The three problems that get conflated
1. Missing or compressed insulation — material no longer doing its job thermally. 2. Wet insulation — material that's absorbed water from below. 3. Failed vapour barrier — the membrane between the home's floor and the insulation that should stop interior moisture migrating downwards. When this fails, even new insulation will wet itself from inside the home.
These have three different causes and three different fixes. Conflating them — particularly fixing #1 when the real issue is #3 — is expensive and short-lived.
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Symptoms and what they suggest
Cold floors only in certain rooms: localised insulation compression or rodent damage.
Cold floors everywhere: full underfloor insulation degradation, or a vapour barrier that's failed and wet the insulation throughout.
Damp patches near floor edges internally: water ingress at the base/skirting, not necessarily an insulation issue at all.
Visible insulation falling from underside (when you look): physical detachment — almost always replace.
Musty smell near skirtings: usually vapour barrier or ventilation issue, sometimes both.
Proper diagnosis before scope
A good independent inspection looks at: condition of the insulation under the home, condition and continuity of the vapour barrier (where visible), condition of the underside membrane, evidence of water ingress at base / skirting level, ventilation grilles around the home (often blocked or insufficient), and the internal floor for cold spots and dampness.
Without this, the typical contractor scope is 'replace all the insulation' — which sometimes is right, but quite often is treating the symptom rather than the cause.
Cost ranges and what to expect
Standalone underside insulation replacement: typically £1,200 — £3,500 depending on home size and access. Replacement with vapour barrier overhaul: £2,500 — £5,500. Combined with skirting / drainage corrections: more.
Get any insulation quote reviewed before committing. The single most useful question to ask the contractor: 'how do you know it's an insulation problem and not a vapour barrier or ventilation problem?' If they can't answer that, ask for proper diagnosis first.