Insulating glass units – basic information

Insulating glass units, also known as thermal glazing, are commonly used in windows. They are multi-layered panels consisting of two or more glass plates separated by a gas-filled cavity.

The basic components of insulating glass units are:

1. Glazing: these are the individual (or laminated) panes that make up the outer layers of the IGU. They can be of different thicknesses, depending on thermal and acoustic insulation requirements.
2. Glazing cavity: this is the space separating the glass panels. It can be filled with gas or vacuum. The most commonly used gas is argon, as it has better insulating properties than air.
3. Inter-glazing frame: this is the metal or plastic profile placed between the glass panels in an insulating glass unit. Its main function is to maintain the distance and stability between the glass panels and to ensure a solid connection between them. The inter-glazing frame can be made of aluminium, stainless steel, plastic or any other material of adequate strengthi.
   Here are some of the functions of the interglazing frame:
   1. Maintaining distance: the interglazing frame ensures that the glass panels are evenly spaced and maintain the correct distance between them. This is important to maintain the correct interlayer space, which contains the gas responsible for thermal and acoustic insulation.
   2. Structural reinforcement: the interglazing frame adds rigidity and stability to the overall glazing unit. It also protects the glazing from deformation and mechanical damage.
   3. Thermal insulation: in some double-glazed units, the inter-pane frame may be made of a material with low thermal conductivity, the so-called warm frame, which additionally improves the thermal insulation of the entire double-glazed unit by reducing the so-called thermal bridges.
   4. Moisture absorbers are placed in the inter-pane frame to control moisture and minimise condensation inside the glass.
      Moisture absorbers are materials that have the ability to absorb and retain water vapour. The most common material used in moisture absorbers is silica gel, which is safe to use and has a high capacity to absorb moisture. When ambient moisture enters the inter-glass space, the moisture absorbers absorb this moisture, keeping it inside themselves and preventing condensation on the glazing surfaces. It is worth noting that not all double-glazed units have built-in moisture absorbers in the inter-glazing frames.
4. Sealing: to maintain gas or vacuum in the space between the panes of glass, IGUs are sealed using special gaskets (butyl gaskets). The seals also protect against the ingress of moisture and dirt.
   Butyl gaskets are widely used due to their excellent sealing properties and flexibility.

DOUBLE-glazed unit (single-chamber)

TRIPLE-glazed unit (double-chamber)

1. glazing   2. inter-pane space   3. inter-pane frame (spacer)   4. butyl seal   5. molecular sieve (moisture absorber)   6. sealing

 

Insulating glass units are widely used in joinery. There are many different types of double glazing units, which can be tailored to individual needs, taking into account aspects such as:

• thermal insulation,
• sound insulation,
• protection against solar radiation,
• protection against ‘peeping’ (frosted glass, ornamental glass)
• security.

In the upcoming posts, we discuss these aspects.

There was information in the description that may be of interest to some readers.

Butyl is a type of synthetic rubber with high resistance to weathering, UV radiation and temperature. It is used as a sealing layer between glass panels in insulating glass units. Butyl seals are flexible and durable, effectively protecting the space between the glass panes from the ingress of air, moisture, dirt and reducing the risk of condensation inside the glass.

The process of sealing insulating glass units using butyl involves applying sealing strips or cords to one surface of a glass panel. When the two glass panels are joined, the butyl seal forms an airtight joint that provides thermal insulation and protects against moisture.

Thermal bridges are areas in a structure where heat conduction is significantly higher than in the rest of the structure. In double glazing, thermal bridges occur where the inter-glazing frame connects to the glass panels.
If the inter-glazing frame has a high thermal conductivity compared to the rest of the structure, it can lead to heat loss and result in a reduction in the thermal insulation of the insulating glass unit in this area. This means that as a result of thermal bridging, the temperature on the surface of the frame can be lower or higher compared to the rest of the glass surface, which can lead to condensation (water vapour) or energy loss.
To minimise the effect of thermal bridging, manufacturers of double glazing units often use frame profiles made of materials with low thermal conductivity, such as plastics with suitable insulating properties, known as warm frames . Such frame profiles help to reduce heat conduction through the frame and minimise the effect of thermal bridges on the thermal insulation performance of the double glazed unit.

Moisture condensation can occur when air with high humidity comes into contact with a surface that has a lower temperature. In the case of thermal bridges, where the temperature on the surface of the frame may be different to the rest of the glass surface (the point of contact between the glass and the inter-glazing frame), there is a possibility that moist room air will condense on this surface. This is particularly likely if the indoor air is humid and the surface temperature of the frame is below the dew point (the temperature at which water vapour in the air condenses to form water droplets). 

The dew point is the temperature at which moist air becomes saturated and is unable to hold all the water vapour it contains in gaseous form. Excess water vapour condenses on cool surfaces where the temperature is lower than the dew point. The dew point depends on the water vapour content of the air. The higher the relative humidity of the air, the higher the dew point.

In practice, condensation on the surface of the frame can lead to various problems such as mould growth, damage to the frame surface, weakening of the structure or loss of aesthetics. It is therefore important to design and select insulating glass units with suitable thermal insulation properties, minimising the risk of condensation on surfaces in the vicinity of thermal bridges.

In order to avoid condensation on the surfaces of double glazing units, it is also recommended that the rooms are properly ventilated, that the appropriate relative humidity is maintained and that the entire window structure is well thermally insulated, including the areas where thermal bridges occur.