Is double glazing worth its money?

The answer is YES, but ONLY if it is installed correctly without a cold bridge (thermal bridge) and if they have a low u-value. 

A window or a door is essentially a hole in the wall and responsible for most of the unwanted heat loss or gain, and even the best window performs not as good as an insulated wall.

However, windows are essential for a house and the comfort and well-being of its habitants, as they let natural light and fresh air into the building and enable views. Appropriate window design, size, location and glazing treatment, combined with shading and internal covers, can significantly reduce the energy required for heating and cooling.

Windows should be orientated to the north where possible. If solar access is good, north-facing windows should be large, but the size also depends on the amount of thermal mass in the building. South and eastf acing windows should be kept pretty small, and windows to the south need to be positioned to enable cooling summer breezes to pass easily through the rooms. Whereas west-facing windows should be avoided where possible, if needed they should be relatively small and well shaded.

Appropriate window sizing, combined with double glazing, and/or close-fitting internal coverings such as drapes with pelmets, can minimise heat loss in winter. Furthermore, it is important not to overshadow windows in winter by the structure of the building itself, as it will reduce the solar access.

The u-value describes the performance of a window. The lower theu-value the better forming the window.

As a general guidance I would not recommend to use a window with a u-value larger than 3.0. Ideally the u-value should be 2.0 or lower.

If you are interested in building to the passive house standard you should be looking at a u-value of 1.1-1.5. in comparison. (for our Melbourne climate)

Another, often underestimated roll in the energy efficiency of a window, is the frame itself, as it can effect negatively on the overall performance.

Some materials, such as metal, glass or aluminium, allow heat to pass through them more easily. A double glazed aluminium window has a u-value between 4-5. And even a thermally broken aluminum frame often has u-value above 3. So better stay away from them.

Generally speaking, PVC and timber frames perform better than metal frames.

My biggest advice is: spend as much money as you can on windows (and insulation)!

Although you might update your kitchen or bathroom again in 10-15 years, you will most likely

never ever touch your windows or the insulation again.