Pitched Roof - Over Rafter

Roof insulation installed over the rafters creates a continuous thermal layer above the structural frame, improving energy efficiency without reducing internal room height.

The SuperFOIL breathable multifoil system is designed for high-performance pitched roof applications in both new build and retrofit projects.

How the Over Rafter System Works

The insulation is installed horizontally across the rafters, starting at the eaves and progressing upwards towards the ridge.

The system is:

• Securely stapled to rafters (maximum 300 mm intervals)

• Installed taut when using counter battens

• Sealed at all overlaps using the integrated breathable membrane tape

• Finished with counter battens and tile battens

This creates a continuous thermal barrier above the rafter line while maintaining controlled water runoff and ventilation.

System Flexibility & Moisture Protection

The over-rafter system can be installed as a standalone insulation layer or combined with additional insulation between rafters, such as mineral wool or PIR, depending on the required U-Value target.

Additionally, multifoil insulation such as SF19+ or SF40 can be installed on the internal side of the structure (beneath the rafters) to further enhance the thermal performance of the system.

Where additional insulation is installed between rafters, adequate airspace must be maintained to ensure correct thermal and moisture performance.

Condensation & Breathability

When insulating over rafters, it is essential that the selected insulation system does not introduce condensation risk.

SuperFOIL Breathable products are W1 watertight, offering high levels of moisture protection while maintaining vapour permeability. This helps manage condensation risk within the roof structure.

The system provides:

• Breathable membrane performance

• Secondary rain barrier protection

• Integrated sealing overlaps

• Installation up to twice as fast as many traditional insulation systems

Designed to protect the structure as well as improve thermal efficiency.

Importance of Airspaces

The performance of multifoil insulation relies on correctly formed airspaces.

To achieve maximum efficiency, air gaps must be maintained on both sides of the insulation layer.

When installed correctly, the system:

• Reflects up to 95% of radiant heat

• Reduces conductive heat transfer

• Minimises convective heat movement

• Enhances overall effective R-Value

Appropriate counter batten thickness must be used to create and maintain the required ventilation cavity.

Critical Detailing Areas

Correct detailing at key junctions ensures long-term durability and weather protection.

Particular attention must be given to:

• Ridge overlaps and membrane sealing

• Hip junction alignment and trimming

• Valley detailing to ensure water runoff

• Eaves integration into tray or gutter system

All overlaps must be sealed using compatible breathable tape to maintain airtightness and water resistance.

Recommended Products for Over Rafter Applications

For over-rafter pitched roof systems, the following SuperFOIL breathable products are recommended:

• SF19BB

• SF40BB

Both products provide high-performance breathable multifoil insulation suitable for external roof applications.

Always follow manufacturer guidance to ensure correct installation, airtightness and long-term performance.

Installation Guide

For detailed installation instructions, fixing requirements and technical specifications, please refer to the official SuperFOIL Installation Guide.

Download Pitched Roof – Over Rafter Installation Guide (PDF)