Warm edge spacers and smart glass

Warm edge spacers reduce condensation in insulated smart glass units, minimising the risk of short circuits and electrical fires.

In a Nutshell

This article is about how warm edge spacers have revolutionised the building facade industry by improving thermal insulation in windows.

They also reduce electrical safety issues in insulated smart glass by eliminating condensation, thus mitigating the risk of short circuits.

Let’s dive in.

Window Edges

A house typically loses 10% of its heat through the windows, of which 80% is lost through the edges.

This subsequently makes the window edges a weak point in the overall insulation of the building.

Windows, doors and skylights nowadays incorporate Insulated Glass Units (IGUs) having two, three or even four panes of glass, and air gaps for thermal insulation. 

This is shown in the diagram below.

Within each ‘air gap’ (normally filled with argon or krypton), we find an ‘edge spacer’ (shown in orange). This seals the window cavity and controls thermal conduction from the interior to the exterior (and vice versa).

Edge spacers in double, triple and quadruple-pane Insulated Glass Unit IGU

Double-, Triple- and Quadruple-pane IGUs with edge spacers (in orange) which seal the window cavity

Aluminium Edge Spacers

Whereas the gas-filled window cavity reduces heat transfer through the centre of the window, the edge spacer should reduce heat transfer at the edge of the window.

Unfortunately, the edge spacers that have been used so far in the industry are made from aluminium. And aluminium conducts heat to the outside, resulting in thermal losses and cost inefficiencies.

Also in the summer months, aluminium edge spacers conduct heat into the building by the same heat transfer mechanism. 

This results in your air conditioned house losing its pleasant coolness, as well as wasting the money you spent cooling it.

Heat Loss through Convection

Convection accounts for heat loss within the glazing cavity. 

In a cold climate, the inner pane warms the gas inside the cavity. This warm air rises and cooler air replaces it, creating a convection current which transfers heat from the inner pane to the outer pane.

If the lower part of the window cavity then drops in temperature due to heat loss through the edge spacer, condensation can form inside the IGU at the bottom edge. This can lead to mould growth and respiratory health risks for asthma sufferers.

This is a common and well-known problem associated with aluminium edge spacers.

Smartglass Safety

Condensation is a particularly important issue for electrically-controlled smart glass. The conductors inside the glass present no problem however, since they are insulated.

The weak point is where the cable exits the smartglass and connects to external power. This is especially true when the driving voltages are 65-110 Vac, such as those found in liquid crystal and SPD smart glass.

Even low-voltage technologies such as electrochromic, electrophoretic and transparent photovoltaic smartglass can suffer if there are any weaknesses in the the exit cable.

Therefore, it is at the smartglass cable exit point where condensation can seep into scratches or cuts in the cable, causing a short circuit and possibly an electrical fire.

Warm Edge Spacers

Now, if the edge spacer is made from a thermal insulator, we can reduce heat loss and draughts due to temperature differentials. This also reduces condensation inside the window cavity and likewise the risk of short circuits in the smart glass.

Since the edge spacer conducts less warmth, it feels ‘warm to the touch’. This is why we call it a ‘warm edge spacer’.

Warm edge spacer materials include:-

  • thermoplastics
  • silicone-based materials
  • steel reinforced polymers
  • glass fibres and
  • structural foam with polysulphide seals

A ‘warm edge spacer’ must meet the requirements of Annex E of DIN EN ISO 10077-1 (for windows). 

For curtain walls, Annex B of standard DIN EN ISO 12631 specifies the necessary requirements.

Window Metrics

Thermal insulation in static and dynamic glass windows can be measured with the following metrics:-

U-Value

The U-Value (or U-Factor) indicates how much heat windows, doors, skylights or curtain walls lose.

The U-Value is expressed in Watts per square metre-Kelvin, or W/(m2.K). This measures the heat transfer through a 1 square metre window with a 1º temperature difference across it.

A lower U-Value indicates a more energy-efficient product.

The U-Value is based on all three forms of thermal loss:-

  • thermal radiation – heat transmitted by electromagnetic radiation
  • thermal convection – heat transmitted through the gas-filled window cavity
  • thermal conduction – heat conducted through a solid, such as the frame

The US National Fenestration Rating Council (NFRC) specifies the U-Value as representing the entire window, including the frame and the edge spacer. 

Below you can find representative figures for the U-Value of various window types:-

Window TypeTypical U-Value, W/(m2⋅K)
Single glazing5.7
Double glazed window3.3
Double glazed window with advanced coating2.2
Double glazed window with advanced coating and frame1.2
Triple glazed window1.8
Triple glazed window, with advanced coating and frame0.8

With an additional warm-edge spacer, we can expect to see a reduction in the U-Value of approximately 12%.

Solar Heat Gain Coefficient (SHGC)

This metric indicates the percentage of the sun’s energy that the window directly transmits into the interior (or absorbs then retransmits as infrared).

  • In warmer climates: a low Heat Gain is preferable since it keeps the interior cool and reduces air conditioning costs.
  • In colder climates: a high Heat Gain is the norm since it provides passive heating by the sun and reduces central heating costs.

Condensation Index

The Condensation Index is the ability of a window to resist the formation of condensation under specific environmental conditions. 

For further details, see the document ANSI/NFRC 500-2020, which guides the design of fenestration systems.

The Condensation Index is a dimensionless number between 1 and 100. Higher values indicate better performance and this depends on the thermal conductivity of the materials used.

Outlook

Warm edge spacers made from insulating materials are now prevalent, replacing the aluminium variety and improving thermal efficiency in buildings.

This advance is especially important for smart glass windows, where condensation in the window cavity can result in short circuits and electrical fires. 

Warm edge spacers also reduce draughts and mould, avoiding health issues.

Better window insulation and the improved solar control that smart glass brings can thus enable a faster transition to global Net Zero. 

The industry is now poised to orchestrate multiple overlapping technologies in the fight against the climate crisis.

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