Multifoil vs Wool Insulation

Different materials for different jobs

It is tempting to line multifoil and wool up as rivals, but they usually answer different questions. Wool — mineral, sheep or a recycled quilt — is the standard way to insulate a loft floor, laid between and across the joists so the heated rooms below stay warm and the loft itself runs cold. Multifoil is a thin, layered foil product most often fitted at rafter level, against the slope of the roof, where you want to keep the loft space itself warmer or where floor depth is impractical. Understanding that difference is the key to choosing well.

How each one works

Mineral wool and other quilts insulate by trapping still air in a thick, soft layer. Performance is straightforward and predictable: it comes from the material and the depth, with 270mm the commonly used benchmark for wool on a loft floor.

Multifoil works differently. It is made of reflective foil layers separated by thin insulating material, and part of its performance relies on reflecting radiant heat and on air gaps left on one or both sides during installation. That makes it thin, which is its main attraction — but it also makes correct installation critical, because without the specified air gaps and sealed joints its real-world performance falls short of the ideal.

Where wool on the loft floor suits

For most homes, insulating the loft floor with wool is the sensible default. It is inexpensive, well understood and effective, and it keeps the largest heat-loss path — up through the ceiling — under control while leaving the loft as a cold, ventilated space. A standard loft insulation installation is floor-based wool in the great majority of cases.

Where multifoil at the rafters suits

Multifoil, or rafter-level insulation more generally, comes into its own when the loft space itself needs to be warmer — for a room-in-roof, a boarded area you want less cold, or where the roof slope is being lined. Its slim profile helps where every millimetre counts, such as awkward or shallow spaces. It is also sometimes chosen in sensitive properties, including some listed and traditional buildings, where the approach to the roof has to be handled carefully — though in those homes any work should respect the building’s breathability and any consent requirements.

The limitations of each

Neither is a cure-all, and it is worth being honest about the constraints.

Wool on the floor does nothing for the loft space above it — that space stays cold by design, which is fine for storage but not if you want a warmer room in the roof. It must also be kept dry and never compressed under boarding.

Multifoil’s headline thinness comes with strings attached: its performance is highly sensitive to installation, needing the correct air gaps and well-sealed joints, and independent verification of its thermal figures should be checked against the specific product. Crucially, fitting it at rafter level changes how the roof breathes, so ventilation has to be planned so moisture is not trapped between the foil and the bituminous roofing felt or membrane above. Getting that wrong invites condensation. This is why foil work and loft ventilation are planned together, a point the Energy Saving Trust echoes in its general guidance.

Can you use both?

Sometimes, yes. Some homes have wool on the floor for the main thermal job and a foil element at the rafters where a warmer roof space is wanted — but this is a design decision that depends on how you use the loft and how the roof is ventilated, not a default.

The honest bottom line

Multifoil and wool are not really competitors; they solve different problems in different places. Wool on the loft floor is the standard, low-cost way to stop heat escaping from the home. Multifoil at the rafters suits a warmer roof space or tight areas, but only with correct air gaps and carefully planned ventilation. The right answer starts with a simple question — do you want the loft cold or the roof space warm? — which a survey can help you settle.