Glass and Glazing

Glass typically makes up most of the surface area of a window, so the glazing you choose has a major effect on thermal performance, solar heat gain, daylight, acoustics, condensation resistance, appearance and the overall weight of the window.

The best glass configuration is not necessarily the same for every opening. Climate, orientation, window size, safety requirements, noise exposure and the balance you want between insulation and solar gain should all be considered together. This page is a short primer on how glass and glazing work. For the GlasFëns ZERO glass packages and their performance, see Glass.

On this page: Quick facts · Vacuum insulating glass · Low-E glass · Solar heat gain · Acoustics · Condensation · Safety glass · The glass edge · Whole-window vs. center-of-glass · Selecting the right glass

Glass and glazing quick facts

• Maximum insulating glass unit sizes vary by manufacturer. Large units need extra thought for fabrication, handling, transportation and installation. Units weighing more than about 300 lb can become difficult to handle safely.
• Heat soaking can reduce the risk of spontaneous tempered-glass breakage. It is an additional fabrication process intended to identify panes containing nickel-sulfide inclusions before the glass reaches the building.
• Heat-treated glass can show roller-wave distortion. It can become more noticeable in large panes, in reflective glass and under certain lighting. Glass size, orientation, coating and viewing angle all affect how visible it is.
• Thin laminated glass plies can produce optical distortion. Where reflected appearance or exceptionally clear views matter, consider glass thickness and laminate construction early in the specification.

Vacuum insulating glass

What is vacuum insulating glass?

Vacuum insulating glass, or VIG, is two panes of glass separated by an extremely thin evacuated space. A typical vacuum gap is only about 0.1 to 0.3 mm, compared with the much wider gas-filled cavities of conventional insulating glass. Tiny support pillars between the panes resist atmospheric pressure and hold the glass surfaces apart.

The support pillars are a normal part of vacuum-glass construction. On close inspection, or in particular lighting, they may be visible as a very fine, regular pattern of dots.

Why does a vacuum insulate so well?

Heat crosses conventional glazing in three ways: conduction, convection and radiation.

Removing nearly all of the gas from between the panes dramatically reduces conduction through the gas and eliminates the convection currents that form inside air- or gas-filled cavities. Thermal radiation can still cross a vacuum, which is why Low-E coatings remain an important part of high-performance vacuum glazing.

VIG is not the same as conventional insulating glass

Both use more than one pane of glass, but their insulating cavities work differently. A conventional insulating glass unit separates the panes with a relatively wide cavity of dry air or an inert gas such as argon. Vacuum insulating glass removes almost all of the gas and uses a cavity measured in fractions of a millimeter.

That allows very high insulating performance without relying only on adding panes and ever-deeper gas-filled cavities.

Less thickness does not mean less performance

Unusually, the insulation of vacuum glazing does not depend on a deep cavity. Double- and triple-glazed units improve performance by combining more panes, Low-E coatings and one or more gas-filled spaces. Vacuum glazing instead removes one of the main paths of heat transfer by removing the gas itself.

As a result, a comparatively thin vacuum-glass construction can deliver thermal performance that would normally take a considerably thicker conventional glazing assembly.

Low-E glass

What does Low-E actually do?

Low-emissivity, or Low-E, coatings are microscopically thin coatings applied to glass to reduce radiant heat transfer.

A vacuum cavity greatly reduces conduction and convection, but a vacuum does not stop thermal radiation. Low-E coatings deal with this remaining path by reducing the infrared heat that passes between the glass surfaces.

The type of Low-E coating, and which glass surface it sits on, also affect solar heat gain, visible light transmission and the exterior appearance of the glass. The right coating depends on more than simply reaching the lowest possible U-factor.

Solar heat gain

The lowest U-factor is not always the whole objective

Glazing design is a balance between heat loss, solar heat gain and daylight.

In heating-dominated climates, solar gain through well-oriented windows can reduce winter heating demand. In cooling-dominated climates, controlling solar gain can matter more. Orientation, shading, window-to-wall ratio, climate and glass coating should be considered together. See U-Value & SHGC for the definitions.

Acoustic performance

More panes do not automatically mean a quieter window

Sound transmission through glazing depends on glass thickness, pane spacing, asymmetry between the panes, laminates, cavity construction and, importantly, the frequency of the noise.

The air cavity in conventional insulating glass behaves partly like a spring between the panes and can resonate at certain frequencies. A vacuum cavity removes that air, which changes the acoustic behavior of the glazing and can give particularly strong performance against low-frequency noise such as road traffic and mechanical equipment.

For acoustically sensitive projects, select the complete glazing configuration from tested acoustic data rather than from pane count or total glass thickness alone.

Condensation and interior comfort

Highly insulating glazing keeps the interior glass surface warmer in cold weather. A warmer interior surface improves comfort near the window and reduces the likelihood of interior condensation.

Condensation cannot be judged from the glass alone, however. Outdoor temperature, indoor temperature, indoor relative humidity, the temperature at the glass edge, the frame construction and air circulation inside the room all affect whether condensation forms. See Condensation Resistance.

Safety glass

Glass requirements change with location and application. Doors, glazing near walking surfaces, large floor-to-ceiling openings, overhead glazing and other hazardous locations may require tempered, laminated or other safety glazing, depending on the applicable code and the installation.

Vacuum glazing can also be built into laminated and other composite assemblies, for overhead glazing, floor-to-ceiling openings and other locations that call for additional safety, acoustic or structural performance.

The glass edge matters

The center of the glass is not the only place heat moves through a window. The perimeter of the glass, the spacer or edge seal, the glazing pocket and the surrounding frame form their own thermal path.

Glazing depth, edge materials and frame geometry all influence the temperatures around the perimeter of the glass, and so affect whole-window thermal performance and condensation resistance.

Exceptional center-of-glass performance only reaches its potential when the glass and the frame work together.

You can see this in the frame sections on our Performance page.

Whole-window vs. center-of-glass performance

Center-of-glass U-factor measures the thermal performance through the central portion of the glazing only.

Whole-window U-factor includes the glass, the glass edge, the frame and the sash. It is the more meaningful number when comparing the energy performance of complete windows.

A very high-performance glass unit does not automatically make an equally high-performance window if the frame or glass edge around it introduces significant thermal bridging. That is why a center-of-glass U-factor and a whole-window U-factor for the same product are always different numbers, and why GlasFëns publishes both.

Selecting the right glass

There is no single glazing configuration that is best for every project. Selection can depend on:

• Climate and design temperatures
• Window orientation
• Heating and cooling loads
• Solar exposure and exterior shading
• Window-to-wall ratio
• Desired visible light transmission
• Acoustic requirements
• Safety-glazing requirements
• Size and weight
• Wind and structural loads
• Frame and sash configuration
• Applicable energy and building codes

Actual energy savings should be evaluated for the complete window in the complete building, not from center-of-glass performance alone. We model the whole window for every project on request. See the ZERO glass packages or ask us for a project calculation.