From the street, an insulated glass unit looks like a single pane of slightly thick glass. Inside the edge, however, is a small assembly of parts, each doing a particular job. If any of them fails, the unit stops working as designed, and the first sign is usually mist between the panes. This article explains what is inside the unit and what each part is for, so that you know what to ask a supplier.
The basic idea
A double-glazed unit is two panes of glass held apart by a spacer, with the gap between them sealed around the edge. The gap contains dry air or another gas. Because the gap resists the flow of heat much better than solid glass does, the unit insulates far better than a single pane, and it also reduces some noise.
Triple units add a third pane and a second gap. The principle is the same, and so are the parts. The key point is that the unit is sealed in the factory and must stay sealed for its whole life. Everything inside it depends on that seal holding.
The spacer and the drying agent
The spacer is a narrow bar that runs around the perimeter between the panes and sets the width of the gap. It is often hollow and contains a drying agent, called a desiccant, that absorbs any small amount of moisture trapped inside when the unit was made or that enters slowly over time. Small holes on the inner face of the spacer let the desiccant reach the air in the gap.
Spacers differ in material. Some conduct heat more than others, which matters at the edge of the glass, where the temperature is lowest and condensation tends to appear first. A spacer that conducts less heat can reduce that effect. The material and the type should be listed in the specification, because two units that look the same from outside can behave differently at the edge.
The edge seals
The panes are bonded to the spacer by sealants, and most units use two layers. The inner layer is a primary seal that acts mainly as a barrier to moisture vapour and to the escape of gas. The outer layer is a secondary seal that gives the unit its mechanical strength and holds the panes and the spacer together.
These seals are the most important part for durability. They face constant temperature swings, moisture, ultraviolet light and the stress of the wind pushing on the panes. A unit whose seal is poorly made, or that sits in water, can fail early. This is why the quality of the manufacturer and the way the unit is set into the frame both matter.
Coatings and the surfaces that carry them
A low-E coating is a thin metallic layer on one of the glass surfaces that reflects heat radiation while letting most light through. It improves the insulation of the unit and can be chosen to favour either keeping heat in or keeping solar heat out. Solar control coatings work on a similar principle and reduce the amount of solar heat entering.
The coating is placed on an inner surface of the gap, where it is protected from touch and weather. Surfaces are numbered from the outside, so the specification should say which one carries which coating. The same coating on a different surface can change the behaviour of the unit, which is one more reason to read a specification closely.
Combining coatings with laminated or acoustic panes gives further choices. One pane of a unit can be laminated for safety or sound, and the other can carry the coating. The specification should list every layer, since the performance of the whole depends on how they are combined. Ask the supplier to give the full build-up in a single line, so that nothing is left to interpretation.
The gap and what fills it
The gap is filled with dry air or a gas, often argon, which conducts heat a little less than air does. The width of the gap also matters, because a gap that is too narrow conducts more, and one that is too wide lets the gas circulate and carry heat. The supplier chooses the width to suit the glass and the coating.
Gas can slowly leak from any unit over the years, and the performance gain from the fill is modest compared with that of the coating. The specification should state the fill and the performance of the finished unit, rather than treating the gas as a promise on its own. Ask what figure applies to the unit as a whole, and what it is based on.
Why the frame matters to the unit
A unit does not live alone. It sits in a glazing pocket in a frame, held by gaskets, glazing beads, setting blocks and sometimes structural silicone. The frame must support the weight properly, keep the edge dry and allow any water that enters the pocket to escape. A sealed unit that sits in standing water is likely to fail early, however good the unit itself.
- Setting blocks carry the weight of the unit and must not block the drainage path.
- Gaskets and sealants around the unit must be compatible with the edge seal of the unit.
- The pocket must drain, and the drainage must be clear of dirt and debris.
- Pressure from the frame on the glass must be even, without point loads.
This is where curtain wall and casement window details come in. Ask to see the glazing pocket on a section drawing, and confirm that the unit and the frame supplier have agreed on the detail. Where the two are supplied by different companies, compatibility has to be confirmed in writing. The drainage guide describes the water path in more detail.
Site practice affects the life of a unit. Units should be stored upright and protected from standing water, because moisture sitting on the edge seal can start to damage it before the unit is even installed. Edges should never be struck or levered during fitting, and the surfaces should be protected from welding sparks, plaster and cement. A small chip at the edge can grow into a crack after installation.
Pressure differences are another concern. A unit made at one altitude or temperature and installed at a very different one can bulge or pull inward, because the sealed gas expands or contracts. Large units and sites at high elevation need the supplier to consider this, sometimes with a way to equalise pressure. Mention the site conditions in the enquiry so that the unit is made to suit them.
Signs of failure and what to do
The usual sign of a failed seal is fogging or droplets between the panes, or a milky film that cannot be wiped off. Because the gap is sealed, the cleaning of the outer faces does not change it. Condensation on the room side or outer side of the glass is a different matter and has other causes, which the article on condensation explains.
A failed unit is normally replaced, not repaired. Replacement needs the original build-up, including the coating, the spacer type and the thickness of each pane. If this information is not on file, matching it can be slow and uncertain. Keep the specification and the supplier's details with the building records, and note the manufacture date marked on the spacer if there is one.
Questions to ask a supplier
- What is the full build-up, from outside to inside, including coatings and surface numbers?
- What spacer and which seals are used, and who manufactures the unit?
- What is the gas fill, and what performance figure applies to the finished unit?
- Is the unit compatible with the gaskets and sealants in the frame?
- How does the pocket drain, and are the setting blocks clear of the drainage path?
- What warranty applies, and what does it exclude?
Good answers show that the unit and the frame have been considered as one assembly. A mock-up review is a good time to check these details on a real section, before the same pocket is repeated across a whole elevation.
General guidance for orientation, not a specification. Applicable standards, values and details are confirmed per project with the facade consultant.
