In article ,
me here writes:
2. Tear down ceiling, insulate with Celotex/Kingspan between joists,
reboard & replaster. Messy, expensive. Need at least 60mm Celotex to
be worthwhile: building regs probably require at least 100mm maybe 150
in new builds, anything over 100mm is probably overkill in our
lifetime for a retrofit.

If you've got no insulation, even 25mm of celotex will make an
enormous difference. Each increase in thickness is a tiny gain
compared with going from nothing to the first inch. You don't
need 60mm to be worthwhile. In this scenario, I would install
a thickness which is the ceiling joist height minus 2 - 3" to
allow for ventilation above (capped at whetever current building
regs require). Cut and fit with the bottom level with the bottom
of the joists (so the gap is above the celotex, not below).
Seal all the joins below with aluminium tape (including between
the celotex and joists) and refit with foil backed plasterboard.

BTW, it's quite easy to tell if you have any insulation up
there. Buy an infra-red digital thermometer (a great toy to
have anyway). Use it to measure the ceiling temperature on a
cold night. If the ceiling is significantly colder than the
body of the room, then there's little or no insulation in it.

You can repeat that around all the external walls/ceiling/floors
of the living accomodation in the house to see where you are
losing heat, i.e. where you would benefit from more insulation.

You can also do it the other way around. On a cold day when
you have the heating on, walk around the outside of the house
looking for the warmer outside surfaces. This is also where
you are losing heat. (Might be difficult to get a measurement
from the top of a flat roof unless you can oversee it some how,
or get a ladder up there.)

--
Andrew Gabriel
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