Flat roof insulation installed under the joists creates a thermal layer at ceiling level, improving internal energy performance while maintaining the existing external roof structure.

This system is typically used in refurbishment projects or where insulation is being added internally without removing the roof covering.

Under-joist installations commonly form part of a cold roof construction, where ventilation above the insulation layer is essential to prevent condensation build-up.

Cold Roof Construction – Critical Ventilation Requirement

In cold flat roof constructions, UK Building Regulations require a clear ventilation gap of at least 50 mm between the insulation layer and the underside of the roof deck.

This ventilation cavity:

• Allows moisture to dissipate

• Prevents interstitial condensation

• Protects the structural timber elements

• Ensures long-term durability of the roof

When additional insulation is installed between joists (such as mineral wool or PIR boards), the remaining joist depth must allow:

• 50 mm ventilated cavity

• Thickness of additional insulation

• Required airspace for the selected SuperFOIL product

The required airspace refers to the minimum continuous cavity created by appropriately sized battens, allowing the multifoil insulation to function effectively as a reflective radiant barrier.

Failure to maintain this ventilation zone may result in condensation risk and reduced structural lifespan.

How the Under Joist System Works

Installation begins by selecting one edge of the ceiling to start the run.

The insulation is:

• Installed horizontally across the underside of the joists

• Run parallel to the previous layer

• Overlapped by a minimum of 50 mm

• Stapled under gentle tension

• Fixed at intervals not exceeding 300 mm

• Fully sealed using SuperFOIL MultiFOIL Tape

Removing slack before stapling is critical to ensure proper performance.

This creates a continuous internal thermal barrier beneath the joist structure.

Counter Battens & Ceiling Finish

To complete installation:

• Timber battens are fixed through the SuperFOIL into the joists

• Battens must be installed at maximum 600 mm centres

• Batten size must match the selected SuperFOIL product thickness

• 12.5 mm plasterboard is then installed onto battens

Timber battens must be appropriately sized to maintain the required internal airspace and to provide a secure fixing base for plasterboard installation.

Importance of Airspaces

Airspaces are fundamental to multifoil performance.

To maximise efficiency:

• Correct batten depth must be used

• Airspace must be maintained beneath the insulation

• Ventilation above insulation must be preserved in cold roof designs

Correct batten depth depends on the selected SuperFOIL product. As a general guide:

Batten depth should generally match or exceed the selected SuperFOIL product thickness in order to prevent compression and to create the required reflective airspace.

Internal Batten Structure

Battens are fixed beneath the insulation to form the required internal airspace and service void.

Battens are installed beneath the insulation to create the required internal airspace and a service void, allowing installation of services without penetrating the insulation layer.

Batten depth must:

• Prevent compression of the SuperFOIL
• Maintain continuous airspace beneath the insulation
• Allow correct positioning of plasterboard

In cold roof designs, ventilation above the insulation must be preserved (typically 50 mm ventilated cavity between insulation and deck).

The final batten depth must be determined based on product thickness and condensation risk design.

Importance of Correct Airspace

Without an adequate airspace:

• Radiant heat converts into conductive heat

• Insulation performance is significantly reduced

Maintaining correct airspaces allows SuperFOIL to reflect up to 95% of radiant energy and achieve its designed thermal performance.

Sealing & Airtightness

To ensure a fully airtight installation:

• All overlaps must be taped

All overlaps must be taped using SuperFOIL Aluminium Tape.
For breathable insulation products, appropriate double-sided adhesive tape may also be used where required to ensure a continuous seal.

• All cut edges must be sealed

• Ceiling, wall and floor junctions must be sealed

• Double-sided tape may be used to enhance sealing

• All penetrations must be airtight

Improper sealing may cause:

• Air leakage

• Reduced thermal efficiency

• Increased condensation risk

Service Penetrations & Electrical Points

Where services pass through insulation:

• Carefully cut minimal openings

• Feed services through

• Seal fully using SuperFOIL MultiFOIL Tape

• Ensure airtightness is maintained

Copper pipes must be wrapped in non-metallic insulation before contact with aluminium foil to prevent reaction.

All electrical work must comply with relevant building and electrical regulations.

Roof-to-Wall Junction

To prevent thermal bridging:

• Overlap at least 50 mm onto wall insulation

• Fix using mastic or double-sided tape (optional)

• Seal with reflective tape

• Secure with perimeter battens

This creates a continuous thermal envelope across junctions.

Window & Roof Light Detailing

Where insulation meets window reveals or roof lights:

• Measure reveal depth

• Cut insulation accordingly

• Seal all cut edges before folding into reveal

• Install battens/shims around frame perimeter

• Ensure total build-up does not exceed reveal depth

All additional strips must be sealed fully to maintain airtightness.

Cutting & Handling

When cutting SuperFOIL:

• Compress insulation before cutting

• Use straight edge for accuracy

• Seal all cut edges with tape

Recommended tools include heavy-duty shears, utility knives or rotary cutters.

SuperFOIL must not be walked on. It does not support body weight and must be handled with care.

Additional Insulation (Hybrid Internal Build-Up)

Where additional insulation is installed between joists:

Ensure remaining joist depth allows:

• 50 mm ventilated cavity

• Thickness of additional insulation

• Required SuperFOIL airspace

Professional condensation risk assessment is strongly recommended for hybrid constructions.

Installation Checklist

Before completion ensure:

• Product cut and trimmed correctly

• Ends sealed before installation

• Staples spaced no more than 300 mm

• All overlaps taped

• Counter battens installed at max 600 mm centres

• Ventilated cavity maintained (minimum 50 mm where required)

• Plasterboard securely installed

Photographic documentation during installation is recommended.

Recommended Products for Flat Roof – Under Joist

For under-joist flat roof systems, the following SuperFOIL multifoil products are typically recommended:

• SF19
• SF40
• SF60

These products provide high-performance internal multifoil insulation suitable for ceiling-level installations.

Always verify required U-Value using the official SuperFOIL U-Value Calculator prior to specification.