Last reviewed: September 2026. This is general information, not a product recommendation. Better Lofts has no connection with any insulation manufacturer.
Choosing insulation is one of the bigger decisions in a loft project. It affects comfort, energy use and, in a converted loft, how much headroom you keep. Multifoil insulation — SuperFOIL is one well-known brand — is sometimes suggested where space is tight. Here's what it is, where it fits and what to check.
What Is Multifoil Insulation?
Multifoil insulation is made of several thin layers of reflective foil separated by wadding. SuperFOIL SF19+, for example, is described by the manufacturer as a roll of multiple foil and separation layers about 40–45mm thick.
Unlike bulk insulation such as mineral wool, which works mainly by trapping air, multifoil combines reflective surfaces with thin insulating layers. The reflective surfaces only work properly when there is a still air gap next to them.
How Well Does It Insulate?
This is the key question, and it's worth being clear about. Thermal resistance is measured as an R-value — the higher the number, the better the insulation.
- The manufacturer quotes a tested R-value of 2.49 m²K/W for SuperFOIL SF19+ in a roof.
- A full 270mm layer of mineral wool, the Energy Saving Trust's recommended minimum depth for loft insulation, gives a much higher thermal resistance than that.
So multifoil is not a like-for-like replacement for a full depth of loft roll. Its advantage is thinness. In a loft conversion it is usually combined with other insulation — for example boards between the rafters — to reach the U-values required by Part L of the Building Regulations. Your designer or building control body should confirm the complete build-up.
Certification to Ask About
For any insulation product in a loft conversion, ask what independent certification it has for the way it will be used. Two common ones are:
- BBA certification — an independent assessment by the British Board of Agrément.
- LABC Registered Details or System approval — details that local authority building control has already assessed.
SuperFOIL says its products hold BBA approval and LABC registration. Certificates cover particular products and uses, so ask your installer or building control body to confirm that the certificate covers your roof and your installation method.
Multifoil vs Other Loft Insulation
Compared with mineral wool (loft roll)
- Where it's used — mineral wool is the usual choice for an unused loft, laid between and over the ceiling joists. The Energy Saving Trust describes this as the cheapest, easiest and most efficient option for most people.
- Thickness — mineral wool needs a lot of depth (270mm recommended), which is fine on a loft floor but hard to fit into a sloping ceiling. That's where thinner products come in.
- Handling — mineral wool fibres can irritate skin, so installers wear protective clothing.
Compared with rigid board (PIR or phenolic)
- Performance — rigid boards insulate well for their thickness and are widely used between and below rafters in converted lofts.
- Fitting — boards must be cut accurately to avoid gaps; flexible multifoil can be easier to fit around awkward timbers.
- Cost — Checkatrade's July 2026 guide puts PIR board at around £35 per m² and a thermal foil layer at around £14 per m² (materials only). Labour and the other layers in the build-up come on top.
Where Multifoil Can Help
- Tight headroom — a thin layer, combined with other insulation, can help a converted loft meet the standards without losing as much height.
- Vapour control — some multifoils are designed to act as part of the vapour control layer when joints are taped as the manufacturer specifies.
- Awkward shapes — a flexible roll can be easier to fit than rigid boards in irregular roofs.
Installation Matters
Whatever product you use, the installation decides how well it works:
- Follow the manufacturer's air-gap guidance — reflective layers need still air gaps to perform as tested.
- Tape the joints — use the specified tape so the vapour control layer is continuous.
- Don't compress it — squashing insulation reduces its performance.
- Keep the roof ventilated — the Energy Saving Trust warns that insulation changes how moisture moves, and poor ventilation can lead to condensation, damp and rot.
- Fix damp first — never insulate a roof that leaks or has damp problems.
Is Multifoil Right for Your Project?
For an ordinary unused loft, mineral wool at joist level is usually the simplest and cheapest option. Multifoil is more likely to come up in a loft conversion where headroom is tight, as one part of a combined build-up. Ask your designer to show you that the whole roof meets building regulations, and get more than one quote.
Better Lofts is an introduction service run by Better Home Hub Ltd — we don't supply or fit insulation. Read about the main loft insulation options, or send us an enquiry and Better Home Hub will try to introduce you to an installer who covers your area.
