What Is a Sealed Glass Unit (IGU)? Anatomy, Lifespan and Warranties
A sealed glass unit, or insulated glass unit (IGU), is the factory-made glass package in a modern window: two or three panes held apart by a spacer, sealed at the edges, and filled with dry air or an inert gas such as argon. It does most of the insulating work in a double- or triple-pane window. Sealed units typically last about 10 to 25 years, and when one fails you can usually replace just the unit and keep the frame.
When a sealed unit fails, you can usually replace just the IGU and keep the frame. A new unit is made to fit your existing frame, usually in 1.5 to 3 weeks.
An IGU is two or three panes, a spacer with desiccant, a two-stage edge seal, and a gas fill, often argon. Low-E coatings sit on a glass face inside the unit.
No edge seal is perfectly airtight. Units fail when the desiccant is saturated, which shows up as fog between the panes.
Published lifespans range from about 10 to 25 years. In one 25-year study of more than 2,400 units, 9.2% had failed by year 25.
In Canada, sealed units are made to CAN/CGSB-12.8, and a national certification program audits manufacturers independently. Certified units carry a permanent mark with the maker and year.
Seal warranties typically run 10 to 20 years, but labour is often covered for only 1 to 2 years, glass cover may be prorated, and many warranties aren't transferable.
On this page
A sealed glass unit, also called an insulated glass unit (IGU), sealed unit or thermal unit, is the glass package inside a modern double- or triple-pane window. It's two or three panes held apart by a spacer, sealed around the edges and filled with dry air or an inert gas such as argon. The frame holds it in place, but the IGU does most of the insulating.
It's also the part that fails most often. When you see fog between the panes, it's the IGU's seal that has given out, not the frame. Knowing how the unit is built helps you judge a quote, read a warranty and see why replacing just the unit, not the window, is usually the fix.
Anatomy of a sealed unit
| Part | What it is | Why it matters |
|---|---|---|
| Glass panes | Two or three sheets, typically 3–10 mm thick | Can be clear, low-E coated, tempered or laminated. Tempered is required in some locations, such as doors. |
| Spacer | A strip that holds the panes apart around the edge | Traditionally aluminum or steel. "Warm-edge" spacers made of foam, plastic or stainless steel cut heat loss at the edge and reduce edge condensation. |
| Desiccant | Drying beads inside the spacer | Absorbs moisture that gets into the cavity. When it's saturated, the unit fogs. |
| Primary seal | Polyisobutylene (PIB) between the spacer and the glass | The main vapour barrier. |
| Secondary seal | Polysulfide, silicone or similar around the outer edge | Holds the unit together and keeps water out. Silicone holds up better to UV and heat. |
| Gas fill | Dry air, argon or krypton | Argon conducts about 67% as much heat as air, and krypton about half as much as argon. |
| Low-E coating | A coating about 500 times thinner than a human hair, on a glass face inside the unit | Cuts glass emissivity from about 0.84 to as low as about 0.02. |
| Label / date code | A permanent mark on the spacer | Certified units show the maker, plant and year of manufacture. |
How wide is the gap?
The ideal space between panes depends on the gas. Natural Resources Canada puts the best spacing for limiting convection at 12–16 mm, and other references give about 16–19 mm for standard units. Heavier gases do the same job in a narrower gap. Krypton performs about as well as argon in one-half to two-thirds of the width, which is why it's used in the narrow gaps of some triple-pane units.
This is why the overall thickness of your existing unit matters when you order a replacement. The new unit must fit the same glazing pocket in your frame.
Is there argon in my window?
Two small plugged holes along the spacer, one to let gas in and one to let air out, are a common sign that a unit was gas-filled. Gas fills are never 100%, and over time argon slowly escapes while air and water vapour get in. Argon is inert, non-toxic and odourless, so a leak poses no health risk, but a leak can be detected only with special equipment.
Why sealed units fail
IGU edge seals are not truly hermetic. There is always a small permeation of water vapour and air into the cavity, and of argon out of it. The desiccant soaks up that moisture for years. When it's full, condensation forms between the panes.
What speeds that up:
- Heat. Sealants are 6 to 8 times more permeable at 60 °C than at 20 °C, so sun-baked units age faster.
- Thermal cycling. Daily expansion and contraction as temperatures rise and fall works the seal loose over the years.
- UV. Sunlight breaks down sealant chemistry and makes it brittle.
- Standing water. Water pooling at the base of the unit, often from blocked weep holes, speeds up seal breakdown.
- Manufacturing and installation defects.
For signs to watch for and simple home tests, see how to tell if your window seal is broken.
How long does a sealed unit last?
Published estimates differ, so it's best to think in ranges:
- 10–25 years, with equator-facing (south in Canada) units often under 12.
- 10 to 20 years before failures typically start, in Canadian conditions.
- 15–25 years, depending on climate, exposure and glass.
The field data is more encouraging than those ranges suggest. In a 25-year study of more than 2,400 units in 140 buildings, 4.9% had failed at 10 years, 7.9% at 15 years and 9.2% at 25 years. Certification made a big difference: in that study, units tested to the highest (CBA) level had a 3.6% failure rate, against 14.0% for lower-level units.
In practice, most houses have a few failed units rather than all of them at once. The south- and west-facing windows usually go first.
A GTA condo note. Early condo window-wall systems had a reputation for sealed units that often fogged after only a few years of service. Requirements have since tightened. GTHA condo completions set a record in 2024, and 29,291 units were completed in 2025. All that glazing will reach typical seal-failure ages over the next decade or two. In a condo, who replaces a failed unit depends on your declaration, so check it or ask management first. Our condo glass guide explains how responsibility usually splits.
Canadian standards and certification
- CAN/CGSB-12.8, "Insulating glass units," is the Canadian standard for sealed units with two or three panes, including coated, muntin and argon-filled units. It tests seal integrity, durability and gas concentration, and permanently vented units are excluded. That description is from the 2017 edition (reaffirmed 2022), which the Standards Council of Canada now lists as superseded by CAN/CGSB-12.8-2026.
- IGMAC certification, established in 1968, requires manufacturers to meet the highest level of CAN/CGSB-12.8. It is administered with independent audits, including unannounced semi-annual plant audits.
When you get a quote for a replacement unit, it's reasonable to ask who manufactures the unit and whether that plant is certified.
Understanding IGU warranties
Manufacturer warranties on sealed units typically run 10 to 20 years. Read the fine print, because common terms include:
- Seal failure covered for 10 to 20 years.
- Labour covered for only the first 1 to 2 years, so a free unit may still come with an installation bill.
- Glass coverage often prorated by age.
- Often non-transferable, which matters if you bought the house from someone else.
- Common exclusions: cracked panes, poor installation and condensation on the room-side surface.
Home insurance is a separate question. It generally covers sudden damage, such as a storm or break-in, but not seal failure from old age, and deductibles typically run $500 to $2,000.
What we'd do: replace the unit, keep the frame
If the frame and sash are sound, a failed IGU can be replaced on its own. Natural Resources Canada notes you can buy a new sealed glazing insert and keep frames in good condition. The glazier removes the glazing beads, lifts out the old unit, cleans the channel, sets the new unit on fresh setting blocks and snaps the beads back. That takes about 20 to 30 minutes per opening, after a 1.5 to 3-week wait for the custom unit.
A replacement is also your chance to choose better glass at the same thickness. Our low-E, argon and triple-pane guide covers what's worth specifying, and our foggy window guide shows how to confirm the frame is worth keeping (it usually is). If the frame is rotted, cracked or warped, that's a window-replacement job, not a glass repair, and we'll tell you if that's where you are.
Next step
If a unit in your home has fogged, cracked or broken, you can usually replace just the IGU. See our sealed-unit replacement service, where we replace the sealed glass unit and keep your frame, or contact us with the size and a photo of the spacer label for a free quote.