Insulating an Existing Wall: Inside, Outside or Cavity

Lining the room face adds a thermal layer in front of the existing construction, so heat that used to warm that construction no longer does. The record is direct about the consequence: the wall behind the lining runs colder and, in most climates, stays damp for longer after rain, because moisture arriving there has less energy available to drive it back out.

Wrapping the weather face does the opposite. The wall runs warmer and takes far less rain, and its drying is then shared between an inward route into the room and an outward route through the insulation and the render, with how much the outward route carries set by the vapour openness of the boards, the base coat and the finish.

Filling a cavity changes neither face. What it changes is the void: the space that was breaking the path water would otherwise follow across the wall is now occupied, and resistance to water crossing passes from an open space to the fill's own behaviour.

Internal lining depends on whether the wall can still dry outward once the inward route is removed. The record names the inputs: the driving rain exposure of the wall, whether anything applied to its outer face would slow outward drying, the state of the pointing, the presence or absence of a barrier at its base, and any timber built into the masonry.

External insulation depends on whether the wall is dry and sound enough to be enclosed, and on where the system can start near the ground relative to splash, paving and any damp-proof provision in the wall. The record calls the base the place where every weakness meets, because water entering behind the boards is beyond the reach of any surface repair.

Cavity fill depends almost entirely on the wall rather than on the material: how much water the outer leaf passes under driving rain, whether the ties are sound and sitting the right way, whether the void is clear of mortar droppings and debris, and whether trays, closers and vents were built as drawn.

An internal lining has to stop somewhere. Where it meets a floor, a ceiling, a partition built into the external wall, a party wall or a window reveal, the insulated line is interrupted by construction that runs on through to the outside, and those interruptions become the coldest interior surfaces in the room. Insulation carried along them lengthens the path but never removes it, and each return consumes reveal depth or room dimension that may not be available.

External insulation moves the problem outward rather than removing it. The thermal line is broken at party boundaries, at attached structures, at thresholds and wherever a neighbour's wall begins, and reveals are the hardest place to maintain it. Those stopping points are decided by ownership and geometry as much as by physics.

Cavity fill leaves reveals, lintels and the wall base uninsulated, so the field is improved while the junctions are unchanged. None of the three routes is a whole-envelope answer on its own.

An insulated lining is indistinguishable from an ordinary wall once decorated, which is the problem. Later occupants drill, fix and chase into it without knowing what is behind, and a puncture of the control layer leaves no visible sign. The record treats a drawing of what was installed, and of where fixings may safely be made, as part of the assembly rather than as paperwork about it.

External insulation adds depth to the wall, so every pipe, bracket, meter box and light currently fixed to it has to be re-fixed through that depth onto something that can carry it. Cavity fill removes the void as a route for later cabling, tie replacement and drainage remedial work, which the record treats as part of the decision rather than a consequence discovered later.

Exposure varies with orientation, shelter and height. The condition of an outer leaf varies with its history of pointing and repair. What a void contains varies with who built it. None of that is a property of any insulation product, and none of it can be read from a product description, which is why the cavity fill record describes the survey rather than the material as the system.

The same logic applies to the other two routes. A survey based on a few openings in one elevation is not evidence about the whole building, and one elevation may be excluded from work that is entirely reasonable elsewhere on the same house.

Both the internal lining and the cavity fill records make the same point about later damp: its appearance does not by itself prove the work failed. It may equally reveal a rainwater or ground moisture problem that the wall previously masked by drying inward, or a defect that an open void was draining around.

That is why the records treat correcting rainwater defects, ground levels and gutter faults as work that belongs before insulation rather than after it, and why a baseline record of the wall's condition before it is covered is what makes any later diagnosis possible.