When speaking about wooden windows, we often think of wood, glass and hinges. We rarely think of the spacer bars between glass panes in thermally insulated glass unit (IGU) or we simply do not pay any attention to it because it is almost unnoticed placed at the edge of IGU hidden in the window casement.
Position of spacer bar
The basic function of the spacer bar is to maintain a constant distance between the glass panes (e.g. 12, 14, 16, 18 mm) and to prevent the release of the inert gas (argon, krypton) out of the cavities between glass panes. Latter is additionally prevented by secondary sealant (e.g. polysulphide or silicone), located below the spacer bar. The basic spacer bars are (were) made of aluminium, which is a very good heat conductor. The combination of glass and aluminium spacer bar in the composition of IGU therefore represents a thermal bridge, which reflects in the low temperature of the edge of the inner glass pane and consequently in the formation of a condensate. Alternatives are so-called "warm edge" spacers. More than 20 different types can be found on the market recently. One of the most commonly used is the TGI spacer, which is made of a combination of stainless steel and polypropylene, which prevents the thermal bridge. Currently, the best available spacer bar on the market is Swisspacer Ultimate, which is made of composite polymer material and is free of metals. The empty space between the walls of all three mentioned spacers is filled with a desiccant.
Aluminium (left), TGI (middle) and Swisspacer ultimate (right) spacer bar
Due to the different materials from which spacer bars are made and due to the different linear thermal conductivity (λ), the use of different spacer bars significantly influence the (I) temperature of the edge of the inner glass, and consequently (II) the formation of condensate and (II) the thermal transmittance of the entire window (Uw).
The influence of the type of the spacer bar on the temperature of bottom edge of inner glass
(outside temperature of -10 °C and 0 °C, inside temperature of 20 °C)
The influence of the type of the spacer bar on the minimal relative air humidity (RH) at which condensate will occur at the bottom edge of inner glass
(outside temperature of -10 °C and 0 °C, inside temperature of 20 °C)
The influence of the type of the spacer bar on the thermal transmittance of 1,23 × 1,48 m big window (Uw) with triple IGU
(model Nature Optimo) and double IGU (model Nature)
