How the two differ in kind
A cavity wall was not built with a gap by accident. The outer leaf is expected to get wet and to let some water through; the cavity is a drained, vented void that stops that water reaching the inner leaf, with trays and weeps to take it back out. Insulation placed in that void has to keep the water crossing while letting the wall stay dry, and whether it can depends on the cavity's width, its cleanliness, the state of the outer leaf and — above all — how much driving rain the wall receives.
That is why exposure assessment is the gate on cavity fill rather than a formality. In a sheltered location with a sound outer leaf and a clean cavity of adequate width, filling it is one of the cheapest and most effective improvements available: no change to the appearance, no loss of internal space, no disturbance inside, and it is done in a day through small holes. In a severely exposed location, or with a cracked render, poor pointing or a cavity bridged with mortar droppings, the same work produces damp patches on internal walls that were dry before.
External insulation takes a different route entirely. It wraps the outside of the wall in a continuous layer, which does two things nothing else does as well: it removes almost all of the THERMAL BRIDGING — at junctions, at floor edges, at the wall head — because the layer is unbroken, and it keeps the masonry on the warm side of the insulation, so the wall stays warm and dry and the building keeps its thermal mass. It works on solid walls, where there is no cavity to fill and the alternative is insulating inside.
The factors that actually differ
Show
| External wall insulation | Cavity wall insulation | |
|---|---|---|
| What wall it suits | Any — solid, cavity, stone, concrete. It is the option that does not care what is underneath. | Cavity walls only, and only where the cavity is wide enough, clean, unbridged and the outer leaf is sound. |
| Exposure to driving rain | Improves it. The wall is protected by a rendered or clad outer layer and stays drier than before. | The governing constraint. Filling a cavity in a severely exposed wall is a recognised cause of penetrating damp, and the assessment is not optional. |
| Thermal bridging | Almost eliminated. A continuous external layer passes over floor edges, party walls and junctions that every other method leaves cold. | Reduced but not removed. Wall ties, floor edges, reveals and the wall head remain bridges. |
| Where the dew point ends up | Outside the masonry, so the wall runs warm and dry. This is the thermally correct place for it. | Inside the cavity, which is why the insulation must be water-repellent and the cavity must drain. |
| Thermal mass | Kept inside the insulation, so the building's mass moderates temperature swings — valuable in summer. | Inner leaf kept inside; outer leaf now outside the insulation and lost to the interior. |
| Disruption | Substantial outside: scaffold, weeks of work, every detail reworked — eaves, sills, thresholds, downpipes, meter boxes. | Minimal. Drilled from outside, filled, holes made good, done in a day or so. |
| Appearance and consent | Changes it completely. Often needs planning permission, and generally impossible on a listed or protected elevation. | Invisible when the holes are made good. |
| Shape of the cost | High, and dominated by DETAILS rather than by area — reveals, eaves, base, junctions. A simple elevation is far cheaper per square metre than a busy one. | Low, and close to proportional to area. Among the cheapest measures per unit of heat saved where it is appropriate at all. |
Which one, and when
Choose external wall insulation when…
- The wall is solid, so there is no cavity to fill and the alternative is insulating inside.
- Exposure is severe, or the outer leaf is in poor condition, so cavity fill is inadvisable.
- Thermal bridging is a real problem — cold corners, mould at floor edges, a building with many junctions.
- The elevation is being renewed anyway: failed render, a re-clad, a wall that needs work whatever happens.
Choose cavity wall insulation when…
- The wall is a cavity wall in reasonable condition and a competent exposure assessment supports filling it.
- Appearance cannot change: a conservation area, a listed building, a terrace where one house cannot be wrapped alone.
- The budget is limited and the objective is the most heat saved per unit spent.
- Disruption has to be minimal — an occupied house where weeks of scaffolding is not acceptable.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- How do I know whether my cavity should be filled?
- By assessment rather than by preference, and the assessment has several parts. Exposure to WIND-DRIVEN RAIN is the first, and it comes from published maps rather than from how sheltered the house feels — a coastal or upland location can be in a severe zone with a hedge in front of it. Then the wall itself: cavity width, whether the outer leaf is sound and well pointed, whether there are cracks, whether the cavity is clean or bridged with mortar droppings, and whether trays and weeps exist and work. A borescope survey answers most of that in an hour. Where any of it fails, the honest answer is not to fill — and a contractor who will fill any cavity without looking into it is telling you something about the survey rather than about the wall.
- What happens when cavity fill goes wrong?
- Damp appears on internal walls that were dry, usually on the windward elevation, usually a season or two later. The mechanism is that water crossing the outer leaf now has a path — through the insulation, through a bridge of material, or down a slumped void — to the inner leaf, instead of running down a clear cavity and out through a weep. Slumping is the other failure: some fills settle over years and leave the top of the wall uninsulated, which shows as a cold band at the ceiling. Extraction is possible and is a serious job: the fill is removed through the outer leaf, which then has to be made good. Prevention is the survey, which is why it is the part of the process worth insisting on.
- Will external insulation cause damp?
- It generally reduces it, and the reason is where the dew point ends up. Wrapping the wall on the outside keeps the masonry on the warm side of the insulation, so its surfaces stay above the dew point of the internal air and condensation has nowhere to form. It also shelters the wall from rain. The risks are at the DETAILS rather than in the field: the base, where the system must stop above ground level and not bridge the damp-proof course; the reveals and sills, where insufficient insulation leaves a cold line around every opening; and any penetration through the system. Internal insulation is the option with the genuine condensation risk, because it makes the masonry colder and puts a dew point inside the construction — which is why it needs vapour control designed rather than assumed.
- What about the window reveals?
- They are where external insulation is won or lost. A wall wrapped in a hundred millimetres of insulation with reveals left bare has a continuous cold line around every opening, and that line is the coldest interior surface in the room — so it is where condensation and mould appear, in a house that has just been insulated. The remedy is a thinner high-performance insulation returned into the reveal and up to the frame, which means the window frames need enough of a face for it to land on. Where they do not, the honest sequence is to replace the windows and the insulation together, setting the new frames forward so the insulation can meet them. That is a larger project and it is the one that actually works.
- Do I need planning permission?
- For external insulation, often — it changes the external appearance and the wall's thickness, and requirements vary by jurisdiction and by whether the building is in a conservation area or listed. Boundaries matter too: a wall on a property line cannot usually grow outward, and a terrace or semi-detached house creates a junction with the neighbour that has to be detailed and sometimes agreed. Cavity fill normally needs no permission, being invisible, though it is generally notifiable work with a guarantee attached. The practical point is that the consent question should be answered before the thermal one, because on many buildings it removes external insulation from the list entirely and the real choice is between cavity fill and internal insulation.
- What about insulating inside instead?
- It is the third option and it is the one with the most ways to go wrong, which is why it is not the subject of this page. It loses internal floor area in every room. It leaves the masonry COLD and outside the insulation, so the dew point moves into the construction and vapour control has to be designed rather than assumed. It leaves every junction — floors, party walls, internal walls meeting external ones — as a thermal bridge that the insulation cannot cross, and those bridges become the cold spots where mould forms. It is done successfully all the time, on buildings where the outside cannot change, and it is a designed assembly with a moisture risk analysis rather than a sheet of insulated plasterboard fixed to a wall.