Foil Backed Plasterboard

What Is Foil Backed Plasterboard?

Foil-backed plasterboard is a conventional gypsum board with a factory-laminated sheet of aluminium foil bonded to its rear face.

Everything you see from the room-side - the ivory paper lining, the gypsum core, the tapered or square edge - is identical to standard board; the only difference is the shiny metallic layer on the back.

Composition and How It Works

Foil backed plasterboard fundamentally consists of three main components working in synergy:

• Gypsum Core: This is the heart of the board, made from calcium sulphate dihydrate. The gypsum core provides the structural integrity, rigidity, and inherent passive fire resistance common to all plasterboards. It's a stable, non-combustible material that also contributes to acoustic damping.
• Paper Liners: Standard paper liners encase the gypsum core, providing a smooth surface for decoration, tensile strength, and facilitating adhesion for jointing compounds.
• Aluminium Foil Backing: This is the distinctive feature. A thin, reflective layer of aluminium foil is permanently laminated to the back face (the side installed facing the cavity or insulation).

Applications

Foil backed plasterboard is particularly beneficial in applications where thermal performance and moisture control are important considerations:

• External Walls (Internal Linings): Widely used as the internal lining for external walls, both in new construction and refurbishment. It helps improve the wall's U-value and acts as a VCL to prevent condensation within the wall cavity, especially in cold climates.
• Ceilings Below Unheated Spaces: Ideal for ceilings beneath unheated roof spaces (e.g., cold lofts) or unheated floors, enhancing thermal insulation and preventing moisture from migrating into the void above.
• Room-in-Roof Applications: Where a loft space is being converted into habitable rooms, foil backed plasterboard is an excellent choice for lining rafters and ceilings, contributing to both thermal performance and condensation control.
• Behind Existing Insulation: Can be used on the warm side of existing cavity insulation or internal frame insulation to add an extra layer of thermal resistance and a critical vapour barrier.
• Garages and Outbuildings (Connected to Habitable Space): Used to line walls and ceilings that separate a heated interior from an unheated garage or utility space, maintaining thermal boundaries and preventing condensation.

Benefits

• Improved Thermal Efficiency: The reflective foil significantly enhances the overall thermal performance of walls, ceilings, and roofs by reducing radiant heat transfer. This leads to better heat retention and reduced energy consumption for heating and cooling.
• Effective Vapour Control: The foil acts as an excellent VCL, mitigating the risk of condensation within wall and roof structures, protecting timber elements and insulation from moisture damage.
• Space Saving: By contributing to the overall thermal resistance, it can sometimes allow for slightly thinner primary insulation materials to achieve required U-values, thereby maximising internal space.
• Cost-Effectiveness: It offers a dual benefit (thermal enhancement + vapour control) in a single board, potentially simplifying installation and reducing the need for separate VCL membranes.
• Versatility: Available in common thicknesses (e.g., 9.5mm, 12.5mm, 15mm) and can be used in various applications where thermal and vapour performance are critical.
• Ease of Installation: Can be cut, fixed, and finished using standard plasterboarding techniques, with minor considerations for the foil layer.

How the Foil Enhances Performance

• Thermal Reflection (Radiant Barrier): The primary mechanism is its ability to reduce radiant heat transfer. The low-emissivity surface of the foil reflects heat radiation, preventing heat from escaping into cold cavities in winter and blocking external heat from entering during summer. For this reflective property to be fully effective, the foil surface typically needs to face an airspace or cavity (at least 20mm wide).
• Vapour Control Layer (VCL): The foil layer acts as a highly effective vapour control layer. It significantly restricts the movement of water vapour from the warmer, more humid interior of a building into the cooler wall or roof cavities. This prevents interstitial condensation, which can lead to timber rot, insulation degradation, and mould growth within the building fabric.

Installation Best Practices for Foil Backed Plasterboard

Correct installation is crucial for the foil backing to perform its intended functions effectively.

• Foil Orientation: The foil-backed side must always face the warm side of the construction and typically needs to face an airspace (at least 20mm wide) to maximise its radiant barrier properties. It should face the cavity or insulation.
• Airtightness and Joint Sealing: To function effectively as a VCL, the foil layer must be continuous and airtight. 
• Joints: All joints between boards must be meticulously sealed using foil-backed jointing tape or a suitable airtight sealing tape. The tape must completely cover the joint and adhere well to the foil.
• Perimeters: All perimeters where the board meets adjacent structures (floor, ceiling, other walls, windows, doors) must be sealed using an appropriate airtight sealant (e.g., mastic, adhesive tape).
• Penetrations: Any service penetrations (electrical sockets, pipes, light fittings) must be carefully sealed using airtight gaskets, grommets, or intumescent collars/sealants to maintain the integrity of the vapour control layer.
• Fixing: Boards are fixed using standard drywall screws to timber or metal framing. Ensure screws are driven just below the surface without tearing the paper or puncturing the foil backing unnecessarily, as this compromises the VCL.
• Finishing: Once fixed, the visible paper face is jointed and finished using standard plasterboard jointing compounds and techniques, ready for decoration.

Any breaches in the foil or unsealed joints and penetrations will significantly reduce the effectiveness of the vapour control layer, potentially leading to condensation problems.