Because the glass area represents such a large proportion of any UPC Patio Doors or French Doors, and glass is just not as good as a solid wall for retaining heat, it has to make financial sense to consider opting for the most energy efficient options available to you at www.french-patiodoors.co.uk/bifold-patio-door
For new French doors you should be looking at two areas: the glass itself and the double glazed unit as a whole.
In terms of glass the most basic would be to opt maybe for tinted glass as this should reduce glare and lower heat build-up inside the room, but it’s not too effective at reducing heat being radiated out or cold penetration, your best option here would be to consider low emissivity glass (low-e) which has a metal oxide coating.
This coating bounces heat & cold and reduces glare.
From the outside, low-e glass reflects solar radiation and from the inside it bounces you heat back into the room, allowing for better ‘climate control.
With the sealed double glazed units, opt for ‘warm edge’ spacers to stop heat & cold being transmitted from one side of the glass to the other & get sealed units which have the space between the glass panes filled with an inert gas, such as Argon.