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.
By Dhruv Agarwal · · 2 min read
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).
Standards referenced
- ECBC 2017 — Building envelope — fenestration
- IS 2553 (Part 1) — Safety glass — architectural