Front door purchase. Plastic door/Aluminum? A thermo carbon door?

Hello,

for a new building (KfW75 energy standard) we need a front door with a U-value less than or equal to 1. Dimensions 1750 x 2135 mm, front door 1100 x 2135 mm, side panel 650 x 2135 mm, clear passage dimension 959 x 2045 mm

Plastic door or aluminum door? Or does it have to be a Thermo Carbon door?

And how do we understand the manufacturer's information about the U-value? These always refer to the door; the side panel (with glazing) almost always has a higher value, which is even overlooked in the offers we have received.

What should we decide on?

I think wooden doors are quite nice as a contrast; it's not just plastic and aluminum. I don't know what Thermo Carbon is. The side panel should actually not have a higher U-value if it has 0.6 glass (triple). The door should be overlapping on both sides, otherwise the Ud value probably won't be achieved. A door glazing should also be triple glazed.

Thermo Carbon: never heard of it. Front doors are subjected to more strain and rougher treatment than windows, I would choose aluminum even in a single-family house without children.

The U-value (formerly k-value) indicates the heat transfer in watts per square meter Kelvin per hour. One Kelvin is a degree Celsius temperature difference, and if 2° C temperature passes through one square meter of surface in one hour, that results in a U-value of 2.0. Of course, the door leaf with foam insulation layer has less heat transfer than the transparent side panel.