Tim W wrote:
David J
wibbled on Friday 27 November 2009 15:42

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

That's about it (ignoring the thickness of the plaster and assuming T1 is
the temperature of the inner face of the thermal block).

Do it like a resistors in series calculation:

Current is analagous to power transmission
Potential difference is analagous to temperature difference.

I can never remember which is U/R/K/whatever is analagous to resistance (in
this case more like resistance per unit area), but a look on Wikipedia:

http://en.wikipedia.org/wiki/R-value_(insulation)

Suggests is the R value. U values are the inverse.

I just look it up in a manual that tells me that the heatloss with CWI
is about a third as without...

Since mots people have dine lofts and double glazed, that means about a
60% saving in fuel bills, as the walls are the dominant loss now.