British walls come in two kinds and they need different answers. A cavity wall has a gap between two leaves and can be filled. A solid wall has no gap and has to be insulated on one face or the other. Knowing which you have is the first step, and it is usually visible from the brick bond and the wall thickness at a window reveal.
Cavity fill and cavity wall insulation beads: when it goes wrong
Filling a cavity with mineral wool, polystyrene beads or foam is quick, relatively cheap and effective in the right wall. The right wall is one in good condition, not severely exposed to driving rain, with a clean cavity and no bridging. Where those conditions are not met, the fill can transfer moisture across the cavity and cause damp on the inside face, which is the well documented failure mode. Beads bonded with adhesive are more resistant to slumping than some older materials. A proper pre installation survey, including a borescope inspection of the cavity, is what separates a good installation from a problem.
Cavity wall insulation extraction, and when it is the right call
Where fill has caused damp, has slumped leaving cold patches, or has been contaminated by a defect, it can be extracted: the wall is opened at intervals, the material is vacuumed out and the wall made good, and the cavity can then be left clear or refilled once the underlying defect is fixed. It is disruptive and it is cheaper than living with persistent damp. Before extracting, establish that the fill is actually the cause rather than a leaking gutter or a bridged damp proof course, because it is frequently blamed for both.
Solid walls: outside or inside
External wall insulation wraps the outside in insulation and a render or cladding finish. It performs best, keeps the wall warm and dry, loses no internal space and transforms the appearance, which is why it may need planning permission and is usually unsuitable on a period frontage. Internal wall insulation lines the inside and is cheaper, can be done room by room, loses internal space, and has to be detailed carefully at floors, ceilings and reveals or it creates cold bridges and condensation exactly where it stops.
Cold bridging, which is where retrofit insulation fails
Insulation only works where it is continuous. The places it stops are the window reveals, the eaves, the floor junction, and any point where a wall meets a structure that carries heat out. Internal insulation is especially prone to this, which is why a competent installation insulates the reveals and returns the insulation at the junctions rather than stopping at the plaster line. A retrofit that produces mould in the corners of a newly insulated room has almost always failed at a junction rather than in the middle of the wall.
Thickness, fixings and the practical detail
Thickness follows the performance the element has to achieve, which the building regulations set when a thermal element is renovated, and more is not always better internally because every millimetre is floor area. Board systems are fixed with mechanical anchors and adhesive, and the fixings are specified for the substrate, which matters a great deal on a soft or rendered wall. None of this is exotic; it is the sort of detail a good installer names in a quote and a poor one does not.
Government grants for external wall insulation: where the answer is yes
Wall insulation is a measure government backed schemes genuinely fund, unlike glazing or a new roof. Eligibility runs through the household's benefits, the property's energy rating and sometimes local authority criteria, and the schemes change names and rules, so the current position on GOV.UK and with your local authority is the only reliable source. Where a scheme is involved it will require a registered installer working to a recognised retrofit standard, which is protection for you given how badly a poor wall insulation job can go.
Other questions people ask
How do I know if I have cavity walls?
Look at the brick bond and the wall thickness at a window reveal. A wall of stretchers only, and a thickness over about 260mm, usually means a cavity; alternating headers and a thinner wall usually means solid.
Can cavity wall insulation cause damp?
In the wrong wall, yes. Severe exposure, a defective outer leaf or a bridged cavity can let the fill transfer moisture inwards. A proper pre installation survey including a borescope check is what prevents it.
Is removing cavity wall insulation possible?
Yes, by extraction: opening the wall at intervals, vacuuming the material out and making good. Establish first that the fill is the cause rather than a gutter or a bridged damp proof course.
External or internal wall insulation?
External performs better, keeps the wall warm and loses no internal space, and changes the appearance and may need permission. Internal is cheaper and room by room, and needs careful detailing at reveals and junctions.
Are there grants for wall insulation?
Insulation is one of the measures schemes do fund, with eligibility through benefits, the property's rating and local criteria. Check GOV.UK and your local authority for the current scheme.