Sealant

Definition and Function in Cladding Systems#

In the context of the UK building envelope sector, a sealant is a material used to fill and seal gaps in building joints, preventing moisture ingress while accommodating differential movement between components. Unlike adhesives, which are primarily designed to bond surfaces together to resist structural loads, sealants are formulated to maintain a physical barrier against environmental elements—specifically wind-driven rain, air infiltration, and particulate matter—while remaining flexible over a sustained period.

Within external wall systems, sealants occupy the interfaces between cladding panels, around window and door apertures (fenestration), and at transitions between disparate building materials, such as the junction between a masonry plinth and a rainscreen system. The efficacy of a sealant is determined by its ability to adhere to the substrates on either side of a joint while stretching or compressing as those substrates expand and contract due to thermal changes or structural settlement.

Technical Composition and Material Selection#

The selection of a sealant for cladding applications depends on the specific performance requirements of the joint and the chemistry of the surrounding materials. In the UK market, several chemical bases are prevalent:

• Silicone: Often favoured for its UV resistance and high movement capability. It is frequently used in glazing and high-movement expansion joints.
• Polyurethane (PU): Known for its toughness and abrasion resistance, often utilised in floor joints or areas where over-painting is required, as standard silicones generally do not accept paint.
• Modified Silane (MS) Polymers: Hybrid materials that combine the weatherability of silicones with the paintability and adhesion properties of polyurethanes.
• Acrylics: Primarily used for internal perimeter sealing where movement is minimal, as they lack the elasticity required for external cladding interfaces.

The performance of these materials is codified under BS EN ISO 11600, which classifies sealants based on their intended use (Type G for glazing, Type F for building construction) and their movement capability, expressed as a percentage. A sealant classified as 25LM, for example, can accommodate movement of up to 25% of the original joint width and has a low modulus (LM), meaning it exerts less force on the bond line when stretched.

Role in Testing and Condition Surveys#

When assessing the integrity of an existing cladding system, the condition of the sealants is a critical metric. During a facade condition survey or a pre-acquisition inspection, professionals evaluate sealants for signs of degradation, which may include 'crazing' (fine surface cracks), loss of elasticity, or adhesive failure (where the sealant pulls away from the substrate) and cohesive failure (where the sealant tears internally).

In the context of weather tightness testing, such as those conducted in accordance with CWCT (Centre for Window and Cladding Technology) standards, sealants are often the primary line of defence. If a building is undergoing a hose pipe test or a dynamic aero-engine test to identify leaks, the failure point is frequently located at a degraded sealant joint or a poorly executed interface. Consequently, testing protocols often necessitate the extraction of sealant samples to verify their current state of flexibility and adhesion compared to the original design specification.

Regulatory Context in England and Wales#

The application and performance of sealants are governed by the Building Regulations, specifically Approved Document B (Fire Safety) and Approved Document L (Conservation of fuel and power). Under Approved Document L, the continuity of the building envelope is paramount for air tightness; degraded or improperly installed sealants can lead to excessive air leakage, compromising the thermal efficiency of the building.

Regarding fire safety, the role of sealants in external walls is subject to the requirements of Regulation 7(2) for relevant buildings over a certain height, and the general functional requirements of Requirement B4. While sealants are often considered 'attachments' or 'minor components' in some regulatory contexts, their combustibility and their role in maintaining the integrity of fire-stopping systems are significant. In many rainscreen applications, the sealant is not the primary weather barrier—this role is often filled by a breather membrane or the backing wall—but the sealant remains vital for managing the drainage and ventilation characteristics of the cavity.

Repair and Replacement Considerations#

When a cladding system reaches the end of its service life or is identified as requiring remediation, sealant replacement (often termed 're-caulking' or 'resealing') is a common maintenance task. This process is more complex than it appears; simply applying new sealant over old material is generally considered poor practice, as the new sealant may fail to bond to the degraded substrate or the contaminated surface of the old material.

Effective replacement involves the total removal of the existing sealant, mechanical cleaning of the joint faces, and the application of a primer if specified by the manufacturer. Furthermore, the use of a 'backing rod' (a foam strip inserted into the joint) is essential to ensure the sealant is only bonded to the two opposing sides of the joint, rather than the back of the cavity. This 'two-sided adhesion' allows the sealant to stretch like a rubber band; if it bonds to the back of the joint (three-sided adhesion), it will tear when the building moves.

Information for Building Owners and Managers#

For those responsible for the management of high-rise or complex buildings, such as RMC/RTM directors and managing agents, understanding the lifecycle of sealants is necessary for long-term financial planning. Sealants are considered 'consumable' components of the facade, with typical lifespans ranging from 10 to 25 years depending on the material quality and environmental exposure.

Professional guidance should be sought to determine if sealant failure is an isolated issue or symptomatic of wider systemic movement or moisture problems within the external wall system.

Common Misunderstandings and Related Terms#

A frequent misunderstanding is the confusion between sealants and gaskets. While both serve to seal joints, gaskets are pre-formed strips of EPDM or neoprene that are mechanically compressed into a joint, whereas sealants are applied in a liquid or paste form and cure in situ.

Another common error is the assumption that a sealant is a permanent waterproofing solution for a failing facade. If the underlying drainage strategy of a cladding system is flawed, or if water is bypassing the sealant via internal structural paths, 'over-sealing' the external joints may actually trap moisture within the wall cavity, potentially leading to interstitial condensation or accelerated corrosion of the sub-frame.

Related terminology includes:

• Adhesion: The ability of the sealant to stick to the substrate.
• Cohesion: The internal strength of the sealant itself.
• Substrate: The surface (e.g., aluminium, brick, stone) to which the sealant is applied.
• Tooling: The process of smoothing the sealant surface to ensure full contact with the joint edges.
• Primer: A chemical coating applied to the substrate to enhance the bond of the sealant.