How should curtain wall drainage and sealant regimes be detailed to prevent water ingress and staining?

Preventing water ingress and staining in curtain walls requires a layered drainage and sealant regime engineered for redundancy. The primary principle is to keep water out of the building interior by using multiple lines of defence: external gaskets and primary seals to repel bulk water, internal drainage cavities and back-pans to collect and discharge any infiltrated water, and pressure equalisation to prevent driving water into joints. Detail horizontal members with continuous internal gutters and weep locations that lead to protected external outlets; slope sills and set internal back-dams to avoid ponding. Sealant regimes must be specified with compatible materials; use structural silicones where structural adhesion is required and high-performance polyurethane or hybrid sealants for movement joints where adhesion plus flexibility is needed. Crucially, select primer and substrate combinations approved by sealant manufacturers and control joint geometry (depth to width ratios) to ensure optimal elongation and adhesion. Avoid placing sealant in direct contact with dissimilar metals that can stain glazing or façade finishes; incorporate sacrificial flashings and drip edges to direct runoff away from visible surfaces. For metal curtain walls, specify corrosion-resistant coatings and ensure that fasteners and spacers are non-staining materials; isolated contact points or protective washers can prevent metal staining from runoff. Provide details for capillary breaks at vertical joints and ensure that façade components are accessible for periodic resealing. Finally, require mock-ups and water tests to validate drainage paths and sealant performance before full installation, and document a maintenance regime for joint inspection and reseal at manufacturer-recommended intervals to preserve long-term watertightness and aesthetic integrity.