Ian Stirling wrote:
The Natural Philosopher wrote:
Grunff wrote:
Andy Wade wrote:

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.

Thanks for the input Andy. While I understand your logic, something
still feels instinctively wrong - can't the same calculation be done to
show that the difference between copper walls and 100mm celotex walls is
negligible? What am I missing?


The fact that celotex traps a far far larger 'boundary' layer - about
100mm in fact ;-)

And what GRUNFF is missing, is that in a 30mph gale, the boundary layer
gets stripped away on copper, and glass, both.


Only on the outside.
Hopefully, on the inside, the gale doesn't get in.

true, but that doubles the U value right away.