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Of all the components in a building, it is through the windows that most heat is lost. This is because glass allows heat to escape more readily than most other building materials.
For example, given the same area of wall and window, the window will allow up to eight times more heat to escape. For this reason, it is important that your windows are as efficient as possible.
Glass
To meet the current regulations a minimum standard of double-glazing with a 12mm air gap and soft low-E glass is necessary. This change is due to the need to reduce energy consumption and the desire to improve comfort.
So why do double glazed windows prevent more energy from escaping? It is mainly because of the space between the two panes of glass. This gap is filled with air, which is a poor conductor of heat, thereby making it more difficult for the heat in the room to bridge the gap to the outside pane.
While standard double-glazing is a great improvement over single, even more advanced glazing systems are now available on the market at affordable prices. Two examples include gas-filled double-glazing and low emissivity glazing.
In gas-filled double-glazing, the cavity between the two panes of glass is filled with an inert gas (usually argon) which conducts less heat than air, therefore improving the window's energy efficiency. This type of system generally costs about 10 - 20% more than standard double-glazing, but will vary from one manufacturer to another.
The low emissivity or low-e type system is more complex. The outside face of the internal glass pane is coated with a special material, which allows light to pass in through it while very little heat is allowed to pass out. When light hits an opaque surface, much of its energy is turned to heat. Heat is similar to light in that they are both forms of energy, which travel in waves. However, heat has a much longer wavelength than light. The specially applied coating on the glass prevents the long wave heat energy from escaping. It also prevents much of the heat generated in a home heating system from escaping. In short, it acts as an invisible layer of insulation.
This will generally cost between 10-15% more than standard double-glazing but is very energy-efficient.
In both Denmark and Germany, this form of glazing is fast becoming the standard installed in all buildings and it is possible that in Ireland it will also become the norm in the not too distant future. Other combinations of glazing systems include low-e gas-filled double-glazing or triple glazing.
Window Frames
In addition to the glazing, the window frame must be considered when addressing the energy efficiency of a window. When viewed in terms of overall heat loss the difference in performance of these materials is not significant.
Air leakage through gaps in window frames can also lead to considerable heat loss. By selecting a well-designed system, further energy and cost savings can be made. All proprietary systems should have draught strips built into the frame.
Other points to consider include ease of opening for ventilation and cleaning, the security features and provision for controllable `trickle' ventilation. The durability of the window frame is important, as poor quality will result in higher maintenance.
With such a selection of new glazing systems, it can be difficult to know which one to choose. If you are thinking of replacing your existing windows you probably have single glazing – up to just a few years ago this or poor quality double-glazing was still being installed in most houses. In recent years, however, the cost of double-glazing has dropped considerably, making it much more affordable. Good quality double-glazed windows should have an air gap of 12 mm or greater. If you are replacing your windows, you should consider installing double-glazing as a minimum. Not only will you save energy but you will also increase your comfort levels immediately.
When selecting double-glazing you should shop around.
There are numerous reputable window manufacturers in Ireland who can supply, and in many cases fit, good quality double-glazed windows.
The extra cost of double-glazing with a low-e coating is not a whole lot and could save you even more money. If your budget allows, you should give this option serious consideration. Not all window manufacturers or installers will be familiar with low-e glazing. However, it will not be long before most manufacturers and installers will be offering it.
Who will install them? The manufacturer from whom you will buy them will probably recommend an approved installer. Otherwise, you will need to find one. Remember, this is a job for a professional.
If replacement windows are beyond your budget then there are also secondary glazing systems available on the market.
These are normally installed inside your existing windows, creating an air space between this and the new `window'. There are a number of different designs - for example those with sliding frames. While these systems can help to reduce heat loss and save you money if installed correctly, provision
for the ventilation of the air gap is important. Otherwise, condensation could become a problem.
In a new home, nothing but double-glazing, as a minimum, will do. However, since you have to install windows anyway, why not consider low-e double-glazing, and start saving from day one? In years ahead, it may even add value to your home. Talk to your building contractor as early as possible to assess the options.
It is important to know that if you are changing from old single-glazed windows to more modern double-glazed systems it is likely that draughts will be greatly reduced. While this will help to save energy, it can cause moisture levels in your house to increase. You should always ensure that there is some controllable means of ventilating all inhabitable rooms.
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