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.

It also gets pretty bad with rockwool in the loft.

Its not often mentioned, but I did get significantly better insulation
when I boarded over mine..noticeable in strong winds..

Likewise my suspended (and insulated) concrete floor is significantly
colder when the wind blows..the underfloor space is a good insulator
when its calm.