GI Duct — Gauge, Construction and Leakage Class

What it is

Galvanised steel sheet ductwork, fabricated into rectangular or round sections to distribute conditioned air. The default air distribution method in Indian commercial buildings.

Specification at a glance

Property	Typical specification
Material	Galvanised steel to IS 277, coating stated
Gauge	By duct size and pressure class per SMACNA
Joints	Slip and drive, or flanged systems
Stiffeners	Spacing by size and pressure
Sealing	Sealant or gasket at every joint
Leakage class	Stated and tested on higher-pressure systems

What the specification must state

• Coating mass of the galvanised sheet, not just "GI"
• Gauge table referenced, by size and pressure class
• Joint system, since flanged systems seal better than slip and drive
• Sealant type and where it is applied
• Leakage class and whether testing is required
• Insulation type, thickness and facing, separately

Where the money is lost

Duct leakage is invisible and expensive. Air lost through unsealed transverse joints, unsealed longitudinal seams and poorly made take-offs is air the fan moved and the chiller cooled for nothing. On a fit-out where the tenant pays the running cost for a decade, sealing is the highest-return detail in the whole HVAC package.

Where projects go wrong

• One gauge used throughout, regardless of duct size.
• Joints unsealed because the ceiling is going up and nobody will see them.
• Flexible duct used for long runs instead of short final connections, adding large pressure losses.
• Stiffeners omitted on large ducts, so they drum audibly under fan pressure.
• Insulation compressed by hangers, losing thickness where it is needed.

Standards referenced

Galvanised sheet under IS 277. Duct construction, gauge and sealing per SMACNA HVAC Duct Construction Standards. Duct sealing and insulation requirements under ECBC 2017.