How Does Double Glazing Reduce Heat Loss?
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Double Glazing Basics
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Normal windows are designed to let in light without allowing air to pass. They are very good at this job, but not so good at insulating. Window panes are thin, and heat can pass through very easily, making it more expensive to heat a building in the winter and cool one in the summer. Double glazed windows are actually two windows in one, separated by a thin layer of air. They let light in like normal windows, but stop heat from leaking in or out.
Double Glazed Construction
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In a double glazed window pane the air is actually sealed in the middle by a frame called a spacer, which goes all the way around the border of the window pane. Silicone sealant is used to stop any air or moisture from leaking in or out from between the windows, and a desiccant absorbs any droplets that may get in. The double glazed windows don't necessarily use particularly thick glass--often each pane is only 4 mm wide--but with the layer of sealed air inside, the whole thing can be 20-30 mm thick or sometimes even thicker.
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How Double Glazed Windows Work
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The window panes themselves insulate somewhat, but most of the insulation actually comes from the trapped air. When molecules are heated, they move more quickly. They conduct this heat to nearby molecules by bumping into them. Glass is a pretty good conductor of heat because it is a solid. The molecules are packed in pretty tightly, so they can easily move heat in or out of the room. The air in the middle, on the other hand, doesn't conduct heat that well. The molecules in gas are spread out much more thinly than in a solid. Although heat will flow through air, it takes much longer. The air in the middle of the glass works as an insulator and stops heat from leaking in or out.
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