Tim Lamb wrote:
Insulation between joists below under floor heating.

Having promised no wall hung radiators I am now stuck with getting the
job done! Ground floor no real problems: grooved insulated sheet to
take the pipes covered with standard chipboard or screed for the new
bit.
Upstairs life gets expensive and difficult. All the suppliers call for
insulation below and in contact with the heated floor. In a chalet
bungalow, there is no such thing as a standard floor joist spacing.
The dormer cheeks are double or triple timbered and the intervening
space filled with ladder work on joist hangers. Elsewhere walls have
double supporting timbers.

I have been considering cross battens but this adds a huge cost and
the suggested sizes (30mm x 70mm) are not found off the shelf and
particularly not untreated.

Pre-grooved chipboard is an alternative and cheaper solution. Both
systems call for insulation between the joists and in contact with the
underside of the floor. PIR foam is not cheap, needs careful
cutting/fitting together with supporting battens. Rockwool is cheaper
and easy to cut and fit but... how do you stop it ending up resting on
the ceiling plaster board, light fittings and electrical wiring?

Chicken wire netting? Some sort of cross batten or?? My floor joists
are 9" deep and Rockwool 6". Perhaps lengths of plain galvanised
fencing wire stapled between the joists? Any impact on wi-fi
transmission?

Insulation isn't needed for upstairs floors, any heat 'lost' isn't actually
lost - it's just transferred downstairs into the living space which will
reduce the amount of power required to heat that space.