The right answer depends almost entirely on how your walls were built. Here is how to tell what you have and what suits it.
People often ask us for 'wall insulation' as though it is one product. It is not. What your home needs depends on whether it has solid walls or a cavity, and the two solutions are completely different jobs.
How to tell what you have
Homes built before roughly 1920 almost always have solid walls. From the 1930s onwards most have a cavity — two leaves of masonry with a gap between. A quick test is to look at the brick pattern: if you can see regular 'header' bricks laid end-on among the stretchers, it is likely solid; if every visible brick is laid lengthways, it is probably a cavity. Wall thickness at a window or door reveal is another clue.
Cavity wall insulation
If you have an unfilled cavity in good condition, injecting it with bonded bead or mineral fibre is one of the cheapest efficiency measures available and is usually done in a day. It is not suitable where the cavity is too narrow, is bridged with debris, or where the wall is exposed to severe driving rain without a good outer leaf.
External wall insulation
For solid walls, or cavity walls that cannot safely be filled, external wall insulation wraps the outside of the building in a continuous insulated layer finished in render or brick slips. It delivers by far the biggest fabric improvement available on a solid-wall house, removes cold bridging and refreshes the appearance, but it is a bigger, scaffolded project.
Internal insulation as a fallback
Where the outside cannot be altered — a listed frontage, a wall tight to the boundary — insulating on the inside is an option. It is more disruptive room by room and slightly reduces floor area, so we usually treat it as plan C rather than plan A.
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