# DGU: What Goes Into a Double Glazed Unit

> A double glazed unit is two panes of glass separated by a sealed, dried air or argon cavity held apart by a spacer bar. The cavity is what insulates; the edge seal is what keeps the cavity dry, and when that seal fails the unit fogs permanently.

## Why it matters on a project

Most conversations about DGU are about the glass. Most DGU failures are about
the edge.

The two panes and the coating determine how the unit performs on day one. The
spacer, the desiccant and the primary and secondary seals determine whether it
still performs in year twelve — or whether it has fogged and has to be replaced
at height, on a sealed facade, at many times its original cost.

## How it is specified in practice

| Component | Typical position | Why |
| --- | --- | --- |
| Outer pane | 6-12mm, coated on surface 2 | Coating reflects solar gain, protected inside the cavity |
| Cavity | 12-16mm, air or argon | Below 12mm insulates poorly; above 16mm gains little |
| Spacer | Aluminium or warm-edge polymer | Warm edge cuts the thermal bridge at the perimeter |
| Desiccant | Inside the spacer | Absorbs residual moisture so the cavity stays clear |
| Primary seal | Polyisobutylene | The actual gas and vapour barrier |
| Secondary seal | Silicone or polysulphide | Structural bond holding the unit together |
| Inner pane | 6-10mm, often toughened | Safety requirement depends on location and use |

## Common mistakes

- **Specifying glass and ignoring the edge seal.** The unit's service life is
  set by seal quality, not by the coating.
- **Using polysulphide behind structural silicone glazing.** Silicone secondary
  seal is required where the DGU is structurally bonded; polysulphide is not
  compatible with that application.
- **Skipping setting blocks or placing them wrong.** The unit must sit on
  correctly positioned blocks. Point loads at the edge crack glass and stress the
  seal.
- **Sealing the glazing rebate with no drainage.** Water sitting in the rebate
  attacks the secondary seal — one of the most common causes of early fogging in
  Indian monsoon conditions.
- **Assuming argon is always worth it.** It gives a modest gain and leaks slowly
  over time. Coating choice usually delivers more per rupee in a cooling climate.

## Standards referenced

Fenestration U-value and SHGC targets by climate zone are set by **ECBC 2017**.
Safety glass requirements, including where toughened or laminated panes are
mandatory, follow **IS 2553 (Part 1)**.

## Frequently asked questions

### Is DGU worth it in India, or is it only for cold countries?

It is worth it for air-conditioned commercial buildings, and ECBC targets in most climate zones are difficult to meet with single glazing. The gain is different from a cold climate though — here DGU mainly cuts cooling load and stops condensation on the inner pane, rather than retaining heat. For naturally ventilated buildings the payback is weak.

### Does the coating go on the outer or inner pane?

For solar control in a cooling climate, the low-E or solar control coating normally sits on surface 2 — the cavity-facing side of the outer pane — so it reflects solar energy before it enters the cavity. Coatings are also fragile and belong inside the sealed cavity, not on an exposed face.

### Why do DGUs fog up after a few years?

Almost always edge seal failure. The desiccant inside the spacer absorbs residual moisture, but once the seal is breached it saturates and moisture condenses inside the cavity. It cannot be cleaned or repaired — the unit is replaced. This is why edge seal quality and correct setting blocks matter far more than the headline glass specification.

## Sources

- [ECBC 2017](https://beeindia.gov.in/) — Building envelope — fenestration
- [IS 2553 (Part 1)](https://standards.bis.gov.in/website/published-standards/department-wise) — Safety glass — architectural

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Source: https://hagerstone.com/glossary/double-glazed-unit-dgu