# 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.

## 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**.

## Frequently asked questions

### What does a warm edge spacer actually improve?

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.

### Why do double-glazed units fog up years after installation?

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.

### Is the spacer worth specifying separately?

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.

## Sources

- [EN 1279](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Insulating glass units — long-term test methods
- [ECBC 2017](https://beeindia.gov.in/) — Building envelope — fenestration U-value
- [ASTM E2190](https://www.astm.org/products-services/standards-and-publications.html) — Insulating glass unit performance and evaluation

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Source: https://hagerstone.com/materials/warm-edge-spacer-bar