Insulation is the highest-return upgrade available to most UK homes, yet the right material depends entirely on where you’re insulating and what you’re insulating against. In 2026, prices range from around £3–£10/m² for mineral wool roll in a loft to £15–£35/m² for professionally installed spray foam, with performance measured in lambda (W/mK) and U-value rather than price alone — so the cheapest option per square metre isn’t always the cheapest way to hit a target.
This guide brings together our insulation library in one place — where heat actually escapes, loft and roof insulation, wall insulation, floor insulation, how the main materials compare, and the grants that can cover some or all of the cost.
Where Heat Escapes & U-Values
Heat leaves a home through its fabric — walls, roof and floor — but also straight through gaps around doors, windows, floorboards and the loft hatch. Draught-proofing is the cheapest fix of all, typically £200–£400 professionally fitted or £50–£80 in DIY materials, and pays for itself within a year or two.
Fabric insulation only works if moisture is managed at the same time. A vapour control layer on the warm side of the insulation stops moist internal air condensing inside the wall or roof build-up, while a breathable membrane on the cold side lets any vapour that does get through escape outward — installing either on the wrong side is one of the most persistent and expensive mistakes in UK construction.
Loft & Roof Insulation
Loft insulation remains the highest-return single upgrade in most existing homes: loft insulation types covers the 270 mm mineral wool depth target, cold-loft versus warm-loft strategies, and what Part L requires in a new build. Smaller roof areas need their own approach — the flat roof over a bay window is one of the most neglected, and insulating a bay window roof covers repair and insulation together before water tracks into the ceiling below.
Wall Insulation
Around a third of UK homes have solid walls rather than a cavity, and the two lose heat very differently — an uninsulated solid wall typically has a U-value of 1.6–2.1 W/m²K, roughly double that of a filled cavity. Where there’s a cavity, cavity wall insulation types compares mineral wool, blown bead and injected foam fills at £8–£22/m² installed by cost and damp risk; where the wall is solid, solid wall insulation sets out the trade-off between internal (£4,000–£13,000 for a semi, cheaper but loses floor space room by room) and external (£10,000–£22,000, pricier but changes the elevation and needs no internal disruption) options.
Floor Insulation
An uninsulated ground floor accounts for around 15% of a typical home’s heat loss, yet it’s usually the last thing homeowners tackle. Underfloor insulation covers both suspended timber and solid concrete floors, with professional suspended-floor installation from around £25–£55/m². Floors and walls both need controlling for sound as well as heat — acoustic insulation explains the Part E targets that apply to separating floors and party walls in flats and converted houses.
Materials Compared
Six materials dominate UK insulation specification, and none of them wins every time. PIR vs mineral wool is the fundamental trade-off — PIR needs roughly half the thickness for the same U-value but costs more per square metre; if you’ve settled on PIR specifically, Kingspan and Celotex boards compares sizes and cost by thickness. On the mineral wool side, rockwool vs other mineral wool explains why stone wool costs more than glass wool but outperforms it on fire resistance and acoustics.
Rigid foam boards split into two families as well: EPS vs XPS covers where the extra cost of moisture-resistant XPS is justified — mainly under slabs and inverted roofs — and where cheaper EPS does the same job. Marketing claims deserve scrutiny too: multifoil insulation tests out at the equivalent of roughly 40–80 mm of mineral wool despite claims of far higher performance.
| Material | Thermal Performance | Typical Cost (2026) |
|---|---|---|
| PIR board | 0.022–0.025 W/mK | £8–£20/m² |
| Mineral wool (roll/slab) | 0.032–0.044 W/mK | £3–£10/m² |
| EPS board | 0.030–0.038 W/mK | £3–£8/m² (100 mm) |
| XPS board | 0.032–0.036 W/mK | £8–£18/m² (100 mm) |
| Multifoil | equivalent to 40–80 mm mineral wool, tested | £5–£14/m² |
| Spray foam | varies by depth and type | £15–£35/m² installed |
Lower W/mK (lambda) means better performance per millimetre of thickness; figures are 2026 UK supply or installed costs as noted.
A mortgage-risk warning applies to one product in particular. Spray foam insulation costs £15–£35/m² installed, but major lenders including Halifax, Nationwide and Santander now refuse to lend on homes where it’s been sprayed onto roof timbers, and removal alone can cost £800–£3,000 or more. Get independent advice before agreeing to it, whatever a salesperson promises about energy savings.
[!tip] Check lambda, not just thickness Two products of the same thickness can perform very differently. Always compare the lambda value (W/mK) — lower is better — rather than assuming “50mm of insulation” means the same thing whatever the material.
Costs & Grants
Not every homeowner has to pay full price. Insulation grants explains ECO4 and the Great British Insulation Scheme (GBIS), which between them can fully or partially fund loft, cavity and solid wall insulation depending on your circumstances and current EPC rating.
Detached structures need their own budget. Insulating a garage — whether for a home gym, workshop or conversion — costs £500–£2,500 in DIY materials or £1,500–£4,500 professionally installed, with roof and door treatment usually giving the best return.
Where to Start
If you only tackle one thing this year, start with draught-proofing — it’s the cheapest measure here and pays back within a couple of years. For the next step, compare loft insulation types against your current depth, and check PIR vs mineral wool before buying board insulation for a conversion or flat roof — the wrong material choice there is expensive to undo.
