Double glazing means two sheets (panes) of glass are sealed together with a 10-12mm gap (cavity) between them and that gap works as insulation, that is, it reduces the heat transfer through the window. It doesn't sound much for the costs you have to pay, but it works. Double glazing is often using special low emissivity glass coating and the gap can be filled with argon instead of air. Both increase the insulating properties of the window, making them work better but the principle is the same as with regular glass filled with air.
Double glazing effect can also be created by fitting a new pane of glass or even a sheet of plastic (perspex) over an existing window and as long as there is a gap between them it will work almost as good as professionally made windows. This is called secondary glazing and there are companies that make these inserts for those who don't want to pay double glazing price or for some reason are unable to replace entire windows.
We also included tightly fit heavy drapes with pelmets because they can trap the air in the gap (cavity) between drapes and the window which is said to work as good if not better than double glazing. Except for the visibility part of course so this solution is best for bedrooms where the light is not as important as in other living areas.
Single glazed windows are no barrier to heat transfer. In winter the heat escapes easily (or the cold enters the house, depends on which side of the window is observed)
The small gap between two panes of glass creates really effective barrier for heat transfer
Drapes with pelmets. The reason why drapes with pelmets are considered more effective than double glazing is that they also seal from heat loss through the frame.
Its important to note that the heat loss in winter or gain in summer can not be stopped completely. Double glazed windows improve average 40% over single glazing and the best ones offer as much as 70% improvement. To see which windows are the best or to check how good are the windows you were quoted you can look them up on WERS database where you can find hot and cold star rating, and % of hot and cold improvement (over 3mm single glazed).
Also note that radiant heat (direct sun) is pretty much "immune" to double glazing. It can be controlled by using tinted glass, external blinds, eaves or verandahs to shield the window from the sun. However care must be taken when selecting the type of cover as in winter we actually do want that direct sun to warm up the house with less or no energy used.
Apart from glass, we should also consider the frame. The material it is made of can also make a difference. A lot of heat can be transferred through metal frame so much in fact that it makes sense to reduce that transfer as well. You can use timber or plastic frames or if you like the aluminium frames makes sure they are "thermally broken". Thermally broken means that the aluminium frame is made of 2 frames, internal and external part, held together with plastic joiner strips and the plastic slows down the heat transfer through the frame. See the picture - a section through double glazed window and frame.
To see how it works in practice, we took infrared pictures of single and double glazed windows.
Single glazed from the outside in winter. Yellow and red colour shows heat escaping from the house.
Single glazed window from the inside in winter. Blue colour shows cold entering the house
To compare we took picture of double glazed window with sliding door opened slightly to show the temperature outside and blinds partially drawn to see any effect of internal covering.
Double glazed window from the outside. The glass is pretty much as cold as the outside which means the heat transfer rate is very small through the glass, more through the aluminium frame.
Double glazed window from the inside. The frame is colder than internal pane of glass. The blinds are not tight fitted so their effectiveness is minimal
We can see the internal glass is warm as the room and the external is cold. The temperature of the glass outside was average 14 degrees while the outside wall is 11.5 so there is still some heat transfer but nowhere near what a single glazed window would transfer and it be reduced further by using better glass or argon fill.
Some concern is a frame. It's aluminium and its cold inside and warm outside which means heat is escaping through it. Another thing to notice is the blind colour over the open section of the door. Yellow indicates the blind is much cooler than elsewhere over the glass. So these blinds are not much help in reducing heat transfer in winter and neither they will be in summer.
There is a rebate for replacing single glazed widows with double glazed products but the rebate is rather small. If you're interested in applying for the rebate (and rebates on any other energy efficiency improvements) visit our site at homelab.com.au where you can read about the rebate process.