Grunff wrote:
Ian Stirling wrote:

The second will win.



You don't say...


As to the leading - it's largely irrelevant.
Glass has a K of 1, so a 6mm pane has a U value of 1*.006= .006.
The air sitting next to the inside of this utterly swamps this in
insulation.

I dunno - lead has a K of ~35. Steel has a K of ~46. If we take one
opening pane, which is 49cm wide by 90 cm high:

Total area: 4410 cm^2
Glass: 3563 cm^2
Steel: 540 cm^2
Lead: 307 cm^2

That means the glass only makes up 81% of the window area, with the rest
of the area being made up of materials that are over an order of
magnitude more conductive.

You can't quite get there from here - as a very significant factor at
this level of conductance is the air next to the window/frame.
Also - the much wider section of the metal, and the fact that it's not
(usually) solid means that it's less conductive.