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This is the most commonly used type of frame in Belgium. It offers good quality heat and sound insulation, making it the best choice for a competitive price. It must be fitted with, at least, a pane of glass with a U value < 1.4 W/m2K.
Offering similar insulation levels to the wood frame, this synthetic material is popular due to its maintainability and slightly lower cost. Also recommended with high insulation glazing.
Very fashionable in contemporary architecture, today's aluminium frames incorporate thermal breaks, nullifying criticisms about their poor insulation performance. Nonetheless, they still do not insulate as well as wood or PVC frames. Higher insulation glazing must be chosen, with a U value of 1.1 to 1.3 W/m2K, if the overall window unit is to achieve the same performance.
Even a well-design, well-chosen window does not insulate as well as a wall. That is why manufacturers have developed composite frames, made by layering different materials (wood/cork/wood, for example) to increase their insulation capacity. The cost of this type of frame is still high.
Formerly called the "K value", the heat transfer coefficient ("U value") indicates the amount of heat that passes from the internal air to the external air through one square metre of a given material, when the temperature difference between the two sides is 1°C. The lower the U value, the greater the insulating capacity of the material.