Arguably, French doors could be considered as retractable as each door can be pulled back. The advantage of retractable doors is that maximum access is possible between the two sides of the aperture whereas sliding panel doors generally overlap unless built to retract into a recess.
Metal frames, usually aluminium, provide strength in compact form. As notoriously good conductors (which is a bad attribute for insulation), frames made from aluminium are thermally-broken, which is a good attribute. It means that the metals on the inside and outside of the door frames are not joined, preventing the temperatures from being transferred between them. A bare metal frame would look completely unattractive so it is powder-coated in a choice of over 100 standard colours, including a white that resembles pvc.
A visible "kitemark" on the glass is the consumers assurance that the double-glazed unit has been manufactured to British standards. The BSI has numerous standards, including: U-value verification, Window Energy Rating and Window Installation. The lower the U-value, the better the thermal performance and most local authorities will require this to be 1.8 or better to meet building regulations.
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