- Round spiral duct represents the most widely-used type of spiral ductwork. According to the Spiral Duct Manufacturer's Association, or SPIDA, round spiral is available in standard sizes ranging from 3 to 60 inches in diameter. This type of duct offers better natural sound attenuation that rectangular duct, but is more difficult to insulate against noise and thermal loss.
- Flat oval duct looks like a round spiral duct, but splayed out to form an elliptical shape. Builders use this material when ceiling height is at a premium, or in existing buildings with space limitations. According to SPIDA, flat oval duct ranges from 4 to 30 inches in height, though custom sizes are also available. Flat oval ducts offer approximately the same rate of air leakage, rigidity, and a similar weight as round spiral ductwork.
- Standard spiral duct features a single layer of metal forming the outer walls, while double-walled varieties include two layers of metal. Manufacturers fill the cavity between these two walls with fiberglass insulation, which helps to reduce acoustic transmission. Insulation also cuts energy loss from the ducts, resulting in a greener building and lower heating and cooling costs. The walls of these ducts can be constructed from the same material, or different materials depending on the application, and the inner wall may be perforated to maximize sound absorption.
- Corrugated ducts offer a more highly-textured finish than standard spiral ducts. On basic units, the raised seams which join each section spiral around the duct, giving it a corrugated appearance. Those who want greater depth and aesthetic appeal can choose corrugated duct, which has an additional raised ridge between each seam. This not only makes the duct more attractive, but also gives it extra strength and rigidity. Multi-corrugated ducts features several ridges between each seam, while single-corrugated units incorporate only a single ridge per section.
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