What holds the two pieces of an insulated glass unit together?

A metal channel sealed with a hot-melt butyl sealant, but not all spacers are the same.

Concept-1 windows come standard with the Intercept Warm-edge Glass Spacer System

• Warmer indoor glass edge temperatures provide excellent condensation resistance.
• U shaped channel vs. box shaped channel - cold and heat take a longer time to travel around the U-shape
• U-shaped spacer is more flexible allowing sealant to expand and contract with less risk of stress cracks or seal failure.
• Tin-plated steel is 88% less conductive than aluminum - maintaining a more consistent glass surface temperature.