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Windows are probably the most complex part of the building fabric. The window space itself is a strong contrast between the large surrounding mass of the uniform wall and glass which is possibly the worst material for heat loss. This is a focus for condensation which slowly deteriorates the window frame on the inside as well as weather from the outside. This lead to draughts and the need for eventual replacement. However windows are vital for well being and ventilation and need a lot of attention.
There are 3 aspects to windows from the energy efficiency point of view
1. Thermal conductivity – heat passing through the windows to the outside
2. Draughts – cold air streaming in from the outside through ill fitting window frames. This is can get worse with strong winds. Also it must be borne in mind that a house needs a certain amount of ventilation but draughts are generally uncontrolled ventilation and should be replaced with controlled ventilation .
3. The heartbreaking misery of condensation leading to deterioration of the window frame, mould and other unpleasantness.
As well as the glass itself, the type of frame will also have a bearing on the performance, wood and vinyl being slightly better than aluminium.
The following table shows the thermal conductivity of different glazing configurations. The suitability and strength of the frame will have a bearing if the glazing is being replaced.
A quick glance at this table shows that simple double glazing almost halves the U value of single glazing. It might be surprising that the improvement is not more. Simple double glazing can be enhanced by several other improvements that will add cost but may be worth the extra.
The factors that can effect price and performance are :
Separation: The larger the distance between the glass the better it will perform. The thickness of the panel will be limited by the frame.
Spacer type: This can be metal or insulating material with insulation being the more efficient choice. This is more important the thicker the spacer becomes.
Coating: The panel can have a special non emitting coating which reduces heat escaping from the building. This is described here https://www.glazingvision.co.uk/blog/improving-glass-performance-with-low-e-coatings/ and here https://claytonglass.co.uk/news/the-advantages-and-disadvantages-of-low-e-glass/. Glazing panels with e-coatings come with a sticker indicating which is the inside and outside.
Gas filling: Replacing the air between the glass with argon will also improve the efficiency.
Simple double glazing may almost double the efficiency of a single pane and adding all the available enhancements may lead to tripling the efficiency so study the table and try and visit a glaziers shop and hopefully speak to an expert.
Secondary double glazing
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