Thermal glass

These days, it’s not even an option - when selecting windows for a new construction or a replacement project, thermal windows are the only way to go. But what exactly is thermal glass? And what are the available options?
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Basic composition
Evolution
Component details
Window Performance
Good to know…

Basic composition

Thermal glass is comprised of two panes of 1/8 in. (3 mm) thick glass, separated by an insulating spacer. The cavity created by the spacer frame provides an "air chamber" between the two panes of glass, which improves the insulating properties of the window. The two panes of glass and the insulating frame form a hermetically sealed unit that prevents the passage of air and humidity.

In theory, the greater the spacing between the two panes of glass, the better the insulating properties of the unit. But in this industry, as in many others, a standard was required in order to satisfy residential construction requirements as well as the principles of mass production, all while providing superior efficiency.

In the end, the industry standard for the spacing between the two panes was set at ½ in. (1.3 cm). Custom units may be ordered, with spacing up to ¾ of an inch (2 cm), but units built with non-standard specifications require additional lead times and are more expensive than standard windows.

Finally, though not readily visible but of critical importance none the less, the desiccant (i.e. the product designed to absorb humidity) is added to prevent condensation between the two panes of glass.

Evolution

Since the introduction of thermal glass windows some fifty years ago, the industry has made quantum leaps in terms of product efficiency and durability. This may help explain why, over this same period of time, product warranties have jumped from 12 months, to 10 years and up…

When you consider that 85% of residential heat loss is attributed to household fenestration, it is easy to understand why manufacturers have invested so much effort in the design and development of energy efficient glass assemblies.

The use of improved sealants, filling the cavity between the panes with argon instead of air, the kind of desiccant used, the application of Low-e coatings and, more recently, the advancements in insulating spacer materials and profiles, are the types of additions and/or modifications that have significantly improved thermal glass efficiency.

Though they may appear somewhat simple, today’s windows are definitely technology-driven products.

Component details

The glass
Obviously the most recognisable part of any window, glass is produced from melted silica sand. Today’s production "recipes" make the material better than ever.

During the production process, glass may be tinted and products may be added to make it "Low-E" (see below).

The insulation value of basic glass (3 millimetres thick) is R 0.8.
The insulating spacer
The insulating spacer is a frame whose primary function is to separate the two panes of glass, which are then hermetically glued in place in order to produce a sealed glass assembly (glass - air - glass) that will have a higher insulation value than basic glass. There are two types of insulating spacers available and, consequently, the type of window is determined by the type of spacer.
  • Traditional insulating spacers: These frames are shaped like an enclosed bar, as indicated on the cross-section here. Small holes generally appear on the tops of these units.

    This type of insulating spacer has two heat conducting channels, illustrated by the two red arrows on the image. These channels generate a small amount of heat loss.

    Originally made of aluminium (a conductor metal), traditional insulating spacers have evolved towards the use of PVC, which conducts less heat (less than wood as well), and, with an insulation value of R 2.0, provide improved efficiency for thermal glass assemblies.

    If the thermal windows on your home were installed more than a few years ago, the insulating spacers are probably traditional spacers made of either aluminium or PVC.
  • "Intercept" insulating spacers: Made of PVC, the cross-section of these frames is U-shaped.

    As illustrated here, this type of insulating spacer has only one heat conducting channel (indicated in green). In combination with PVC, a material with limited conductivity, this type of insulating spacer further reduces heat loss and provides an insulation value of R 2.0 to R 2.2.

    Over the last few years, thermal glass manufacturers have migrated towards this type of insulating spacer, which is more efficient than traditional units.

    It is important to note that the PVC used today is stable: its colour will not fade, nor will it be altered when exposed to the sun’s ultraviolet rays. This explains why manufacturers now rely on the material for much of their production needs.
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