Mineral wool rolls, rigid PIR boards and thin multifoil products such as SuperFOIL can all insulate a home. They work in different ways and suit different places. This guide explains the differences so you can ask the right questions. SuperFOIL is a brand of multifoil insulation; Energy Crisis UK is not connected with the manufacturer.
How Each Type Works
Mineral Wool
Glass wool and rock wool trap air in a mass of fibres, slowing heat loss. They need depth to work well — the recommended minimum for loft floors is 270mm — and must not be squashed. Mineral wool is widely used, straightforward to lay between and across joists, and also reduces sound.
PIR Board
PIR (polyisocyanurate) is a rigid foam board that gives more insulation for its thickness than mineral wool. It is commonly fitted between rafters in loft conversions, in walls and under floors. Joints and gaps need sealing carefully.
Multifoil
Multifoil insulation is made of several reflective foil layers separated by wadding or foam. The foil reflects radiant heat and the layers add resistance. It is much thinner than mineral wool, but its performance depends on the product and on installation details such as air gaps next to the foil and taped joints.
Thickness and Performance
Comparing thickness alone can mislead. Building Regulations set the U-value a roof, wall or floor needs to reach, and the right question is what build-up reaches it in your space. Multifoil is often combined with PIR board or other insulation to meet the requirement in a slim construction.
Ask any installer for a U-value calculation for the whole build-up they propose, and check the product has independent certification for that use.
Where Each Fits
Mineral Wool Suits
- Open loft floors, where there is plenty of depth
- DIY projects in accessible, damp-free lofts
- Places where sound reduction helps
PIR Board Suits
- Between rafters in loft conversions
- Walls and floors where a rigid board is practical
- Where you need more insulation in less thickness
Multifoil Suits
- Rooms in the roof and sloping ceilings where headroom matters
- Shallow rafters and refurbishments where thick insulation will not fit
- Extensions, garden rooms and outbuildings
Cost
Prices change often and depend on the product, the area and the labour involved, so we do not quote per-square-metre figures. As a general rule, mineral wool is the cheapest way to insulate an open loft floor. For a whole-loft comparison, the Energy Saving Trust puts professional mineral wool loft insulation (0 to 270mm) at around £650–£1,200 depending on the house. Get quotes that show the full build-up, not just the material.
Which Saves More?
If two build-ups reach the same U-value, they should keep in similar amounts of heat. So the real question is which option can reach the performance you need in the space you have, installed properly. We have not found independent, published savings figures specifically for multifoil products, so treat any headline savings claim with care.
The Bottom Line
Mineral wool is usually the best value for open loft floors. PIR board gives the most insulation for its thickness in rafters and walls. Multifoil earns its place where space is tight. Whatever you choose, ventilation and moisture control matter, so use an installer who will explain their design — and read our loft insulation guide before you start.
Sources: Energy Saving Trust, “Roof and loft insulation” (fuel prices as of July 2026). Last reviewed September 2026.