All double glazed windows are much better insulators than ordinary single pane windows - that's why modern building regulations require their use in all new houses.
But there are various features which you can choose to add, which increase the energy efficiency of your new windows still further, albeit at extra cost.
One such feature is "low e glass", also known as "energy efficient glass" or "energy saving glass".
But what exactly is low e glass, and when should you specify it for your own windows? The "e" in low e glass stands for "emissivity" which is a measure of how much heat energy a material emits.
Ordinary glass has very high emissivity - ie it emits a lot of thermal energy - not ideal if you're trying to stop the warmth escaping from your home through the windows.
Low e glass has been treated by depositing a very thin layer of either tin oxide or silver onto the surface of the glass, to reduce the amount of heat it emits.
To give you an idea of the difference this can make: Untreated glass typically absorbs 84% of the heat energy directed towards it, and then re-radiates that energy towards the cold outside air.
Low e glass can absorb as little as 15% of the heat energy, reflecting the other 85% back into the room.
Obviously this makes a double glazed window with low e glass, much more efficient at preventing heat escaping from your home.
There is another use for low-e glass: By coating the other side of the glass, you can keep heat out just as effectively - that could be extremely useful in a conservatory for example, helping to stop the room overheating when the sun shines, without having to fit expensive blinds which also block your view of the sky.
Nothing's perfect of course, and low e glass has a downside too: It costs more than ordinary glass.
So when should you consider choosing low e glass? If you're buying it to prevent heat loss, it can save you several thousand pounds off your bills over 20 years, so you should be able to get a return on your investment if you plan to stay in your home for the long term - but if you are doing up your house with a view to selling it within a few years, you probably won't get your money back before you sell.
If you want to use it in a conservatory to make it more habitable in the summer, then you may well consider the extra expense to be worthwhile over a short timescale.
In conclusion, low e glass is a very interesting technology that has the potential to save you a lot of money on your bills, but the extra cost may only be worthwhile if you plan to stay in your property for a long time.