I am running standard twin & earth PVC cable. Various cables will be run
from the CU it as follows:
clipped to the wall for a short space
then under the downstairs floorboards in free space
then up the wall buried in the plaster (no conduit)
then run between the joists (between downstairs ceiling and upstairs
floorboards, laying above the ceiling)
At one point, two cables will be passing through the same hole in a
standard size joist.

The cables will not be grouped together at any point other than passing
through the joist - they will be side by side when going up the wall.

Does the thermal insulation factor come into this anywhere (e.g. when
buried under the plaster?). Does the grouping factor come into account for
where the two go through the joist together?

Thanks

Roger