An IGU — short for insulated glass unit — is a sealed assembly of two or more glass panes separated by a spacer and filled with an insulating gas, most commonly argon. It is the glass portion of virtually every modern window sold in Canada today, and it does the heavy lifting when it comes to keeping heat in during winter and out during summer.
Key takeaways
- IGU stands for insulated glass unit — a factory-sealed assembly of multiple glass panes.
- The space between panes is filled with argon or krypton gas to slow heat transfer.
- Low-E coatings applied to inner glass surfaces reflect infrared heat without blocking light.
- A warm-edge spacer along the perimeter reduces condensation and improves the edge U-value.
- IGU failure shows up as fogging or cloudiness between the panes and cannot be fixed by cleaning.
What goes inside an IGU?
Think of an IGU like a sandwich. The outer panes are the bread, the gas fill is the filling, and the spacer is the crust holding everything together around the edge. Each component has a job.
The glass itself can be standard clear float glass, or it can carry a Low-E (low-emissivity) coating — a microscopically thin metallic layer that bounces long-wave infrared radiation back toward its source. During a Toronto winter, that means radiated body heat and furnace warmth stays inside rather than escaping through the glass.
The spacer is a hollow frame, typically made from aluminium, steel, or a composite warm-edge material, that runs around the perimeter of the unit and keeps the panes at a fixed distance. Warm-edge spacers — including foam-based types like Edgetech Super Spacer — have lower thermal conductivity than metal, which reduces heat loss at the edges and cuts the risk of condensation forming along the bottom of the glass.
The gas fill sits in the cavity between the panes. Argon is the standard choice because it is denser than air, which slows convective heat transfer across the gap. Krypton is denser still and performs better in narrower cavities, making it common in triple-glazed units where space is tight.
Double-pane vs triple-pane IGUs
Most Canadian replacement windows use either a double or triple IGU configuration. The table below compares them on the metrics that matter most.
| Feature | Double IGU | Triple IGU |
| Glass panes | 2 | 3 |
| Gas cavities | 1 | 2 |
| Typical U-factor (W/m²K) | 1.6–1.9 | 1.0–1.3 |
| Noise reduction | Moderate | Higher |
| Weight | Lighter | 30–50% heavier |
| Cost premium | Base | Approx. 20–30% more |
A lower U-factor means less heat escapes — so the smaller the number, the better the window performs in cold weather. For Ontario homes, triple IGUs are particularly effective in north-facing rooms or in houses that run high heating bills. You can read more about energy-efficient window replacement to see how IGU spec ties into the bigger picture of a window upgrade.
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How Low-E coatings work with the IGU
Low-E coatings are applied to one or more of the inner glass surfaces — the surfaces facing into the gas cavity, where they are protected from abrasion. The position of the coating changes its function.
- Surface 2 (inner face of the outer pane) — reflects solar heat outward, reducing summer cooling loads.
- Surface 3 (inner face of the inner pane) — reflects interior heat back in, cutting winter heat loss.
- Surface 4 option — an interior i89 coating adds extra reflectivity, reducing heat loss by an estimated 10–15%.
Cardinal Glass, whose units TotalSeal uses, designates their coatings as LoĒ-180, LoĒ-272, and LoĒ-366. The number refers roughly to the solar heat gain coefficient range the coating targets. The 366 variant, for example, blocks a higher portion of solar energy while still transmitting daylight, making it a good fit for south- and west-facing windows in GTA homes.
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IGU performance ratings explained
When shopping for windows, you will encounter a few specific numbers tied to IGU performance. Here is what each one measures.
| Rating | What it measures | Better value |
| U-factor | Rate of heat flow through the entire window assembly | Lower |
| SHGC | Solar heat gain coefficient — solar energy admitted | Depends on orientation |
| VT | Visible transmittance — daylight let through | Higher for light |
| STC / OITC | Sound transmission — airborne noise reduction | Higher |
ENERGY STAR™ certified windows in Ontario must meet Climate Zone 2 requirements, which sets a maximum U-factor of 1.22 W/m²K for the whole window. An IGU with a well-specified Low-E coating and argon fill can get a double-pane unit close to that threshold; a triple-pane unit comfortably clears it. If rebates through the Canada Greener Homes initiative are part of your plan, confirming ENERGY STAR™ certification on the window label is the first step. You can also find out what the Canadian grant for window replacement currently covers.
How IGU failure happens
An IGU is factory-sealed with a primary and secondary sealant along the spacer. When those seals degrade — from age, water intrusion, or thermal cycling — the inert gas escapes and humid air gets in. That moisture then condenses on the cold inner glass surfaces, producing the cloudy or foggy appearance between panes that homeowners often notice first in winter.
Once an IGU seal fails, there is no practical repair short of replacing the glass unit itself or the whole window. Drilling holes to inject desiccant or a cleaning solution is sometimes marketed as a fix, but it does not restore the thermal performance of the original sealed unit. If you are seeing condensation between glass panes, that is a reliable sign the IGU has reached end of life.
Common causes of early IGU failure include:
- Poor installation that allows water to pool along the sill and reach the sealant
- Low-quality primary butyl sealant that hardens and cracks over time
- Excessive thermal stress from direct sun exposure on dark-framed windows
- Mechanical stress from a frame that has racked or settled
- Inadequate weep holes that trap moisture inside the frame cavity
How the frame affects IGU performance
An IGU does not perform in isolation — it works together with the frame. A thermally broken or multi-chamber vinyl frame reduces the overall U-factor of the window assembly beyond what the glass alone achieves. TotalSeal’s windows use multi-chamber uPVC frames with five to seven internal chambers, which significantly cut conductive heat loss through the frame section.
Warm-edge spacers inside the IGU complement this by reducing the psi value (linear thermal transmittance) at the glass edge. That edge zone is where most condensation starts, so matching a warm-edge spacer with a well-insulated frame reduces the risk across the whole assembly. Insulated frames are worth understanding if you are comparing window specifications across brands.
Expert tips
“When comparing IGU specs, most homeowners focus on the centre-of-glass U-factor, but what really matters is the whole-window U-factor — that includes the frame and edge. A cheap frame with a good IGU can still perform poorly overall. Always ask for the NFRC or NRC-rated whole-window number, not just the glass.”
— Marcus Hollingsworth, BCIN-registered designer and building envelope consultant, Hamilton, ON
“In my experience inspecting Ontario homes, IGU failures in windows under fifteen years old are almost always a workmanship issue — either a damaged sealant during installation or improper shimming that puts lateral stress on the glass. A quality IGU installed correctly should comfortably last twenty-five years or more.”
— Sandra Pryce, RHI (Registered Home Inspector), OAHI member, Mississauga, ON
When to replace your IGUs
If your windows are fogging between the panes, rattling in the frame, or letting in noticeably more noise than they used to, it may be time to look at replacement. How often windows should be replaced depends on the original glass spec, installation quality, and how well the frames have held up.
For newer homes where only one or two units have failed, a glass-only swap is sometimes possible if the frame is structurally sound. For older windows with aluminium spacers, single-pane, or dated low-E coatings, full window replacement — with a current ENERGY STAR™ certified unit — is usually the better investment. You can compare replacement windows and frames options to see what makes sense for your situation.
When full replacement makes sense, getting the best deal on replacement windows starts with understanding what you’re buying — and now you know exactly what to look for in an IGU.
Frequently asked questions
What does IGU stand for in windows?
IGU stands for insulated glass unit. It refers to a factory-sealed assembly of two or more glass panes separated by a spacer and a gas fill, forming the glass portion of a modern window.
Can a failed IGU be repaired without replacing the whole window?
In most cases, no. Once the seal fails and fogging appears between panes, the gas fill is gone and thermal performance is compromised. The IGU itself needs replacing, though the frame may be reusable.
How long does an IGU typically last?
A properly manufactured and installed IGU typically lasts 20–25 years. Lifespan depends on sealant quality, installation workmanship, exposure to direct sun, and how much thermal cycling the unit experiences.
What is the difference between argon and krypton in an IGU?
Argon is denser than air and is the standard gas fill for double and triple IGUs. Krypton is denser than argon, performs better in narrower cavities, but costs more. It is typically used in high-performance triple-pane units.
Does an IGU need to be ENERGY STAR™ certified on its own?
No. ENERGY STAR™ certification applies to the whole window assembly — frame, spacer, and IGU together. The rating is based on whole-window U-factor and SHGC values, not on the glass unit alone.
