Insulated Glass Units: Spacers, Gas Fills and Edge Seals

What makes up an insulated glass unit

An insulated glass unit, often called an IGU, is a sealed assembly of glass panes that traps a layer of air or gas between them. The main components are the glass lites, the spacer that holds them apart, the primary seal that keeps moisture and gas out of the cavity, and the secondary seal that gives the unit its structural strength. Each part plays a distinct role.

When you specify glazing for a metal frame, it helps to think of the IGU as a system rather than a single product. The frame, the glass build-up and the way the unit is fixed into the opening all influence how well the assembly performs over the years.

Spacers and why they matter

The spacer sets the width of the gap between panes and influences both thermal performance and how the unit handles stress. Traditional aluminum spacers conduct heat readily, which can create a cold line at the edge of the glass where condensation may appear. Warm-edge spacers, made from stainless steel or polymer-based composites, reduce that heat transfer.

• Aluminum: strong and widely used, higher edge conductivity
• Stainless steel: lower conductivity, good durability
• Warm-edge composites: low conductivity, lighter weight

Spacer choice should be confirmed with the glazing supplier against the frame profile and the project's thermal requirements.

Gas fills and cavity choices

Air is the simplest fill, but many high-performance units use argon or krypton because these gases conduct heat less readily than air. The gap width also matters: a gas-filled cavity performs best within a certain range, and wider is not always better. Gas leakage over time is a real consideration, so the seal quality directly affects how long the fill remains effective.

Exact gap widths and fill levels depend on the glass build-up and the target performance, so these details are best settled with the glazing manufacturer during design.

Primary and secondary edge seals

The primary seal, usually a polyisobutylene layer, is the first barrier against moisture vapor and gas loss. The secondary seal, commonly silicone, polyurethane or hot-melt butyl systems, bonds the panes to the spacer and carries the structural load. A well-chosen combination keeps the cavity dry and the unit stable through temperature swings.

Seal failure usually shows up as fogging or condensation between the panes. Specifying a recognized seal system from the glazing supplier and protecting the edges during installation reduces this risk considerably.

How the metal frame affects the unit

In steel and aluminum frames, the glazing rebate must accommodate the full IGU thickness, the bite of the glazing gasket or structural sealant, and the expansion of the glass. Thermal breaks in aluminum profiles reduce the conduction path that would otherwise undermine a warm-edge spacer.

Our workshop at 37 Rival Street, Tel Aviv, produces the frames and rebates to suit the glass build-up you specify, so drawings should include the final IGU thickness and the sealing method from the start.

Specifying glazing with the architect in mind

A clear specification prevents most glazing problems. Include the overall glass build-up, the spacer type, the gas fill, the edge seal standard, the required performance values and the glass types for each opening. Note any safety or acoustic requirements in the same document so the glazing and the frame are designed together.

• Confirm the IGU thickness before detailing the rebate
• Name the spacer and gas fill explicitly
• Request the supplier's seal and warranty documentation in writing

Frequently asked questions

Why does condensation appear at the edges of an insulated glass unit?

Edge condensation often comes from a conductive aluminum spacer creating a cold line where warm indoor air meets the glass. Switching to a warm-edge spacer usually reduces this effect.

Is argon always better than air in an IGU?

Argon generally conducts less heat than air and improves performance, but the benefit depends on the gap width and the seal quality. The right choice should be confirmed for each project.

What is the difference between primary and secondary seals?

The primary seal blocks moisture and gas migration, while the secondary seal bonds the panes and provides structural strength. Both are needed for a durable unit.

Should the frame be designed before or after choosing the glass?

The glass build-up should be decided first, because the final IGU thickness determines the rebate depth and gasket details in the frame.

Common mistakes to avoid

A common mistake is detailing the frame rebate before the final IGU thickness is confirmed, which leads to glass that does not seat properly. Another is leaving spacer and gas fill unspecified, so the supplier substitutes the cheapest option.

Our recommendation

Send your glass build-up, spacer type and gas fill to the metal fabricator at the drawing stage so the rebate and thermal break are designed around them. Ask the glazing supplier to confirm the seal system in writing before ordering.

Summary

Spacers, gas fills and edge seals determine how an insulated glass unit performs, so specify each one clearly and design the frame around the final glass build-up.

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