Last update: 06.09.2019
Understanding Glass

Insulated glass

A series of factors and physical rules define the characteristics of insulating glass as it is used in thermal insulation and solar protection applications.

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To achieve thermal insulation properties, several float glass panes should be combined with at least one low E coating to create an insulating glass unit.

Two or more panes of the same size are aligned with each other at a defined distance and glued together. The resulting hermetically sealed interspace is filled with especially effective thermal insulating inert gas. No vacuum is generated, as laypersons often mistakenly assume.

The width of the pane interspace depends on the inert gas that is used. Argon is most frequently used, krypton more rarely. To reach its optimum thermal insulation efficiency, argon needs an interspace of 15 - 18 mm; krypton needs only 10 - 12 mm for better insulating results. The gas filling rate is typically about 90%. Krypton is many times more expensive than argon since it is more rare.

The spacer that permanently separates the panes has some influence on the insulating performance and consequently on the dew point at the edge of the glazing. For the past few decades, aluminium spacers have been the industry standard. These are being replaced today by systems with lower heat conductivity (warm-edge technology).

Colour rendering index

Colour rendering is not only relevant for the physiological perception of the observer, but also for aesthetic and psychological aspects. Sunlight that falls through an object or is reflected by it is changed relative to the nature of the object.

The colour rendering index (Ra value) describes how much an object’s colour changes when it is observed through glazing. It defines the spectral quality of glass in transmission, and the value can range from 0 to 100. The higher the colour rendering index, the more natural the reflected colours appear. An Ra value of 100 means that the colour of the object observed through the glazing is identical to the original colour.

A colour rendering index of > 90 is rated as very good and > 80 as good. Architectural glass based on clear float glass generally has an Ra value > 90, and body tinted glass usually has an Ra value between 60 and 90.

Colour rendering index

The colour rendering index is determined according to EN 410.

Dew point and condensation

Condensation in the space of insulating glass units indicates a defect edge seal, missing desiccant or incorrect manufacturing.

Condensation in the interspace of the unit

This rarely occurs with today’s insulating glasses, since they are hermetically sealed and filled with dried gases. Condensation in the space of insulating glass units indicates a defect edge seal, missing desiccant or incorrect manufacturing.

Condensation on the interior surface of the unit

This occurs on poorly thermally insulated windows or those with single glazing. Warm air cools suddenly near windows and transfers humidity to the cold inside pane – the temperature in winter is often below the dew point of the ambient air. Today, the inside pane in insulating glass stays warm longer so that condensation rarely occurs.
If the relative air humidity is very high, for example due to cooking, washing or proximity to a swimming pool, panes may condensate more often. One way to correct this is to exchange the air by means of short and direct ventilation.
The outside temperature at which the glazing on the inner side condensates (= formation of condensation water = dew point), can be determined using the dew point graph.

Dew point graph

Recorded examples (see dew point graph):

  • room temperature 20°C
  • room humidity 50% 
  • outdoor temperature 9°C

Dew points at*:

  • Ug = 5.8 W/m²K    →   9°C
  • Ug = 3.0 W/m²K    →  -8°C
  • Ug = 1.4 W/m²K    →-40°C
  • Ug = 1.1 W/m²K    →-48°C

*condensation formed at the displayed temperature

Condensation on the exterior surface of the unit

This effect has appeared with the advent of modern insulated glass, and is particularly noticeable during the early morning hours, when the moisture content in the outside air has sharply increased during the night.
The excellent insulating quality of these glass surfaces prohibits heat transfer to the outside, so the outer pane remains extremely cold. When the sun’s rays start to heat the outside air faster than the temperature of the pane, it may lead to condensation, depending on the orientation of the building and the environment. This is not a defect, but proof of the excellent thermal insulation of the insulating glass.

Triple glazings and roof windows typically show a much higher tendency for condensation on the outer surface in cold areas with higher humidity.

Glazing condensation

The graph shows a typical situation. In the critical zone is the environmental temperature close to the dew point (red zone).

With ClimaGuard Dry, GUARDIAN offers a special coating that ensures a clear view through glazing even during the morning hours.

Heat radiation

The reflected heat radiation on surface #1 (ClimaGuard Dry coating) back into the IGU leads to a temperature increase of the outer pane and a significant reduction of the tendency for condensation.

ClimaGuard Dry
Edge Seal

The edge seal of insulating units typically consists of a two-barrier-system.

Sealant systems

The edge seal of insulating units typically consists of a two-barrier-system:

Primary seal

  • “gluing string“ extruded on both sides of the spacer bars.
  • Material: polyisobutylene (Butyl).
  • Good adhesion on glass and spacer.
  • Avoids penetration of moisture and escape of the filling gas.
  • Main seal, responsible for the durability of the insulated glass unit.

The primary sealing material Butyl provides the lowest levels of gas permeability (< 0,002 g/m²h) and moisture permeability (< 0,1 g/m²d) according to EN 1279-4.

Secondary seal

  • Glued connection between glass and spacer.
  • Mechanical strength of the edge seal.
  • Protection of the primary seal.
  • Additional diffusion barrier.

The following materials are most commonly used as secondary sealants:

  • Polysulfide (2-component organic polymer).
  • Polyurethane (2-component organic polymer).
  • Hotmelt (1-component organic polymer).
  • Silicone (1- and 2-component).

The gas and moisture permeability of the organic polymers are much lower compared to silicone.
Secondary sealants based on organic polymers do not resist UV-A radiation and must be protected. Any insulating unit with edges exposed to UV should be equipped with silicone as secondary sealant. Due to the bonding strength, silicones resist UV impact. 


The thermal properties of insulating glass refer to the centre area of the panes without any influences from the insulating glass edges.

Until very recently, the majority of insulating glass was produced using aluminium spacers. More demanding requirements have created thermally improved alternatives that are gaining ground in insulating glass production. In the meantime, the aluminium spacer bars were increasingly replaced by other materials using the so-called “warm-edge-technology“.

With the linear heat transfer coefficient (Ψ value), the spacer material directly influences the heat transmission coefficient of the window Uw. 

Stainless steel

Extremely thin stainless-steel profiles with considerably reduced heat conductivity when compared to aluminium are the most common alternative. They are similar to aluminium, however, in terms of their mechanical stability and diffusion capability.

Metal / plastic combinations

Another option is plastic spacers, which offer excellent thermal insulation but do not have a sufficient gas diffusion density to ensure the life-cycle of an insulating glass.
Consequently, combinations of plastic with gas-impermeable stainless steel or aluminium films are available.

Thermoplastic systems (TPS)

A hot extruded, special plastic substance, which is placed between two panes of glass during insulating glass production and which guarantees the required mechanical strength, as well as gas diffusion density after cooling down, replaces the conventional metal. The desiccant is incorporated. There is a wide range of disposable alternatives today that provide important reductions of the Ψ value, the unit of the linear heat transfer coefficient in the perimeter zone, when they are directly compared with each other.

Insulating glass effect – climatic loads

A component of every insulating glass is at least one hermetically enclosed space: the interspace. Since this space is filled with air or gas, the panes react like membranes that bulge in and out in reaction to varying air pressure in the surrounding air.

A component of every insulating glass is at least one hermetically enclosed space: the interspace. Since this space is filled with air or gas, the panes react like membranes that bulge in and out in reaction to varying air pressure in the surrounding air.

Insulating glass effect

Under extreme weather conditions, unavoidable distortions may appear, despite the plane-parallel glazing. This can also occur due to extreme changes in air pressure, and influencing factors include the size and geometry of the pane of glass, the width of the interspace, and the structure of the pane of glass itself. With triple insulating glazing, the middle pane remains nearly flat, which is why the impact on both outer panes is stronger than on double insulating glazing. The two gaps of the triple glazing have the same effect as one large gap of the same overall thickness. These deformations disappear without effect once the air pressure normalises and, far from representing a defect, are an indication of the edge seal density.

The deflections caused by high pressure differences (climatic loads) can lead to high mechanical loads on the glass panes of the insulating glass unit but also in the spacer area. Particularly critical are asymmetrical build ups where the thinner glass deflects more than thick or laminated glass and small units where the glass can’t follow the volume change of the filling gas. 

Asymetrical glazing

Guardian recommends a climatic load analysis for triple glazing with wide gaps, unfavourable dimensions and asymmetrical build-ups.

In case of critical load scenarios, the glass should be heat-treated. It is also advisable to check the sealant depth in order to ensure the necessary mechanical strength of the secondary edge seal of the insulating glass unit.

Interference phenomena

When several parallel float glass panes exist, very specific lighting conditions can cause optical phenomena to appear on the surface of the glass. These can be rainbow-like spots, stripes or rings that change their position when one presses on the glazing - phenomena also referred to as Newton rings. These so-called interferences are of a physical nature and are caused by light refraction and spectral overlap. They rarely occur when looking through the glazing, but in reflection from outside. These interferences are no reason for complaint but rather are a proof of quality with regard to the absolute plane parallelism of the installed float glass.


The insulating glass panes are glued together using the dual-barrier system, in which a spacer is used to keep the two panes separated, and a continuous string of butyl adhesive is applied around the edges of the spacer to keep both panes of glass glued together. The space that is created is filled with a desiccant that keeps the interspace permanently dry. During the gluing process, it is important that the coated side of the pane of float glass faces the interspace and that the adhesive is applied to this side. Some types of coatings need to be removed mechanically before the adhesive can be applied properly. Removing the coating before the adhesive is applied increases the bonding strength and protection against corrosion.

The functional layer is now hermetically sealed and permanently protected. The butyl adhesive sealant, also called the primary seal, prevents water vapour from forming and the inert gas from escaping. After the two panes of glass are bonded together, a gas pressure press is used to withdraw some of the air from between the panes and replace it with a defined amount of inert gas. Finally, the insulating pane receives its second sealant and adhesive level (secondary seal) by filling in the hollow between the installed spacers and the outer edges of the panes. The materials most frequently used are polysulfide and polyurethane. 

Insulating glass panes

As an alternative to these adhesive materials, a UV-resistant silicone is used in special installations that have exposed insulating glass edges or require structural functionality.  

Insulating glass panes production