In article ,
Andy Hall writes:

For example, in one house that we had, an old lady lived next door and
we knew that she only heated one room. I therefore treated that
party wall as being external.

In a more rigorous calculation, one should also account for internal
heat transfers if the rooms are maintained at different temperatures.
In the same house, I had a large bathroom which had a wall adjoining a
bedroom. THe bathroom design temperature was 23, while the bedroom was
16. In that scenario, the transfer was significant. The radiator
manufacturer programs take heatlosses via this route into account but
not usually heat gains - presumably because you'll buy a larger
radiator.

A couple of related things...

In a multi-zone system, don't forget that you might choose to have
an adjacent zone switched off, which could radically change the
temperature differential.

Normally, a good amount of the upstairs heat actually comes through
the ceiling/floor from downstairs. However, this takes time from a
standing start (i.e. at least the time for the downstairs to warm up),
so if you want to be able to get the upstairs up to temperature without
having to wait for the downstairs to first heat up, you should probably
assume the floor of the upstairs is rather more like a suspended
downstairs floor.

These points are probably most relevant where you have a zoned heating
system controlled by occupancy where rapid response is required, rather
than just a heating system blindly switched by a timeswitch with no
idea if or when or which parts of a house are occupied.

--
Andrew Gabriel