# GI Duct — Gauge, Construction and Leakage Class

> GI duct is galvanised steel sheet ductwork, fabricated into rectangular or round sections to distribute conditioned air in commercial buildings. Its sheet gauge and stiffeners follow duct size and pressure class, and sealing every joint matters because leaked air is capacity paid for and never delivered.

## 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**.

## Frequently asked questions

### What gauge should ductwork be?

It follows from the larger duct dimension and the operating pressure class, not from habit. Larger and higher-pressure ducts need thicker sheet and closer stiffeners, and SMACNA tables set the relationship. Using one gauge throughout a project is common on Indian sites and produces oversized ducts that drum and flex at low gauge, or unnecessary cost at high gauge on small runs.

### Why does duct leakage matter so much?

Because air escaping before it reaches the room is capacity paid for and not delivered. Leakage of 10 to 15 percent is common in poorly sealed systems, which means the chiller, the fan and the electrical supply are all sized for air the occupants never receive. Sealing joints properly is one of the cheapest efficiency measures available on any fit-out, and ECBC 2017 sets requirements for it.

### Should ducts be factory fabricated or made on site?

Factory fabrication on automated line machinery gives more consistent gauge, cleaner seams and tighter tolerances than hand fabrication on a site floor, and it is faster. Site fabrication persists because it is flexible when coordination is late. On projects where the ceiling void is congested, coordinated shop drawings and factory ducts almost always finish better.

## Sources

- [IS 277](https://standards.bis.gov.in/website/published-standards/department-wise) — Galvanised steel sheets
- [SMACNA HVAC Duct Construction Standards](https://www.smacna.org/) — Gauge, reinforcement and sealing by pressure class
- [ECBC 2017](https://beeindia.gov.in/) — HVAC — duct sealing and insulation

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Source: https://hagerstone.com/materials/gi-duct