In a recent post, we explained the benefits of using DIY low-emissivity coatings to insulate your current windows at the fraction of the cost of replacing them.  But if you are designing a new sustainable building or home and have the opportunity to select your windows from the cutting edge options available today, these general guidelines will help you make logical decisions.
There is a true science behind low-e coatings that bares the necessity of mentioning that no two low-e coatings are created equal.  However, this is not to say that some products are universally better than others, but that selecting the right window depends on the climate it is being installed in and orientation to the sun.  In order to receive maximum energy efficiency, each window on a building should be analyzed independently and fitted with the appropriately insulated window.

Many superinsulated window manufacturers take these factors into consideration to offer a lineup of windows that are optimal for each application. 

In general, for colder climates, it is desired to encourage long-wave solar heat gain and not let the reradiated heat out of the interior space.  For this application, a low-e hard coat on the inner-most surface of the window would be optimal.  A hard coat is a layer of indium tin oxide that gets floated into the glass when it is still molten.  This allows for more durable surface hence the name hard coat. Compared to soft coats this allows 20-30% more light transmittance, which decreases its insulating R-Value, but encourages passive solar heat gain.  This combined with its application to the inner-most window surface makes it the appropriate choice for colder climates.

For warmer climates, it would be desired to use a soft low-e coating that is typically a spattered layer of silver on the already-hardened pane of glass.  This layer is less durable compared to the hard coat of indium tin oxide, but delivers less light transmittance, thus rejecting more radiant energy from the sun that will contribute to unwanted heat gain in this particular case.  For optimal results this layer should be applied to the outer most surface of the window, but due to the delicate nature of the soft low-e coatings, can be applied to surface #2, just inside the air-space of the window. 

For moderate climates that see a fair share of all four seasons, selecting energy efficient windows becomes more complicated.  In this scenario, it is important to consider each window and its orientation to the sun.  In the northern hemisphere, northern facing windows will never see too much direct sunlight.  Because of this it is not as necessary to have a low light transmittance soft coating, but a well-insulated double or triple pain air-space with a hard low-e coating that will help keep the interior climate constant.

On the contrary, south facing windows might want to encourage solar heat gain for winter months and inhibit solar heat gain in warmer months.  In this case, a well-insulated window is most effective in controlling interior temperatures and the use of low-e coatings is necessary to insulate but the difference in using a soft or hard coat and placement of the coating within the window will be very minimal when evaluated over the course of a year.  There are other measures to be taken in order to create the most energy efficient interior spaces.  Using louvered sun shades and strategically placed operable windows may help to deliver better results as the seasons change.