Warm Edge Spacer Bar in Insulated Glass Units

A warm edge spacer bar is the low-conductivity 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. Compared with an aluminium spacer it keeps the glass edge warmer, reducing edge condensation, and it largely decides how long the unit lasts.

By Dhruv Agarwal · · 1 min read

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

PropertyTypical specification
MaterialStainless, thermoplastic, or composite
ConductivityLow; "warm edge" class stated
DesiccantMolecular sieve within the spacer
Primary sealPolyisobutylene, the vapour barrier
Secondary sealSilicone or polysulphide, the structural bond
TestingEN 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.

Standards referenced

  • EN 1279 — Insulating glass units — long-term test methods (CEN-CENELEC)
  • ECBC 2017 — Building envelope — fenestration U-value (Bureau of Energy Efficiency)
  • ASTM E2190 — Insulating glass unit performance and evaluation (ASTM International)

Frequently asked

It reduces heat flow around the perimeter of the unit. An aluminium spacer is a highly conductive metal bridge connecting the inner and outer panes, so the edge of the glass runs close to outdoor temperature. In an air-conditioned Indian office in humid weather that cold edge attracts condensation, which occupants report as a leaking window. A low-conductivity spacer keeps the edge warmer and largely removes the problem.

The edge seal has failed and moisture has entered the cavity faster than the desiccant can absorb it. The usual causes are a poor secondary seal, water sitting in an undrained glazing rebate attacking that seal, or ultraviolet reaching the seal because the glazing detail leaves it exposed. Once a unit fogs it cannot be repaired; it is replaced.

Yes. It is invisible on a quotation and it determines both the condensation behaviour and the service life of every unit on the building. A specification that names the glass build-up but says nothing about the spacer or the sealant system leaves the longest-lived failure mode to whoever is quoting cheapest.

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