Desiccant (IGU spacer)
Desiccant is silica gel inside the IGU spacer bar absorbing cavity moisture. When the perimeter seal fails, fogging appears between the panes.
Ask Chalkline about this →Desiccant in an insulated glass unit (IGU) is molecular-sieve silica gel beaded into the hollow spacer bar that runs around the perimeter of the unit. The desiccant absorbs the residual moisture left in the cavity at the time of manufacture and any small amounts that diffuse through the perimeter seal during normal service. As long as the perimeter seal remains intact, the cavity stays dry and clear for the design life of the unit (commonly 20-25 years).
Why a sealed cavity still needs a desiccant. Even with butyl primary sealant and polysulphide or silicone secondary sealant at the perimeter, IGUs are never truly air-tight on a long timeframe. Tiny amounts of water vapour migrate into the cavity from the seasonal atmospheric pressure and humidity swings. Without the desiccant, this moisture would condense as visible internal fogging on the inside face of the cold-side pane on cold mornings. The desiccant adsorbs the vapour and holds it indefinitely, until either it saturates (end-of-life) or the seal fails catastrophically (early failure).
Construction:
| Component | Function |
|---|---|
| Spacer bar (aluminium or warm-edge polymer/composite) | Hollow rectangular section that separates the panes 6, 9, 12, or 16 mm |
| Perforated inner face | Lets the desiccant communicate with the cavity air |
| Desiccant beads (molecular sieve, 3A grade, ~1.5 mm diameter) | Packed inside the spacer hollow |
| Primary seal (PIB butyl, 0.3-0.5 mm thick) | Vapour-tight bond between glass and spacer side face |
| Secondary seal (polysulphide, polyurethane, or silicone) | Structural bond + secondary vapour barrier |
End of life: visible fogging. The desiccant has a finite adsorption capacity. Once saturated, any further moisture condenses on the cold-side pane and the householder sees:
- A misty band along the bottom of the unit (most common, gravity pulls water there).
- Streaks or “comet tails” of dried minerals on the inside of the glass.
- Persistent foggy areas that don’t wipe off (because they’re inside the cavity).
Once visible fogging appears, the IGU is non-repairable in residential practice. The unit must be replaced. Some specialist firms offer drilling-and-redrying services on heritage IGUs, but the practice is rare in Australian residential.
Common defects:
- Damaged primary seal in transit: a corner knock breaks the butyl bond, the cavity loads up with moisture, fogging within 6-24 months.
- UV-degraded secondary seal on west-facing windows: silicone is more UV-stable than polysulphide; polysulphide can fail in 7-10 years on direct sun.
- Edge-glazed in wet glazing channel without drainage: water sits against the seal, accelerates failure.
- Argon-fill loss alongside desiccant saturation: as the seal fails, the argon fill leaks out and atmospheric moisture diffuses in; both signal the same end-of-life.
Builder takeaway:
- Inspect IGUs at delivery and reject any with visible internal moisture or damaged corners. Once installed, the warranty path is harder.
- Specify UV-stable secondary sealant (silicone) on west and north faces in Australia.
- The desiccant is invisible during normal life; the only time you “see” it is when it has failed.
Also known as: drying agent; silica gel (loose term); molecular sieve; cavity desiccant; spacer-bar fill.
Category: Materials.
Related
See also
Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.