Duct reinforcement is usually needed for heating, ventilating and air conditioning ("HVAC") duct and, even more likely, for an integrally reinforced rectangular duct system.

It is accepted that longer and larger cross-section HVAC ducts require transverse reinforcement spaced along their lengths.

The specific reinforcement requirements depend on many factors, including joint type, system air pressure, gauge of material and finished duct size, and are set by industry standards and building codes.

An example: The manual, HVAC Duct Construction Standards, Metal & Flexible, published by SMACNA (Sheet Metal and Air Conditioning Contractors' National Association, Inc.). The specific SMACNA standards require systematic duct reinforcement. When such reinforcement is needed, it should be in accordance with SMACNA or other applicable standards necessary.

Duct reinforcement may be necessary either at the ends of a duct section—in the form of duct section connecting joints—and/or at spaced intermediate locations along the length of a given duct section. If short duct sections are employed with the corresponding increase in the number of inherently-reinforced joints, additional intermediate reinforcement may be reduced or eliminated.

Many forms of intermediate duct reinforcement are in common use today. The most common form of reinforcement is conventional angle, zee, or channel iron. It is pre-fabricated into rectangular reinforcement shapes or brackets, then positioned around the duct, transversely, to the direction of air flow.

The duct sub-sections, e.g. the L-shaped duct components, may be nested and shipped (knocked down) to the job site for final assembly and installation. This reduces shipping costs and shipping damage.