The last post on Vacuum Glazing reminded me of another fact when looking at glazing. Most new glazing have a few energy saving features:
1. Glass coating (commonly known as low E glazing)
2. Low conductivity spacers (the material that keeps the panes of glass apart at the edges)
3. The gaseous fill in the glazing (argon and krypton are examples)
1. Glass coating is a good way of keeping heat in during the night time and winter months and the heat out out during the sunny summer days. The coatings can be applied as either a hard or soft coat, but given that the coating is applied to the inner glazing panel that is within the glazing unit there should be no real wear and tear to disturb its workings. These glass coatings are therefore a cheap and easy way to make your house a little more energy efficient (but the effect is quite minimal in the grand scheme of things).
2. These are physical structures that can help reduce cold bridging between the glazing sheets. This can be reasonably important when looking at high performance windows (but then the high performance window manufacturers know this and will fit them as a matter of course). But if you are specifying more standard glazing it is worth ensuring that you get these edge spacers as they are a physical attribute that will not decrease in their energy saving value over time.
3. The fill of windows actually makes quite a big difference to their performance. So having an argon fill is worthwhile, however be aware that the gaseous fill will, over time, dissipate. So after around 10 years the window will not be performing as well as it did when it was originally fitted. There is nothing that can be done about this, so maybe it is worth thinking about it all in a different way.
One way of getting similar performance to gas filled double glazing is to fit air filled triple glazing. The insulation values between the two are quite similar. Of course a gas filled triple glazed unit will be even better for the first 10 years or so.
Triple glazed windows are more expensive of course, this is because of the additional glass, weight and normally an enhanced frame size. The costs may never recouped from the additional heat load, however if you are trying to get a building to perform well and consistently then it might be worth investigating.
Note that triple glazed windows cut out some of the light coming into the building and if this is important then it might be worth considering installing double glazed windows to the more south facing aspects of the building (this will allow more heat and light into the building) and then have triple glazed to the north. The north facing windows will only even let heat out and so having these are higher performance windows makes more sense.