Interior and finishes · Glass & Glazing
Insulated Glass Unit (IGU) - Sealed Glazing Unit
Explains the sealed unit as an assembly rather than a type of glass, describes what each part of the perimeter does, and sets out why seal failure is the defining maintenance question.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What insulated glass unit is
An insulated glass unit is not a type of glass but a factory-made assembly. Two or more panes are held apart by a spacer bar around the perimeter, the space between them is sealed, and the whole thing is delivered and handled as a single component. The panes themselves may be plain, coated, heat-treated, laminated or patterned, in any combination the fabricator has specified.
The engineering sits at the perimeter. The spacer holds the panes apart and normally carries a desiccant that takes up residual moisture inside the cavity. A primary seal between spacer and glass restricts movement of gas and vapour, while a secondary seal around the outside bonds the assembly together and does the structural work of keeping the sandwich as one piece. Spacer materials differ between products and change how the edge of the unit behaves.
Because it is a closed system, a sealed unit ages at its seals rather than at its glass. Once the seal is breached, humid air enters the cavity, the desiccant eventually saturates, and condensation forms on the internal faces of the panes - misting that cannot be cleaned because it is on surfaces nobody can reach. The remedy is replacing the unit, not repairing it, and how long a seal fares depends heavily on the frame, the drainage of the rebate and the glazing method.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- IGU
- Insulating glass unit (US term)
- Sealed unit
- Double glazed unit
- Double glazing unit
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Glazing in windows and external doors, supplied by a fabricator to sizes taken from the frames.
- Rooflights and glazed roofs, where the unit sits at an angle and the edge detail becomes more demanding.
- Glazed walls, sliding and folding door sets, where large units are built to the door manufacturer's specification.
- Conservatory and garden room glazing, where units make up a large share of the envelope.
- Replacement glazing in existing frames, where the frame stays and only the unit is renewed.
- Internal glazed screens where a specifier has asked for a sealed unit rather than a single pane.
Consideration
Where it is commonly considered
- Where an existing window has misted between the panes and the frame itself remains sound and serviceable.
- Where new frames are being installed and the glazing is being specified alongside them from the outset.
- Where the panes within the unit need different treatments on each face, which a fabricator can build in.
- Where a designer is comparing glazing options with a professional and needs the unit build set out in writing.
- Where an existing frame can take the unit thickness the fabricator proposes without alteration.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where the existing frame rebate cannot accept the unit build without modification that changes how the frame works.
- Where the rebate does not drain, since a unit sitting in water is the classic route to early seal failure.
- Where the unit would be installed at a slope or overhead without a detail designed for that by a professional.
- Where weight or size exceeds what the existing hinges, frames and lifting arrangements can handle.
- Where a local requirement may govern the glass within the unit, which a qualified professional must determine.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The single defining issue is seal integrity. Once the perimeter seal is breached and the desiccant saturates, moisture condenses between the panes and cannot be reached, and standing water in the rebate is the most common accelerator.
- Repairability
- A sealed unit is not repairable in any meaningful sense. Attempts to drill and vent a misted unit change what the assembly is, and the normal outcome of seal failure is replacement of the whole unit.
- Thermal behaviour
- The whole point of the build is what happens across the cavity, but the figures for any particular unit belong in the fabricator's technical documentation, and what they mean in a building is a matter for a qualified professional.
- Installation dependency
- Setting blocks, glazing tape, drainage of the rebate and the glazing method all decide how the unit is loaded and whether the edge stays dry. Poor glazing practice shortens seal life far more reliably than exposure does.
- Appearance
- The spacer is visible around the edge of the unit, and its colour and profile become a design detail on slim sightlines. Very large units can also show faint reflected distortion as the cavity pressure changes.
- Maintenance
- Care concentrates on the frame rather than the glass: keeping drainage paths clear, keeping seals and gaskets intact and dealing with frame decay before it lets water sit against the edge of the unit.
- Documentation
- Worth asking for a written build for each unit - which panes, which coatings on which faces, which spacer and which cavity fill - since two units that look identical can be entirely different assemblies.
Exposure
Exposure and environmental conditions
- Does the frame rebate drain and ventilate, or does it hold water against the bottom edge of the unit?
- Is the unit installed at a slope, overhead or fully vertical, since the edge detail differs for each?
- How much direct sun does this elevation take, and is any part of the unit permanently shaded?
- Is the interior of this room prone to high humidity, such as a bathroom, kitchen or utility space?
- Has external surface condensation been observed on units on this elevation on cold clear mornings?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The frame governs almost everything: rebate depth, drainage, gasket type and how the unit is supported.
- Setting blocks position the unit so its weight is carried correctly and its edge is clear of standing water.
- Glazing sealants and tapes have to be compatible with the secondary seal of the unit being installed.
- Where the frame is a door or an opening light, the hardware has to suit the weight of the finished unit.
- Structure around large glazed openings has to be designed by a qualified professional rather than assumed.
Appearance
Appearance and finish considerations
- Spacer colour and profile show at the edge of every unit and are worth choosing rather than accepting by default.
- Where the unit contains a coated pane, the coating gives the glazing a slight tone that is visible in reflection.
- Sightlines between frame and glass change when unit build changes, which shows on a run of matching windows.
- Large units can show subtle reflected distortion as the sealed cavity responds to weather and temperature.
- Mixing old and new units across one elevation usually produces a visible difference in tone and reflection.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- How old are the existing units, and is misting appearing in some positions rather than across the whole building?
- Is water sitting in any of the frame rebates, and have the drainage paths ever been cleared?
- Were the units glazed with materials compatible with their secondary seal when they were installed?
- Is any frame decay or movement transferring loads into the edge of a unit?
- Are sloped or overhead units being reviewed separately, since they work harder than vertical ones?
Upkeep
Maintenance and repair considerations
- Are the frame drainage slots being kept clear as part of routine upkeep?
- If a unit mists, is the frame sound enough that only the unit needs replacing?
- Who takes the sizes for a replacement unit, and are those taken from the frame rather than the old glass?
- Are gaskets and seals around the glazing being checked before they fail rather than after?
- Is there a written record of the unit build for each opening so replacements can be matched?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Units must be glazed onto setting blocks rather than resting directly on the rebate base.
- Rebate drainage has to be present and unobstructed before the unit is bedded into the frame.
- Sealant and tape compatibility with the secondary seal needs confirming with the unit fabricator.
- Handling and lifting arrangements have to match unit weight, which rises quickly with size and build.
- Sloped and overhead installations depend on a glazing detail designed for that orientation by a professional.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- A written build specification for each unit, including which pane carries any coating and on which face.
- The fabricator's guidance on glazing method, compatible sealants and setting block positions.
- Recorded unit sizes and weights so frames, hardware and lifting can be checked against them.
- The technical data sheet for the unit, which is where any performance figures properly belong.
- Terms of the fabricator's warranty, particularly what it says about seal failure and about installation.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What exactly is inside the unit being quoted, pane by pane and face by face?
- Can the existing frames take this unit build, or does the rebate need altering?
- Does the rebate in these frames drain properly, and has that been checked rather than assumed?
- Which sealants and tapes will be used, and are they compatible with the unit secondary seal?
- What does the technical documentation say about how this unit behaves in this orientation and exposure?
- How are large units going to be lifted into position, and is access adequate for that?
- If units mist within the warranty period, what does the fabricator cover and what does the installer cover?
- Are any of these units sloped or overhead, and has the detail for those been designed separately?
Blind spots
Commonly overlooked points
- Misting between the panes is a seal issue, not a dirt issue, and no amount of cleaning reaches those faces.
- A blocked drainage slot in the frame does more damage to unit life than almost anything happening on the outside.
- Replacing only the failed units on an elevation often leaves a visible difference in tone against the older ones.
- Condensation on the outer face of a unit on a cold clear morning is a surface effect and not evidence of seal failure.
- The unit is a sealed system, so anything that punctures or drills it converts it into something else entirely.
What this page does not do
- Build Design Hub does not test, certify or recommend glazing units, and nothing here confirms that a particular unit suits a particular opening.
- Thermal, daylight and weather figures for a sealed unit belong in the fabricator's technical documentation and vary by build.
- Where local requirements may govern the glass used within a unit, that is determined by a qualified professional and must be confirmed for the specific opening.
- Removing, handling and disposing of glazed units is heavy work with real injury risk and is not a task to improvise.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
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