How foil insulation is installed: the principles that matter

1. Identify the product

Bubble foil, multifoil, foil-faced PIR and reflective membranes are installed differently. Find the product's current datasheet and third-party certificate; that document controls everything below. Which is which.

2. Confirm the build-up

Write the layers down, room to sky, before starting: internal lining, service void, quilt, air space, rafters (with or without PIR), ventilated void or breathable membrane, covering. Check it matches a build-up on the certificate and reaches any target U-value. U-values.

3. Ventilation

Establish how the cold side breathes — eaves and ridge vents, tile vents, or a breathable membrane — and how the installation will keep that path open. Blocking the eaves with quilt is the commonest error. Condensation.

4. Air spaces

Form the spaces the certificate specifies, commonly around 20–25mm on each face, with battens or the rafter depth. Sealed, still, and not sagging into. Air gaps.

5. Orientation

Reflective faces towards air spaces. Single-sided products: the reflective face towards the space; in a roof usually towards the cold side, but the datasheet decides. Double-sided multifoils want a space on both sides. Breathable multifoils go over the rafters, not under.

6. Joints and taping

Overlap 50–100mm, tension across the rafters, staple every 100–150mm, tape every joint and every edge with the manufacturer's tape. Generic aluminium tape fails within a few seasons. Rolls and tape.

7. Penetrations and services

Cables, pipes, flues, soil stacks and downlighters each need a detail: cut, dress, tape, and keep required clearances from heat sources. Run cables in the service void rather than through the quilt.

8. Battens and lining

Battens through the quilt into the rafters form the internal air space and carry the plasterboard. Board thickness follows the certificate's fire requirement. Fire performance.

9. Manufacturer instructions and regulations

Whether Building Regulations approval is required depends on the nature and scope of the work. Some insulation work within existing roof spaces may not itself require an application, while renovation of a thermal element, structural alterations, conversions or other controlled work can trigger requirements. Transitional arrangements and the date and scope of a project can affect which edition of the guidance applies. Check property-specific projects against current Building Regulations guidance and, where appropriate, local building control.

Which way should foil insulation face?

Towards the air space it is meant to work across. That is the whole rule; the product's datasheet applies it to the specific build-up.

Questions homeowners ask