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When fitting windows to a property they need to be framed correctly. Any heat gained by using expensive glazing technologies such as double or triple glazing can be lost by a poorly insulated, poor fitting window frame.

Window frames come in three main materials PVC-U, or UPVC, wood and metal.


In metal frames aluminium is normally used due to its light weight and corrosion resistance. Aluminium frames can be extruded into any size or shape and can be anodized in different colours to match the requirements of the build. Aluminium has some major drawbacks when it comes to green building.

Aluminium is mined as bauxite and often has to travel large distances by boat and truck before being made into windows adding CO2 mileage onto a project's carbon footprint. Metals are also very good conductors of heat which when used as an insulating material is not desired. This means that aluminium framing will transmit heat from inside a building through the frame and release it to the outside world. Thermal breaks can be added to combat this, these involve using two separate frames one internally and one externally. This forces the heat to move into poorer heat conducting air between the two frames and can give a boost to an aluminium framed window's insulation rating by 50%. Metal frames with double glazing have a typical U-rating of 1.8 (an energy efficiency rating of D).


Timber frames are the most traditional method of window frame construction. Wood is widely available and easy to mill into the complex shapes required to make windows. Wood will require painting in order to make it weatherproof and to prevent rot, this should be done every 3-5 years making wood a higher maintenance option then either metal or PVC frames. Wooden frames generally insulate better then metal but not quite as good as PVC and are less secure then either due to the comparative weakness of wood to impacts. Where wood is used it is essential to make sure that it comes from FSC approved sustainable to ensure its green credentials. Timber frames with double glazing have a typical U-rating of 1.5 (an energy efficiency rating of C).

The cost difference between PVC and timber frames is so slight that the natural alternative is a serious consideration. Wood has far less of an impact on the environment and is a sustainable natural material making it the clear choice for a green builds window frames.


PVC-U (or UPVC) frames are made from polyvinyl chloride (PVC) which is a very versatile and durable plastic. Frames made from PVC-U offer a high security and insulation value as well as requiring little maintenance throughout their lifetime. As PVC is a plastic paint is not required and the material provides a very good barrier to moisture. To further increase the insulating properties of PVC-U frames many manufacturers are creating frames with internal chambers to act as air pockets within the frame slowing heat loss. A recent development to the PVC-U frame is the inclusion of insulating foam within the frame to provide even greater heat loss resistance. PVC requires a lot of energy to produce and uses many materials including oil. PVC production also produces a by-product of dioxins which Greenpeace rate as a hazard to health and the environment PVC-U Frames with double glazing typically have a U-value of 1.4 (energy efficiency rating of B).


Composite frames contain some sort of mixture of the three common materials. Wood-PVC composites allow the outside face of the frame to be clad in PVC for low maintenance and high weather protection while still allowing the inside of the frame to have the look and feel of wood. Fibre glass frames are also available which have the benefit of being filled with insulating foam, however these are often weaker then wood, metal or PVC in the area of security.
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See also our DIY Building general advice on double glazing >

For more technical details on ratings etc, ee also our DIY Building Glazing page >