I have been wondering how to measure the 'effectiveness' of adding CWI
to a domestic house, apart from seeing the resultant change in energy
costs at a much later date.

It is clearly not good enough to simply measure the surface
temperatures of the inside and outside of the exterior walls, as one
is the source and the other is the sink of the heat transfer through
different materials. And the exterior wall temperature will always be
that of the ambient climate temperature.

I have read about R and U factors, but that does not help. I have also
read that thermal transfer (in buildings) is analagous to electrical
flow through conductors, so what really is required is to obtain the
thermal drops across the wall sections.

So, taking a typical modern exterior wall construction, 4 temperatures
are needed as follows.

T1. - surface temp of inside inner wall (plaster)
T2. - surface temp of outside inner wall (thermal block)
T3. - surface temp of inner outer wall (brick)
T4. - surface temp on outer outer wall (brick)

With the CWI installed and thermal transfer reduced, under steady
state there will be no change to T1 and T4, but an increase in T2 and
a decrease in T3 with the majority of the thermal drop now occuring
across the cavity insulation.

Does the above reasoning make sense?

David