Building codes require duct system joints to be made substantially which of the following?

Building codes mandate that duct system joints must be made substantially airtight to ensure the efficient operation of the HVAC system. When duct joints are airtight, it prevents the loss of conditioned air, maintains air pressure within the system, and reduces the potential for air leakage. This is crucial not only for energy efficiency but also for maintaining proper airflow and indoor air quality. An airtight duct system helps to prevent the introduction of contaminants that can lead to health issues or decreased comfort levels in a building.

While waterproofing, insulation, and secure fastening are also important factors in ductwork installation, they do not address the primary concern of air leakage, which is why being airtight is a specific requirement set by building codes. Waterproofing can be relevant in specific contexts, such as in areas prone to moisture, but it is not a general requirement for all duct joints. Insulation is essential for energy efficiency and controlling the temperature of the air within the ducts, whereas securing the ducts is vital for structural integrity and stability, but these aspects do not take precedence over the need for the joints to be airtight.