Interior and finishes · Glass & Glazing
Vacuum Insulated Glazing (Evacuated Cavity Units)
Explains how an evacuated cavity differs from the gas filled cavity of a conventional sealed unit, and why the slim build up, visible pillars and rigid edge seal change what a window frame has to accommodate.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What vacuum insulated glazing is
Vacuum insulated glazing is made from two sheets of glass separated by a very narrow cavity from which the air has been drawn out. Because a vacuum carries almost no heat by conduction or convection, the two panes need only be held a short distance apart. A regular grid of tiny pillars keeps them from touching, and the perimeter is closed with a rigid hermetic seal, often formed from a glass solder or a metal, rather than from the flexible sealants used elsewhere.
That is the real difference from a conventional insulated glass unit. A sealed unit relies on a wide cavity filled with air or an inert gas, kept apart by a spacer bar carrying desiccant and closed with organic edge sealants, and it has to be deep to work. An evacuated cavity does not work by depth, so the panes sit close together on their pillars and the build up stays slim enough for a frame that would never accept a sealed unit, which is why it comes up so often for slim traditional sashes. What it achieves in insulating terms belongs to the specific product's own documentation.
The trade offs belong to the same physics. The support pillars are visible on close inspection as a faint regular pattern. The edge seal is rigid and unforgiving, so edge cover, handling and the way the unit is bedded matter more than usual. The glass is normally heat treated and cannot be cut or drilled afterwards. Hybrid arrangements also exist, in which a vacuum unit is combined with a further pane, and these behave differently again.
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.
- vig
- vacuum glazing
- evacuated glazing unit
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Slim traditional sashes and casements where a deeper sealed unit will not fit the existing rebate.
- Upgrades to single glazed windows in buildings where the original frames are being retained.
- New windows designed around a slim sightline, where frame depth is being kept to a minimum.
- Rooflights and roof glazing where weight and build up depth are both constrained.
- Hybrid units combined with a further pane where a specialist has designed the whole build up.
Consideration
Where it is commonly considered
- Where the existing frames are being kept and the rebate cannot take a conventional sealed unit without alteration.
- Where the relevant authority has an interest in the appearance of the windows and the frame profile must not change.
- Where a slim sightline is a stated design aim and the window designer has raised this option.
- Where an owner is weighing this against secondary glazing and wants the differences set out by a professional.
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 frame or rebate cannot give the edge cover the manufacturer asks for, since the edge seal is the vulnerable part.
- Where units would have to be cut, drilled or adjusted on site, which is not possible once they are made.
- In openings that will see impact or heavy handling, where the design of the whole window is a matter for a professional.
- Where the visible pillar pattern would be unacceptable to the client, which is better discovered from a sample than after installation.
- Where an installer has no experience of this product and no access to the manufacturer's own instructions.
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.
- Thermal behaviour
- The insulating effect comes from the evacuated cavity rather than from the depth of the build up, which is why the unit stays slim. Actual performance figures belong to the specific product's own documentation and vary between manufacturers.
- Appearance
- The support pillars form a faint regular pattern that becomes visible at close range and in certain light. Some clients never notice it and others find it distracting, so a physical sample is worth seeing before the order.
- Installation dependency
- The rigid edge seal governs everything about fitting: edge cover, setting blocks, bedding and the way the unit is handled on site. The manufacturer's instructions are specific to the product and are not interchangeable.
- Moisture behaviour
- Water sitting in a rebate is a hazard for any glazed unit, and drainage and ventilation of the rebate belong to the window design rather than to the glass. A professional details this for the specific frame.
- Repairability
- A unit that loses its vacuum cannot be recovered, and the whole unit is replaced. Where original frames are being retained, that replacement route is worth understanding before the first unit is ordered.
- Documentation
- Because this is a comparatively recent product family, the manufacturer's own literature, warranty terms and installation instructions carry more weight than any general expectation of how a double glazed unit behaves.
Exposure
Exposure and environmental conditions
- How exposed is the window to driving rain, and how does the frame drain water away from the glazing rebate?
- Does the frame sit in deep shade or in strong sun, and does the manufacturer say anything about that?
- Will part of the unit be shaded by a glazing bar, a reveal or a shutter while the rest is in full sun?
- Is condensation currently a problem at this window, and has anyone established why?
- Will internal shutters or heavy curtains sit tight against the glass and change the conditions at the pane?
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 existing frame and rebate govern what can be fitted, and a survey of the actual frames comes before any decision.
- Edge cover is set by the manufacturer and dictates how the beads and putty line are detailed.
- The condition of the timber or metal frame matters, since repairs to the frame come before new glazing rather than after.
- Where glazing bars divide the opening, each small unit is a separate item with its own edge seal and its own constraints.
Appearance
Appearance and finish considerations
- The pillar pattern is inherent to the product and cannot be removed, so it is a decision to make with eyes open.
- Slim units keep the sightline of an original window, which is often the whole point of choosing them.
- An edge seal detail may be visible at the perimeter, and the bead or putty line is designed to cover it.
- Reflections differ from single glazing, so a run of upgraded windows can read differently from any that remain original.
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.
- What does the manufacturer's warranty cover, and what does it specifically exclude?
- How has this product performed in comparable installations, and can the supplier point to that experience?
- What handling and storage does the unit need between delivery and installation?
- Who is responsible if a unit fails after the frames have been redecorated around it?
- How will the units be protected while other work continues in the building?
Upkeep
Maintenance and repair considerations
- How are the frames to be decorated without disturbing the glazing seal or the edge detail?
- What cleaning materials does the manufacturer accept at the glass and at the perimeter?
- Who holds the record of unit sizes so a replacement can be ordered without a fresh survey?
- How long does a replacement unit take to obtain, and what happens to the opening meanwhile?
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.
- Every unit is made to a measured size, so an accurate survey of each opening precedes manufacture.
- Frame repairs, decoration preparation and any ironmongery work are sequenced before the glazing goes in.
- Setting blocks and bedding materials are those the manufacturer names, not the ones a glazier happens to carry.
- Handling on site is planned around a rigid unit that cannot be trimmed, tapped into place or eased.
- Where the relevant authority is involved in the building, approvals are settled before units are ordered.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's installation instructions for the specific unit being supplied.
- The product literature setting out the performance information for that unit.
- Written warranty terms, including what happens if a seal fails after installation.
- Handling, storage and cleaning guidance for the units before and after fitting.
- The survey record of each opening, kept for future replacement orders.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Is the existing frame in good enough condition to receive new glazing at all?
- What edge cover does this product require, and does the rebate provide it?
- Has the client seen a sample and understood the pillar pattern before committing?
- How does this option compare with secondary glazing for this particular building?
- Who is carrying out the installation, and what experience do they have of this product?
- What does the relevant authority say about changing the glazing in these windows?
- What is the replacement route if a unit fails several winters from now?
Blind spots
Commonly overlooked points
- The frame usually needs work before it can take new glazing, and that cost and time is easy to leave out of early thinking.
- The pillar pattern is a permanent visual feature of the product and surprises clients who have only seen photographs.
- The rigid edge seal makes handling and bedding less forgiving than a conventional sealed unit.
- Condensation may move rather than disappear, appearing instead on the coldest remaining surface such as the frame itself.
- A window is a system, so improving the glass while leaving gaps around the frame changes less than expected.
What this page does not do
- Performance figures are not given here; the manufacturer's documentation for the specific unit is the only reliable source and products differ substantially.
- This is a product where the manufacturer's own installation instructions govern, and a glazier working from general habit can compromise the edge seal.
- Whether a particular window can or should be altered at all is a question for a qualified professional and, in protected buildings, for the relevant authority.
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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