2.1 The structure of triple glazing
There is much more to triple glazing than just assembling 3 sheets of glass.
To maximize the insulating effect the optimum gap required between the panes is about 16mm. Too close together results in radiant heat loss, too far apart leads to creation of gas currents which causes convection current heat losses.
The gap should be filled with inert gas as this provides greater insulation than air due to low thermal conductivity. Argon is commonly used as it is cheap to produce and is about 1/3 better as an insulator than air. Krypton is used in the highest specification units as it has double the insulation property of air.
At least the inner, and ideally also the middle pane should have a low emissivity coating. Also known as K glass, this has a metallic coating on the inner side of the unit. The effect of the coating is to allow short wave length sunlight to enter a room and heat it and to achieve a large reflection on the long wave length sunlight, thus the window minimizes the existing heat loss that exists around all windows. The low emissivity coating is normally made up of a thin layer of metal, preferably indium-oxide, rod-oxide, copper, silver or gold. The low emissivity coating is normally applied straight onto the glass or in certain cases onto a plastic film that then in turn is attached to the glass. Depending on what metal the coating is made of the triple glazing gets different characteristics.
Most glazed units use glass panes of the same thickness, but this is not a necessary requirement. To reduce shear effects on the sealed glaze unit, most standard production units are restricted to a thickness difference of 1mm between consecutive panes.
The key to the structural integrity of the sealed units is the frame. This has to be rigid because any twisting in the unit will shatter the glass.
Early double glazing in the UK used aluminum frames, which are both light and strong. However as a metal, it conducts heat out, so undermines the thermal efficiency of the units. Most double glazing units are now uPVC, generally white, with aluminum internal support, which has better insulation properties.
Triple glazing is usually found with wooden frames, because of its great insulation properties, and relative cheapness in the cold climate countries.
The panes of glass are separated by spacers. Originally metal spacers were employed, but these were effectively cold bridges and reduced the efficiency of the sealed unit. There was also an increased risk of condensation. Now the spacers are usually made from glass fibre.
2.2 Illustrated picture of the structure behind triple glazed windows.
A triple glazed window is so much more that just a few layers of glass and a frame. Below is a detailed description of what a typical triple glazed window looks like. This will make it easier in understanding the terminology of triple glazing.
2.2.1 Bottom Rail
The bottom horizontal part of the sash window.
2.2.2 Top Rail
The top horizontal portion of the sash window.
An opening part of a window.
2.2.4 Window Latch
Metal hook that locks the sash window in the closed position.
2.2.5 Insulating Glass
Two or more glass panes that are hermetically sealed. The gap between the glass panes contains either dry air or gas.
2.2.6 Coupled Window
Two windows separated by a mullion. (i.e. four panes of glass to clean)
The part that is attached to the wall.
2.2.8 Fixed/Picture Window
A window with no moving parts or sashes. (i.e. a window that does not open)
2.2.9 Overall Frame Size
Accurate dimensions of the frame, the width is always specified first.
Example: 985 x 1185 mm = one module dimensions (10x12).
The lower horizontal piece forming the bottom of the window frame.
The vertical structural center pillar which divides adjacent windows.
Decorative trim around interior frame of window.
Device for locking a casement window - normally mounted on the vertical frame.
2.2.14 Muntin (Muntin Bars)
Strips (of wood or metal) separating and holding panes of glass in a window.
2.2.15 Casement Window
A window that opens from one side, usually attached to its frame by one or more hinges - like a door.
2.2.16 Double glazing or Triple glazing
The number of layers of glass in each window. Triple glazing being more energy efficient.
2.2.17 Cross Post
The horizontal center pillar between the two sash windows.
2.2.18 Tilt and Turn Window
A window system that opens inwardly in two ways. Tilt and turn windows swing completely inwards allowing for easy cleaning. They also tilt at the top to provide good ventilation without leaving the window fully open.
The directory of triple glazing suppliers.