Thermal separation of insulated glazing to optimise the Uw value

Why windows steam up from the inside and how this can be prevented at the building planning stage

The energy efficiency of buildings has a major part to play nowadays. In order to achieve a reduction in energy consumption, it is extremely important not only to have modern heating and systems technology but also to insulate windows, doors and façades. Regardless of which material is used to make the frame, the glass must also be correctly insulated. This is important for the Uw value of the window. To keep the latter as low as possible, nowadays Warm Edge spacers are incorporated between the panes of glass.

What is insulating glass made from and why are spacers needed at all

Insulating glass consists of two or three panes of glass. Depending on the desired function, the panes have different kinds of coatings. The inner pane, for example, is additionally equipped with a Low-E thermal insulation coating. For improved thermal insulation in general, the spaces between panes are filled with a noble gas. In order to guarantee the functionality of the glass element long term, the edge bond must hermetically seal the space between panes. Neither moisture may penetrate nor gas escape. But on the edge bond specifically, heat can flow unimpeded. Consequently the panes must be thermally separated here by what are known as spacers. As the name suggests, they keep the panes of insulated glazing at a distance from one another. This interrupts the conduction of heat at the point of transition from the glazing on the frame, and hence reduces it. The thermal insulation in the transition area between the glass and the frame is significantly influenced by the thermal conduction capacity of the insulating glass spacer which − depending on the structure of the glazing – joins the two or three individual panes on their outer edges to form one insulating glass unit. The task of the spacer is to safeguard, on a long term basis, the pre-defined space between the individual panes and, in combination with a primary seal made from butyl going all the way round and a secondary seal that uses a sealing material, to reliably close off the space between panes from the outside. The climate loads acting on the insulating glass place stresses on the edge bond throughout the pane’s lifetime. The edge bond must withstand these stresses long term.

Cold edge leads to high Uw value for the window

Traditional spacers are made from aluminium – referred to as a “Cold Edge”. The downside to this material is its high thermal conduction capacity. When incorporated into an insulating glass edge bond, the aluminium profile forms a highly thermally conductive link between the inner and outer pane. As a result, significant linear thermal bridges are generated in windows and façades. In the peripheral area of the pane, valuable thermal heat is thereby conducted to the outside. The glass edge in the room cools down. Condensation forms on the glass edge, which can result in mould formation. This is a particular problem with wood when condensation can also damage the frame construction in the long term.

Conversely, with air-conditioned buildings, conventional insulated glass spacers can result in increased energy consumption for cooling. Furthermore, the huge thermal conduction capacity of aluminium leads to a high Uw value (the Uw value for the entire window is composed of the U value for the frame Uf, the glazing Ug and the Psi value). The glass, therefore, is not optimally insulated. This increases the energy costs and has a negative impact on the energy footprint of the building. Warm Edge spacers have proven their worth when it comes to preventing all of this.

Reduction in the UW value with Warm Edge spacers

The term Warm Edge is used in the context of insulated glazing to describe an edge bond in which the spacer between the panes of glass consists of materials with a low thermal conduction capacity. Here products made from stainless steel, insulating plastic in combination with stainless steel or structured silicone foam, for example, significantly reduce the heat losses at the glass edge. Thus on the edge bond on the room side, higher surface temperatures are achieved and the glass has what is known as a “Warm Edge”. This saves on thermal heat, increases comfort in the room and reduces the risk of condensation being formed on the glass edge. At the same time, Warm Edge spacers mean that it feels less cold near the window. The Uw value and the Ucw value (thermal transmission coefficient of façades) are significantly reduced. Through the use of the Warm Edge, a window’s thermal insulation can be considerably improved. The Psi value can be reduced by up to 60 % and thereby the U value of the construction as a whole reduced. The extent to which the spacer’s influence makes a difference to the rating of the component as a whole essentially depends on the size and shape of the insulation glazing. Visually, the Warm Edge spacers made from plastic blend in discreetly with the glass – in contrast to the silvery-looking, shiny metallic spacers.

Warm Edge spacer made from high-performance plastic in the glass edge zone

“Warm Edge” spacers, for example Thermix® from the German plastics specialist Ensinger, consist of high-performance plastic (special plastic TECATHERM® PP), which has a thermal conduction capacity that is around 700 times lower than that of aluminium. A wafer-thin diffusion barrier made from stainless steel which has a thermal conduction capacity over 10 times lower than that of aluminium ensures long-term gas-tightness. By using these spacers in the glass edge bond, the Uw value of the window is improved by 0.1 to 0.2 W/(m2K) compared with aluminium spacers.

Providing exceptional rigidity without compromising bendability, Warm Edge spacers can be used to produce insulating glass using established methods – regardless of whether it is manufactured with corner keys or bent frames. Specially developed corner keys and straight connectors ensure well-fitting and secure connections. This speeds up the production process and enhances productivity and economy. This makes Thermix® suitable even for passive houses. And the Warm Edge spacers can even be used in unusual designs and shapes.

Something else which has a positive impact on the design is for example the muntin bar. It allows windows to look like genuine muntin windows. The thermally optimised hollow chamber profiles are UV-resistant, fogging free and simpler to process than duplex solutions made of spacer profiles.


To lower the Uw value and the Ucw value effectively and thereby achieve the required energy efficiency for the entire structure, Warm Edge spacers are recommended. In this way window constructors can achieve optimum Uw values for their windows. Planners, architects and building owners can thereby safeguard the long-term quality and energy efficiency of their building. Thermal parameters, high functionality and aesthetic criteria are the key factors in the selection process for the correct spacer. Here, experienced manufacturers such as Ensinger from Germany are offering tried-and-tested Warm Edge spacers made from highly-insulating plastic in several sizes and colours for the thermal separation of insulated glazing, which minimise the risks of condensation for the building’s structure. It is advisable to incorporate this form of window insulation into the planning at an early stage.

Insulating glass with spacers

Picture caption: The spacer – in this case Thermix from the German company Ensinger – keeps the three panes of insulated glazing at a distance from each other. Œ (1) float glass with coating  (2) Thermix spacerŽ (3) adsorbent  (4) primary seal  (5) secondary seal ‘ (6) edge bond.

High loss of heat in an edge bond with aluminium spacers.

Low loss of heat with thermal separation

Low loss of heat with thermal separation of the edge bond using Warm Edge spacers.

Calculation or Uw-Value

Alongside excellent thermal properties, tangible energy savings and high quality, the new Thermix® spacers from Ensinger offer a whole range of additional benefits.

A warm environment with big windows

Modern bars lend the window and consequently the whole building a unique design character. In this type of window, the Thermix® muntin bar from Ensinger ensures a high level of energy efficiency.
Picture courtesy of: virtua73 –