Pitched Roof - Under Rafter

Roof insulation installed under the rafters creates a continuous internal thermal layer beneath the structural frame, improving airtightness and overall energy efficiency without altering the external roof covering.

The SuperFOIL multifoil system is designed for high-performance pitched roof applications where internal insulation is preferred for retrofit or new build projects.

How the Under Rafter System Works

SuperFOIL is installed directly beneath the rafters, creating a continuous insulation layer across the internal face of the roof structure.

The system is:

Installed either horizontally or vertically
Fixed directly to rafters using appropriately sized staples
Installed with gentle tension to remove slack
Overlapped by a minimum of 50 mm
Sealed at all joints, cuts and penetrations using SuperFOIL MultiFoil Tape
Battens must be installed at maximum 600 mm centres and sized appropriately (typically 25–90 mm depth) to create and maintain the required ventilation cavity and airspace performance.
Completed with plasterboard or chosen internal finish

Multifoil insulation can also be installed between rafters; however, its performance is typically lower compared to a continuous installation beneath the rafters. This approach may be used where preserving exposed roof timbers is desired while optimising the available internal space.

Airtightness & Sealing

To ensure optimum performance:

All overlaps must be taped
All cuts must be sealed
Penetrations (pipes, cables, vents) must be fully airtight
Joints to walls, ceilings and floors must be sealed

Correct sealing prevents:

Air leakage
Thermal bypass
Condensation risk

Importance of Airspaces

Multifoil insulation relies on correctly formed airspaces.

To maximise performance:

Correct batten depth must be used
Airspace must be maintained between insulation and internal finish
Battens must be installed at maximum 600mm centres

Without airspace:

Radiant heat becomes conductive
Insulation efficiency reduces

Maintaining airspace allows the system to:

Reflect up to 95% of radiant energy
Reduce conductive heat transfer
Minimise convective heat movement

Services & Electrical Considerations

When installing under rafters, particular attention must be given to service penetrations.

Where cables, pipes or outlets pass through insulation:

Cut small openings only as required
Fully seal around penetrations using SuperFOIL tape
Wrap copper pipes in non-metallic insulation before contact with foil

All electrical work must comply with relevant regulations.

Critical Detailing Areas

Proper detailing ensures long-term performance and durability.

Special attention should be given to:

Ridge detail (minimum 50mm overlap, taped)
Eaves integration
Roof-to-wall junction
Purlins
Dwarf walls
Window & roof light detailing
All penetrations

Installation Checklist Section (Professional Touch)

Before completion ensure:

Product trimmed correctly
Staples max 300mm spacing
All overlaps taped
Correct batten depth applied
Battens max 600mm centres
Internal finish installed

Recommended Products for Under Rafter Applications

For under-rafter pitched roof systems, the following SuperFOIL multifoil products are recommended:

• SF19+ – slim profile solution for space-restricted applications
• SF40 – enhanced thermal performance for standard build-ups
• SF60 – high-performance solution for demanding U-Value targets

These products are designed for internal installation beneath rafters, providing airtightness, radiant barrier performance and effective condensation control when installed correctly.

In specific project scenarios where additional moisture protection or enhanced vapour permeability is required, breathable SuperFOIL products (such as SF19BB or SF40BB) may also be specified as part of a hybrid build-up. This is typically considered where condensation risk analysis or design requirements demand increased moisture management.