Ian Stirling wrote:

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.

Yes, for single glazing it's the air boundary layers that dominate.

1/U = Rsi + Rso + sigma(t/k)

Where
Rsi is the resistance of the inside air layer
Rso is ditto for outside
sigma(t/k) is the sum of the thickness/conductivity ratios of
the intervening layers

For horizontal heat flow Rsi = 0.12 m^2K/W, Rso = 0.06 m^2K/W. The sum
of these is 0.18 m^2K/W.

Taking a 4mm (t = 0.004 m) pane of glass with k = 1 W/mK (given earlier
in the thread) t/k is 0.004 m^2K/W so contributes negligibly to the sum.
The U value is 1/0.184, i.e. about 5.4 W/m^2K

Replacing the glass with a sheet of 16 swg copper (t = 0.0016 m, k ~ 200
W/mK) only increases U to about 5.5 W/m^2K.

--
Andy