Warm Edge Spacer Bar in Insulated Glass Units

What it is

The perimeter bar that separates the two panes of an insulated glass unit, holds the desiccant that keeps the cavity dry, and carries the seals that keep moisture out.

It is the component that decides how long a double-glazed unit lasts.

Specification at a glance

Property	Typical specification
Material	Stainless, thermoplastic, or composite
Conductivity	Low; "warm edge" class stated
Desiccant	Molecular sieve within the spacer
Primary seal	Polyisobutylene, the vapour barrier
Secondary seal	Silicone or polysulphide, the structural bond
Testing	EN 1279 or ASTM E2190

The two seals do different jobs

This is the detail that matters and is routinely misunderstood:

• The primary seal (PIB) is the moisture barrier. It stops water vapour entering the cavity.
• The secondary seal holds the two panes together structurally and protects the primary seal.

For structurally glazed facades the secondary seal must be silicone, because polysulphide is not UV stable and structural glazing exposes the edge seal to sunlight. Using a polysulphide unit in a structurally glazed facade is a known, repeated and expensive failure.

Where projects go wrong

• Aluminium spacers in air-conditioned buildings, then condensation complaints that get blamed on the AC.
• Polysulphide secondary seal in a structurally glazed or exposed-edge application.
• Undrained glazing rebate. Standing water attacks the secondary seal and the unit fogs in a few years.
• Spacer unspecified, so the cheapest option is supplied silently.
• Units stored flat or in sun on site before installation, stressing the seal before the building is even complete.

Standards referenced

Insulating glass unit durability testing under EN 1279 and ASTM E2190. Fenestration U-value targets under ECBC 2017.