When it comes to insulating your home, you have more options than ever. The traditional choice — thick rolls of mineral wool — has been joined by high-performance alternatives like SuperFOIL multifoil insulation and rigid PIR board. Each has distinct advantages depending on where and how you are insulating.
In this comparison, we break down the differences between SuperFOIL, mineral wool, and PIR board across the factors that matter most: thickness, thermal performance, cost, and best use cases.
What Is SuperFOIL Multifoil Insulation?
SuperFOIL is a multifoil insulation product made from multiple layers of reflective aluminium foil separated by layers of wadding (typically polyester or foam). It works by reflecting radiant heat back towards the warm side of the building, while the wadding layers provide additional thermal resistance.
The key advantage of multifoil insulation is its extreme thinness. Where mineral wool requires 270mm of depth to meet Building Regulations, SuperFOIL products achieve comparable thermal performance in just 40–60mm of material thickness.
SuperFOIL products are BBA (British Board of Agrément) certified and can be used to comply with Building Regulations when correctly installed with the required air gaps.
What Is Mineral Wool Insulation?
Mineral wool — including glass wool (e.g. Knauf, Isover) and rock wool (e.g. Rockwool) — is the most widely used insulation material in UK homes. It comes in rolls or batts and works by trapping air in a fibrous matrix, slowing the transfer of heat by conduction and convection.
Mineral wool has been used for decades and is well understood by builders, surveyors, and building control officers. It is inexpensive and effective, but requires significant depth to achieve good thermal performance.
What Is PIR Board Insulation?
PIR (polyisocyanurate) is a rigid foam board insulation with excellent thermal performance for its thickness. Brands like Celotex and Kingspan are the most common. PIR boards are typically 50–150mm thick and offer the best thermal conductivity of any mainstream insulation material.
PIR is commonly used between rafters in loft conversions, in wall cavities, and under floors. It is rigid, easy to cut, and provides consistent performance.
Thickness Comparison
This is where the differences are most dramatic. To achieve approximately the same thermal performance (a U-value meeting current Building Regulations for a roof):
- Mineral wool: 270mm
- PIR board: 100–150mm
- SuperFOIL (with air gaps): 90–110mm total build-up (40–60mm of product plus 25mm air gaps each side)
In a loft conversion, where every millimetre of headroom counts, the difference between 270mm of mineral wool and 90mm of SuperFOIL is enormous. That 180mm saving can mean the difference between a usable room and one that feels cramped.
Thermal Performance Comparison
Thermal performance is measured by thermal conductivity (lambda value, in W/mK — lower is better) and by thermal resistance (R-value — higher is better).
- Mineral wool: Lambda 0.038–0.044 W/mK. Requires significant thickness to achieve high R-values.
- PIR board: Lambda 0.022–0.025 W/mK. The best thermal conductivity of any mainstream material.
- SuperFOIL: Works differently — combines reflective and resistive properties. The effective thermal performance depends on installation (air gaps are essential for the reflective element to work). BBA-certified performance varies by product.
It is important to note that SuperFOIL’s performance is measured differently from conventional insulation. Its reflective layers work by reducing radiant heat transfer, which is not fully captured by a simple lambda value. This has historically caused debate in the construction industry, but BBA certification provides independently verified performance data.
Cost Comparison
Material costs per square metre (approximate, 2026 prices):
- Mineral wool (270mm): £3–£6 per m²
- PIR board (100mm): £12–£25 per m²
- SuperFOIL SF19: £8–£12 per m²
- SuperFOIL SF40: £12–£18 per m²
Mineral wool is clearly the cheapest option on a materials-only basis. However, the total installed cost also depends on labour time, fixings, and any additional materials needed. SuperFOIL can be quicker to install than mineral wool in rafter applications, which can offset some of the higher material cost.
For open loft floor insulation (between joists), mineral wool is almost always the most cost-effective choice. The space savings of SuperFOIL are irrelevant when the loft is not being used as living space.
Installation Differences
Mineral Wool
Mineral wool is rolled out between joists or rafters and cut to fit. A second layer is often laid across the joists at right angles. It is itchy to handle and requires protective equipment (mask, gloves, goggles), but the installation technique is simple.
PIR Board
PIR boards are cut to fit tightly between rafters or studs. Gaps must be sealed with expanding foam or tape to prevent air leakage. Boards are typically held in place with friction or battens. Installation is clean but requires precision cutting.
SuperFOIL
SuperFOIL is stapled or nailed to rafters, then counter-battened to create the essential air gap before the internal finish (plasterboard) is fitted. All joints must be taped with SuperFOIL tape to create a continuous vapour barrier. The material is lightweight and clean to handle, but the installation requires careful attention to air gaps and sealing.
When to Use Each Type
Choose Mineral Wool When:
- Insulating an open loft floor (between and across joists)
- Budget is the primary concern
- Depth/space is not a constraint
- You want to DIY the installation
- Acoustic performance is important (mineral wool is excellent for soundproofing)
Choose PIR Board When:
- Insulating between rafters in a loft conversion
- You need the highest thermal performance per millimetre
- The application requires a rigid, self-supporting material
- Under-floor insulation between joists
- Partial loft conversions where some space saving is needed
Choose SuperFOIL When:
- Space is at a premium — loft conversions where every centimetre of headroom matters
- You need a vapour barrier — SuperFOIL acts as both insulation and vapour control layer
- Retrofitting rafter insulation — where the rafter depth is too shallow for sufficient PIR board
- Refurbishment projects — where adding 270mm of mineral wool is impractical
- Extensions and garden rooms — where a slim build-up is desirable
Can You Combine Different Insulation Types?
Yes, and this is often the best approach. A common combination for loft conversions is PIR board between the rafters + SuperFOIL across the underside. This achieves excellent thermal performance in a relatively thin build-up, with the SuperFOIL acting as both an additional insulation layer and a vapour barrier.
For open lofts, you might use mineral wool between joists + a layer of SuperFOIL on top to add reflective performance without needing the full 270mm depth of mineral wool.
Which Saves More on Energy Bills?
If all three products are installed to achieve the same U-value (as required by Building Regulations), the energy savings are essentially identical. The insulation does not care what material it is made from — it cares about the total thermal resistance of the build-up.
The real question is not which saves more, but which is practical for your situation. The best insulation is the one that gets installed properly. A perfectly installed SuperFOIL system in a loft conversion saves far more than mineral wool that was never fitted because it would have cost too much headroom.
The Bottom Line
There is no single “best” insulation material. Mineral wool wins on cost for open lofts. PIR board offers the highest performance per millimetre for between-rafter applications. SuperFOIL wins when space is the primary constraint — particularly in loft conversions, shallow rafter depths, and refurbishment projects.
For most homeowners, the decision comes down to what you are insulating and how much space you have. A qualified installer can assess your property and recommend the right material or combination for your specific situation.