Reflective foil insulation reduces radiant heat transfer and scores application-dependent in our independent assessment. Here’s how it works, where it adds real value, and how it compares to mineral wool.
Reflective foil insulation (also called multifoil or reflective bubble insulation) works by reducing radiant heat transfer — the transmission of heat as electromagnetic radiation from a warm surface to a cooler one. A layer of reflective foil reflects radiant heat back toward its source.
This is different from how conventional bulk insulation (like mineral wool) works. Mineral wool acts as a thermal barrier by trapping still air within its fibres, slowing conductive and convective heat loss. For maximum performance, foil and mineral wool can be used together.
Foil fixed to the underside of roof rafters in a cold loft provides a radiant barrier that reflects summer heat gain back upward and winter heat loss back downward. Most effective when installed with an air gap either side.
When converting a loft, foil between rafters combined with rigid insulation boards and mineral wool between ceiling joists is a common and effective approach for meeting building regulations.
Foil membranes are used in flat roof build-ups and pitched roof extensions to complement structural insulation and manage radiant heat in summer.
Thin reflective foil installed on the wall behind radiators reflects heat back into the room rather than losing it through the wall. A low-cost improvement with modest but measurable benefit.
This is a false choice for most applications. The two materials work differently and complement each other. The question is more accurately: do you need foil, mineral wool, or both?
| Attribute | Foil insulation | Mineral wool | Combined |
|---|---|---|---|
| Heat loss mechanism | Radiant (reflected) | Conductive + convective | All three |
| Best for | Radiant barriers, summer heat | Winter heat retention | Year-round performance |
| U-value improvement | Modest alone | Significant | Best combined |
| Thickness | Thin (5–50mm) | 270mm recommended | Both layers needed |
| Mortgage risk | None | None | None |
| Cost | Lower | Slightly higher | Both together |
| Our recommendation | Good complement | Primary choice | Best approach |
For a cold loft, 270mm of mineral wool is the primary recommendation. Adding foil insulation to the rafter undersides can provide additional performance benefits, particularly in summer heat management. Foil alone, without bulk insulation to the floor, does not meet the 270mm mineral wool standard typically needed to qualify for ECO4 or to achieve current building regulations U-values for new work.
Foil insulation is generally lower cost than bulk mineral wool because less material is used. Installed costs vary by area covered and accessibility:
| Application | Typical cost | Notes |
|---|---|---|
| Loft rafter foil (semi-detached) | £200–£400 | Often installed alongside mineral wool |
| Behind-radiator foil sheets | £20–£50 DIY | Simple DIY installation |
| Flat roof foil layer | £300–£800 | Usually as part of full roof build-up |
| Warm loft conversion | £500–£1,200 | Combined with rigid boards + mineral wool |
Foil insulation and mineral wool address different heat transfer mechanisms and work best together. Foil reduces radiant heat transfer while mineral wool reduces conductive and convective heat loss. For a typical cold loft, mineral wool at 270mm depth should be the primary measure, with foil as a complementary addition. Using foil alone, without bulk insulation, does not achieve the thermal performance of mineral wool.
Foil insulation can create a vapour barrier that traps moisture if not installed correctly. The critical requirement is to maintain air gaps on both sides of the foil membrane and to ensure adequate ventilation in the roof space. Improperly installed foil that traps moisture can lead to condensation on timber structures. Following the manufacturer’s installation guidance is essential.
Foil can be used in wall applications, particularly in timber-frame construction where it serves as a vapour control layer as well as a radiant barrier. In masonry cavity walls, standard cavity fill materials are more appropriate. Specialist advice is recommended for wall applications.