In modern architecture, fiber cement boards are widely used as curtain wall materials. So, what are their specific types? And what specifications should be met during design? On this subject, we have compiled relevant information below. Let’s take a look.
1.1 The width of sealant-filled joints must accommodate:
Horizontal lateral displacement of the main structure and supporting system,
Board displacement due to temperature variation, and
Thermal expansion of the board itself.
Adjacent boards must not come into contact under these conditions. The minimum joint width shall be 6 mm.
1.2 Sealant selection shall be based on the curtain wall board material and joint design requirements. Joint depth shall not be less than 6 mm.
1.3 The bottom of board joints shall be filled with backer rod to prevent three-sided adhesion of the sealant.
Rationale:
Joints in closed fiber cement board curtain walls must provide sufficient width to allow for deformation of curtain wall components and the main structure.
Commonly used joint sealants include: Silicone, Modified silicone, and Polyurethane sealants. Silicone or polyurethane sealants compatible with fiber cement boards may be selected. Pre-application contamination testing is recommended to avoid board staining and ensure aesthetic integrity.
Per ASTM C1193-00, Standard Guide for Use of Joint Sealants, typical joint widths for fiber cement board curtain walls range from 6–13 mm. Section 1.2 therefore specifies the minimum sealant depth, though actual dimensions shall be determined based on project-specific conditions.
Three-sided bonding of sealant reduces its displacement capacity and increases susceptibility to tearing. The width-to-thickness ratio of the sealant bead affects its performance: too thin—low waterproofing ability; too thick—reduced displacement capacity and higher risk of rupture. A recommended width-to-thickness ratio is 2:1.
Given the relatively thin profile of fiber cement boards, special attention shall be paid to the distance between the outer surface of the sealant and the board face when designing deep joints. If this distance is too small, appearance may be compromised; if the sealant layer is too thin, bond strength, waterproofing performance, and joint integrity may be reduced, leading to potential leakage.
Therefore, the distance between the sealant surface and board face should be determined according to the board thickness and joint width.
The seam design for gasket-sealed engineered board curtain walls shall comply with the following requirements:
2.1 Sealing gaskets shall be made from EPDM, neoprene, or silicone rubber products, and shall conform to the current national standard Sealing Strips for Building Doors, Windows, and Curtain Walls (GB/T 24498).
2.2 “Cross” joints between curtain wall boards or unitized boards shall be specially designed with necessary structural measures to prevent rainwater infiltration.
2.3 For sealing joints in unitized boards, it is advisable to use multiple gaskets for sealing.
Rationale:
Rubber seals used for board joints must possess good weather resistance. The “cross” joint between curtain wall boards or unitized boards is a location prone to water leakage. Special attention must be paid to the design of “cross” joints, and appropriate waterproofing measures shall be taken.
Using multiple sealing strips in unitized board joints is an effective structural measure to improve the watertightness of curtain walls.
The design of open joints in engineered board curtain walls shall comply with the following provisions:
3.1 The joint width between curtain wall boards shall not be less than 8 mm.
3.2 The cavity behind the boards shall be designed to prevent water accumulation and shall include effective drainage measures.
3.3 Insulation materials installed on the back of the boards shall be water-resistant and ventilated to allow moisture vapor to escape effectively.
3.4 Supporting structures and metal connectors shall be provided with effective corrosion protection or be suitably thickened to allow for corrosion allowance.
The design of board seams for rain-screen engineered board curtain walls shall meet the following requirements:
4.1 The minimum overlap width between boards shall satisfy anti-penetration requirements to prevent significant rainwater ingress into the curtain wall. The cavity behind the boards must be designed to avoid water accumulation, with effective drainage measures provided.
4.2 Adequate clearance for relative displacement shall be maintained between the board and the embedded support strips.
4.3 Vertical board joints shall be designed as lapped (shiplap) connections, while horizontal joints shall be designed as open-drained joints or lapped joints.
4.4 When both vertical and horizontal board joints employ lapped (shiplap) connections, ventilation openings of adequate area shall be provided at the bottom and top of the curtain wall to establish an effective pressure-equalized and ventilated cavity.
4.5 Insulation materials installed behind the boards shall be water-resistant and vapor-permeable to allow for effective moisture dissipation.
4.6 Supporting structures and metal connectors shall be provided with effective corrosion protection or be suitably increased in thickness to account for material loss due to corrosion.
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