How Double Glazing Seals Work
A sealed unit has two barriers against moisture and gas loss. The primary seal is a thin bead of polyisobutylene (PIB) applied directly to the spacer bar — this is the gas-tight barrier that keeps argon in and moisture out. The secondary seal is a structural sealant (polysulphide, silicone, or polyurethane) that bonds the glass to the spacer and provides mechanical strength.
When the primary PIB seal is breached, gas begins to escape and moisture enters. The desiccant inside the spacer bar absorbs moisture initially — this is why seal failure doesn't produce immediate misting. Once the desiccant is saturated (typically weeks to months after seal breach), visible condensation appears.
Why Seals Fail
The most common causes of premature seal failure are: UV degradation (the secondary seal breaks down under prolonged UV exposure, especially on south-facing elevations), thermal cycling (daily temperature swings create mechanical stress at the seal-glass junction), water standing on the seal (poor frame drainage allows water to sit on the seal, accelerating degradation), and manufacturing quality (poor PIB application or contaminated glass surfaces during assembly).
Specifying Longer-Lasting Replacements
When replacing a failed unit, specify improvements that extend the seal life of the new unit:
Warm-edge spacers reduce thermal stress at the seal junction by up to 65%. Less thermal cycling means less mechanical stress on the PIB seal — directly extending seal life.
Polysulphide secondary seal (rather than hot-melt butyl) provides superior UV resistance and structural adhesion. All our units use polysulphide as standard.
Proper drainage — advise your customer to ensure frame drainage is clear. Standing water is the number one accelerator of seal failure. Check weep holes during installation.