Weathertightness and Failure Mitigation

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In a series of bitesize articles Wintech’s Expert Witness Director Nadha Dawood, draws upon her extensive experience in the field and shares her insight on some of the challenges impacting the façade industry. In this instalment, we delve into the complexities of watertightness failures in building façades and explore the critical factors that contribute to their occurrence.

Weathertightness failure is a frequently observed defect in building façades, often stemming from poor design, poor installation, insufficient maintenance, or deterioration to name a few. As much as weathertightness holds considerable significance in façade performance, it frequently garners the most complaints when it occurs. Whilst the Building Regulations stipulate performance requirements for weathertightness in functional terms, the precise methods to achieve these standards often lack clear definition.

Usually, its importance and failure tolerance are dependent on the occupants and buildings’ use. However, in all cases, it is not the failure that is to be considered, but its effect on its occupants and subsequent damage to the building.

Weathertightness Failures

Typically, weathertightness failures are likely to occur due to defective design at interfaces and/or failure of:

• Cladding systems
• Joint seals and membranes
• Door and window systems

These deficiencies can give rise to a range of issues, which include:

• Water Infiltration: When water penetrates the building envelope, it can damage building materials, structural components, and interior finishes. This can result in mould growth, rot and deterioration of the building’s components.

• Air Leakage: Inadequate watertightness can increase the amount of uncontrolled air infiltration into the building. This can lead to increased energy consumption as heating or cooling systems must work harder to maintain indoor comfort.

• Structural Damage: Prolonged exposure to moisture due to water ingress can compromise the structural integrity of a building. This can lead to the accelerated corrosion of metal components, deterioration and weakening of structural elements thus reducing their design life. The unseen corrosion of such elements can lead to safety critical failures.

• Aesthetics and Finishes: Water ingress can damage interior finishes, such as drywall, paint, and flooring, leading to expensive repairs and maintenance.

• Health Concerns: Mould and mildew growth resulting from air and water ingress and subsequent condensation poses health risks to occupants, particularly those with respiratory issues or allergies.

Face Seals without Secondary Weather Defence

Face seal designs rely upon a single line of weathertightness, often demonstrated by a silicone seal of specified dimensions. This seal is positioned at cladding panel joints, interfaces between window/door openings and adjoining cladding components, and between coping panels at the roof level.

This approach functions effectively as long as the silicone seal remains intact and performs its intended role. However, in cases where the silicone seal deteriorates due to defects or disrepair, there is no secondary defence against the infiltration of air and water. Consequently, this can result in damage to interior finishes and a decline in performance.

Best Practice

While not mandated by industry standards, it is considered best practice to incorporate two lines of weather defence strategy. This consists of a primary seal, typically an external silicone seal, and a secondary seal, such as an internal membrane or a cavity seal with a drained cavity in between.

This dual approach helps prevent the ingress of air and water that might bypass the primary seal. Generally, a well-installed secondary seal tends to have a longer service life as it benefits from protection against external environmental factors.

Detailing of Membranes at Interfaces

When membranes are applied at the interfaces between openings and the cladding structure, insufficient attention to detail during installation can result in gaps that permit the ingress of air and water.

Frequently, even though waterproofing membranes are correctly installed at the jambs and sills, a crucial weathertight corner feature, such as overlapping membranes or a preformed corner profile as noted below, is often overlooked or absent. Consequently, despite the potential presence of a secondary defence layer, this deficiency can still result in a failure to maintain weathertightness.

Defective Sealants/Membranes leading to Failure

Sufficient attention to detailing and installation alone does not guarantee a weathertight interface. Equally crucial is the compatibility of the products used to seal these interfaces with the adjacent membranes, finishes, and surfaces, along with proper surface preparation. The lack of compatibility between these materials or inadequate surface preparation frequently results in adhesive failure, effectively undermining the weathertightness.

Adhesive failure becomes evident when the sealant/membrane loses its adhesion to one of the surfaces, leading to delamination. While these failures are often visible externally, allowing for remedial action, in some cases, they remain concealed, going unnoticed until water leakage becomes apparent internally, typically at a distance from the initial point of entry.

Design of Joint Widths

When a building experiences excessive movement beyond what the applied sealant is designed to accommodate, it can lead to both adhesive and cohesive failures within the sealant material itself.

This type of failure typically occurs when the cladding and/or sealant is not engineered to elongate or compress adequately during thermal expansion and/or building movements. In such cases, even a correctly adhered sealant or installed panel can undergo tearing or separation.

All anticipated tolerances and movements should be considered in the design of the joints to mitigate against premature failure due to lack of movement accommodation.

Construction and Installation Tolerances

When construction and installation tolerances deviate from the specified limits on the construction site, the joints between panels or at interfaces may exhibit varying widths. These variations might not align with the design parameters of the sealant, which has its own limitations. It is imperative to install sealant joints with two edges to allow adequate movement, typically achieved by the use of a backing rod/bond breaker tape.

Unfortunately, on-site when either larger or smaller gaps exceeding tolerance limits are identified, this often goes unnoticed as a design variation. Instead, in most cases, more sealant is applied to fill the larger gap, or, in the worst case scenario, a small gap may be left unsealed. Foe example, low modulus sealants cannot be applied within joints less than 6mm.

In both situations, these practices have adverse effects on performance. Furthermore, in the absence of a secondary defence mechanism, this defect can ultimately result in weathertightness failures.

Conclusions

For a successful diagnosis and remediation of weathertightness issues, experience in and understanding of several critical elements is essential, which include but is not limited to:

• Cladding details and interface design
• Properties and compatibility of materials used in the cladding structure
• An understanding of the environmental conditions and external influences on joint seals
• Functionality of joint seals within the building structure and building movements
• Specific properties and attributes of sealing materials
• Remedial measures and knowledge of the available solutions for addressing weathertightness concerns

To mitigate and prevent weathertightness issues, it is crucial to utilise sound building design, construction methodologies, and materials that exhibit resistance to environmental elements. Consistent maintenance and inspections serve to identify and rectify potential concerns before they escalate into substantial problems. Building codes and industry benchmarks typically incorporate specifications pertaining to weathertightness, ensuring that new constructions conform to the prescribed performance standards. However, retrofitting existing buildings with enhanced weathertightness measures can also effectively address defects and augment the overall performance and durability of the structure.

Related Resources:

Glass Risk Assessment →

The Run-Up to Gateway 3 →