Multifoil, PIR and mineral wool are genuinely different products built for different jobs. Used in the right application, each one delivers. Used in the wrong one, you can easily spend 40% more than you needed to, or lose 150mm of precious headroom for nothing. This is the straight walk-through our surveyors use on site for a loft or sloping rafter.
The three products, side by side
| Property | Multifoil (Actis Hybris) | PIR (Celotex GA4000) | Mineral wool (Knauf Earthwool) |
|---|---|---|---|
| Thermal conductivity (λ) | 0.034 W/mK equiv | 0.022 W/mK | 0.040 W/mK |
| Thickness for U=0.18 W/m²K | ~110mm with air gaps | ~120mm | ~270mm |
| Cost per m² (material) | £32 - £40 | £18 - £24 | £8 - £14 |
| Cost per m² (installed) | £60 - £80 | £40 - £55 | £25 - £40 |
| Vapour permeable? | No (foil layer) | No | Yes |
| Fire rating | Class B s2 d0 | Class E (combustible) | Class A1 (non-combustible) |
| Install difficulty | Medium (air gaps critical) | Easy (cut and friction-fit) | Easy (roll out) |
| Best for | Room-in-roof, headroom-tight spaces | New-build rafters, loft conversions | Open lofts, ceilings |
Open loft (over the joists)
Mineral wool wins, no contest. 100mm between the joists, 170mm cross-laid over the top, U-value 0.16 W/m²K, total cost £700–£1,200 for a 3-bed semi — fully ECO4-funded if you qualify. PIR is wasteful here: you'd be paying for headroom you don't need to save. Multifoil is an unnecessary step up when you've got full unobstructed loft space to work with.
Loft conversion — up the rafter line
PIR usually wins here. With 150mm rafters you can hit U=0.15 W/m²K using 130mm of PIR plus a 15mm insulated plasterboard finish, leaving a 20mm air gap above for ventilation. Multifoil works too, but needs more space for the air gaps and tends to cost ~30% more per m². The exception: if your rafter depth is already locked at 100mm or less (common in 1970s conversions), multifoil is the only product that will meet regs without ripping the roof off.
Room-in-roof retrofit
Multifoil wins, often by a long way. Room-in-roof retrofit — insulating the slopes, dwarf walls and ceiling of an already-converted loft without ripping out the plasterboard — is the exact application multifoil was designed for. PIR can't fit in the typical 50–75mm of available rafter depth without losing 100mm+ of headroom. Mineral wool won't fit at all. ECO4 routinely covers the £1,800–£2,800 install cost for eligible households.
Walls and floors — quick guidance
- Cavity wall — none of these three. Use foam beads (EPS) or blown mineral fibre.
- Solid wall internal — PIR-backed plasterboard wins.
- Solid wall external — typically EPS or mineral wool render system.
- Suspended timber floor — mineral wool or sheep wool wins on cost and breathability.
- Solid concrete floor — PIR boards under a screed.
Worked example: 3-bed semi room-in-roof
| Solution | Achieves U=0.18? | Cost installed | Headroom lost |
|---|---|---|---|
| Multifoil 60mm + 25mm air gaps | Yes | £2,300 | 110mm |
| PIR 100mm | Yes | £2,750 | 120mm |
| Mineral wool 270mm | Yes — but won’t physically fit in the rafter depth | N/A in this application | N/A |
Arrange a free survey that recommends the right product for your specific space — not the one with the highest margin. Book online or ring 0800 229 4094. One of our own surveyors spends 45 minutes on site and you will have a fixed written quote within 48 hours. We will tell you honestly if it isn't the right fix.
Want to find out how much you could save?
Book a free 45-minute survey. We’ll check every active grant, measure every wall, and write you a plan. Yours to keep whether you book or not.
The plan is yours to keep. Take it to two other installers if you like.