Commercial insulated glass units — what's actually in the cavity.

An insulating glass unit is two or more lites of glass separated by a sealed, gas-filled cavity. That's the whole idea — trap a still layer of gas between two panes and heat has a much harder time moving through the assembly than it does through a single sheet of glass. Everything else in the spec is detail on top of that one principle. For the plain-English version of this, see our IGU glossary explainer; this page is the commercial spec and replacement reference.

Three components decide how a given IGU performs and how long it lasts:

• The spacer. A frame — aluminum, or a "warm-edge" material like stainless steel or a polymer composite — that holds the two lites the correct distance apart around the perimeter and carries the desiccant that keeps the sealed cavity dry. Spacer choice affects edge-of-glass thermal performance and, over the unit's life, seal durability.
• The gas fill. The cavity is filled with dry air or an inert gas — argon is the standard choice on commercial work today, with krypton reserved for narrower cavities where argon's benefit drops off. The gas slows convective heat transfer inside the sealed airspace.
• The low-E coating. A microscopically thin coating on one glass surface, generally facing into the sealed cavity, that reflects infrared radiation while letting visible light through. It's standard on essentially all commercial IGU glass today — the question is which surface it sits on and which formulation, not whether to spec it. We're not publishing specific U-factor or SHGC figures on this page; those numbers are glass-package- and project-specific and belong in the glazing submittal, not a web page.

The perimeter itself is a two-stage seal: a primary butyl seal against moisture and gas loss, and a secondary structural seal (polysulfide, polyurethane, or silicone) that holds the two lites together and resists the building's structural and thermal movement. Both seals have to hold for the unit to keep working.

A fogged or failed IGU is a sealed-unit problem, not a whole-frame problem, in most cases. On storefront, curtain wall, and punched-opening systems where the frame is sound, the standard scope is a "lite" replacement — pull the failed IGU out of the frame, verify the pocket and glazing stops, and set a new unit built to match the original's makeup as closely as the field verification allows. What that scope actually involves:

• Field verification of the existing glass pocket, glazing bite, and setting-block condition before ordering — a new IGU has to physically fit the existing frame and rest correctly on its setting blocks.
• Matching thickness, tint, coating, and overall unit makeup as closely as the original allows, so the replaced lite doesn't stand out next to its neighbors.
• New glazing gaskets or wet seal as required by the frame system — reusing degraded gaskets on a new unit is a common way to shorten the replacement's life before it starts.
• Disposal of the failed unit and site cleanup, particularly relevant on occupied buildings where the work is happening around tenants or the public.

Some third-party services offer to "defog" a failed IGU in place — drilling the unit, cleaning the cavity, and resealing. It's a lower-cost stopgap, but it doesn't restore the original hermetic seal; most of those repairs fail again, and on a commercial building where the unit is also carrying structural, code-compliance, or life-safety performance, replacement is the more defensible scope. If you're weighing repair against replace on a specific building, send us photos and we'll give you a straight answer.

Vision glass in a commercial building envelope is almost always an IGU today, whichever aluminum system holds it. On a storefront system, IGUs sit in a single-story, non-load-bearing aluminum frame — typical for retail, restaurant, office lobby, and medical entrances. On a curtain wall system, the same IGU logic applies across a multi-story, structurally engineered frame carrying its own dead load and wind load independent of the floor slabs. The glazing principles — spacer, gas fill, low-E position, seal integrity — don't change between the two; the framing engineering and installation sequencing do.

Not every panel in a commercial façade is vision glass. Spandrel glass — the opaque panels that conceal floor slabs, structural members, columns, and mechanical spaces between floors on a curtain wall — is also frequently built as an IGU or as a monolithic opaque lite, depending on the design. Where spandrel is built as an insulated unit, the same seal-failure and fogging issues apply, but they're often caught later because the panel isn't something anyone's looking through. On a punched-opening building like a government or institutional facility, the same vision-glass IGU logic applies at each window opening — see our Haines City Public Safety Complex & EOC case study for an example of IGU glazing across a mixed storefront-and-punched-opening façade on a public building.