A double glazed sealed window comprises of two panes of glass. These two glasses are held apart by a 'spacer bar'. There is an air gap in between each glass which is sealed around the perimeter. Glass is a good conductor of heat. So, just a single glazed or single pane window will not insulate your house very well. Air is a poor conductor of heat, and therefore has superior insulating properties. The air gap between the two sheets of glass reduces the heat conduction between the panes. This helps keep more heat inside your house during winter and outside your house during summer.
The overall thickness of the sealed unit, therefore, includes the air gap plus the thickness of the two panes of glass. The air gap between the panes is generally between 12mm to 20mm thick. The thickness does not impact the insulating properties substantially i.e. a 12mm gap sealed unit is almost as good an insulator as 20mm sealed unit. In fact, a 20mm air gap sealed unit allows increased air circulation within, which impacts the insulation negatively. A 16mm air gap is considered the optimum thickness and is usually recommended by most users.
The gap between the two panes can also be filled with inert gases, like argon. Argon is popular because it is economical and it is a gas that is most suitable for domestic purposes. Other gases, like Krypton and Xenon, are very expensive. Therefore, they are used when higher degrees of insulation are required in smaller and narrower spaces. For home use, argon is the gas to use.
These sealed units are available in various glasses. You can choose a glass according to your taste - clear, toughened, patterned, laminated, Georgian wired and low tinted which is available with enhanced thermal coating. You can also use aluminium, wooden or plastic (PVCu) frames for these units. In UK, they should conform to British Standard kitemark BS EN 1279.
Most double glazing units come with a minimum of a ten-year guarantee. Sometimes the sealed units can start misting and become faulty. This happens when a hole or a small puncture develops somewhere in its perimeter. Then, moisture creeps inside and between the two panes of glass. This leads to condensation of moisture between the two panes of glass. Condensation affects the clarity and visibility of the glass in the sealed unit.
You must take care of such problems while installing the unit, especially if you use aluminium and PVCu frames. With wooden frames, misting occurs when the sealed units are not drained properly. It can also happen if the perimeter of the sealed unit is not ventilated effectively. In most cases the fault is repairable. Sometimes, though, you may need to replace the unit.
Double glazed units have many advantages like thermal insulation that works as a barrier to cold winds and draught. Effective insulation leads to lower energy consumption, so such units save you considerable heating and fuel expenses. Double-glazing also keeps out unwanted noise disturbances while boosting the market value of a property. Double glazed units result in comfortable living and working conditions even in the worst British climates.