Insulated glass units (IGU)
Two panes of glass, sealed around the edge with a spacer, with a captive layer of gas trapped in between. That sealed gas gap does almost all the insulating work in a modern window. Here is how it goes together, what each part does, and why it eventually fogs up.
What is an IGU?
A single pane is a terrible insulator. It hands heat straight through, so its U-factor usually sits around 1.0 or worse. Add a second pane with a sealed gas gap between them and that number falls to about 0.48 for a basic double, and once you add a Low-E coating and an argon fill it can drop to 0.25–0.30 or lower.
Almost all of that improvement comes from the gap, not the glass. The trapped gas (air, argon, or krypton) conducts heat far worse than glass does. The gap is kept narrow enough that the gas cannot set up convection currents. And with a Low-E coating facing into the gap, the radiant heat trying to cross it gets knocked down too. Three different heat paths, all throttled, from one sealed cavity.
Components of an IG unit
Glass lites
The lites in an IGU can be anything: clear, tinted, Low-E, tempered, laminated, or a mix. The one facing the weather is the outboard lite; the one facing the room is the inboard lite. Usually both are the same thickness (6mm is typical), but you will see mismatched lites when the unit is tuned for sound or structure.
Spacer
The spacer is the frame around the edge that holds the two lites apart at a set gap. Old-school aluminum spacers conduct heat well, which is bad news here: they create a "cold edge" around the perimeter, exactly where condensation likes to form on the inside of a window in winter. Warm-edge spacers swap in low-conductivity materials (stainless steel, thermoplastic, foam composites) to cut that edge loss and lift the whole window's performance.
The spacer also carries the desiccant, a moisture-hungry material (usually molecular sieve) that mops up whatever trace humidity got sealed inside during assembly, so the unit does not fog from day one.
Primary seal
The primary seal is a soft, sticky bead, almost always polyisobutylene (PIB), squeezed between the spacer and the glass on both sides. PIB barely lets water vapor through, so it is the main line of defense keeping moisture out of the gas space. It is not there for strength; its one job is blocking moisture.
Secondary seal
The secondary seal fills the channel around the outside of the spacer, between the two lites. This one pulls double duty: it structurally bonds the sandwich together and adds a second barrier against moisture and gas escaping. Two sealants show up most often:
- Silicone: Excellent UV and weather resistance; required for structural glazing applications where the secondary seal is exposed; slightly higher moisture vapor permeability than other options.
- Polysulfide: Lower moisture vapor permeability than silicone; preferred in non-structural applications for better long-term seal integrity; not UV resistant, must be shielded from direct sun.
Gas fill options
The gas you pump into that gap swings the thermal number more than you might expect. Your options:
| Gas | Thermal conductivity | Performance vs air | Cost |
|---|---|---|---|
| Air | Baseline | Baseline | Free |
| Argon | ~34% lower than air | U-factor ~10–15% better | Low |
| Krypton | ~64% lower than air | U-factor ~20–30% better; best in narrow gaps | High |
| Xenon | ~80% lower than air | Excellent; rarely used commercially | Very high |
Argon is the near-universal pick for homes and commercial work: a real bump in performance for almost no money. Krypton only earns its keep in very narrow gaps (under 9mm), where its lower conductivity really shines, which is why you mostly see it in triple-pane units where keeping the overall thickness down matters.
What causes IGU seal failure
IGU seal failure, that haze or misting inside the glass that no amount of cleaning will touch, is the number-one reason homeowners replace glass. Here is the slow-motion sequence: moisture creeps through the seals over the years, the desiccant quietly soaks it up, and then one day the desiccant is full. After that, the next cold snap condenses moisture on the inside faces of the glass, and because it is sealed in there, you cannot wipe it away.
Common causes of premature seal failure:
- Inadequate edge deletion: Low-E coatings extend to the glass edge, preventing the primary seal from bonding to bare glass. Edge deletion removes the coating from the perimeter band before assembly.
- Polysulfide exposed to UV: Secondary seal degradation from UV if the sealant pocket is not shielded by the frame.
- Physical damage to seals: Improper handling during installation, excess sealant contact with incompatible materials, or pressure from improperly glazed frames.
- Normal aging: All IGU seals eventually allow some moisture transmission. Well-made units from IGMA-certified manufacturers typically achieve 20+ years of service life under normal conditions.
Triple-pane IGUs
Triple-pane units add a third lite and a second gas gap. They are the norm in high-performance and Passive House projects and in cold-climate homes. A good one, two Low-E coatings plus krypton, can hit a center-of-glass U-factor as low as 0.10–0.15.
That extra glass and gas also quiets the room and cuts the odds of interior condensation. The costs are real, though: the unit can weigh 50% more than a double, it needs a beefier frame, and it is pricier. In mild or cooling-dominated climates, triple-pane usually is not worth it.
IGMA certification
The Insulating Glass Manufacturers Alliance (IGMA) runs a certification program for IGU makers. Certified manufacturers get their products tested and their production audited against ASTM E2190. On commercial or high-performance jobs, calling for IGMA-certified units is a simple way to buy documented, consistent seal durability instead of taking it on faith.