In article ,
David J writes:
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.

That's not the case. The outside wall must be warmer than the
ambient temperature in order to lose heat to it. Similarly,
the internal side must be cooler than the air temperature in
order for the room to lose heat to the wall. The difference
in temperature between the air and the wall is (roughly)
proportional to the energy loss at the inside. This is also
true at the outside but factors such as wind-chill and dampness
will affect the figures and make comparison readings impossible,
so you'll need to measure this on the inside where wind-chill
and evaporation are hopefully a constant (zero).

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?

It should be more like this...

T0. - ambient indoor temperature
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)
T5. - ambient outdoor temperature

Assuming T0 and T5 remain the same...
There will be an increase in T2 and a decrease in T3 as you say.
However, there will also be a (smaller) increase in T1 and decrease
in T4, because less energy is being drawn though the wall.

Measuring the difference between T0 and T1 (or T0 and T2, or T1 and
T2, as none of these thermal elements change) will give you a figure
which is proportional to the temperature loss through the wall.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]