"NT" wrote in message
...

A simple option is to plug a heater of known power in, let temp
stabilise, and measure temp diff between indoors and out. This takes
account of the whole picture.

You van make the maths easier by using an angle grinder to cut a 1 m square
section out of the wall.
Mount this ay one end of a well insulated box containing the heater.
Let it run until it stabilises and then measure the temps.