixing on its own terms before insulation is added. Insulating over an active damp problem generally makes it worse, not better.
Get a wall insulation assessment
We'll survey your wall construction and any planning constraints before recommending internal or external insulation, with the moisture strategy specified in writing.