Their shape, quality and function
This window is an important element of thermal insulation system. It is also an element of the facade decoration and decoration of the interiors.
While buying the windows you should primarily take care of the heat transfer coefficient- U, because this is the most important thing- protection of house against heat loss. It will be much more economical energy and thus cheaper to maintain.
When our are sure that the windows have appropriate parameters you need to take care of their shape and the material from which they are made. If to your elevation better fit wooden windows but you prefer PVC windows- you do not have to worry about, because on the market is a lot of plastic windows imitating wood. The most important thing is to fit them to hour house perfectly.