additional insulation in attic conversion

Triangle

I'm looking at increasing my insulation in an existing attic conversion.

I'm looking at two areas
1) Floor of the cavity space (Green in attached pic). Placing more rockwool on top of the existing, ofc leaving space around the water tank.
2) The cavity walls (Yellow in attached pic) have existing rockwool in between the joists. I'm looking to increase this, my current thinking is to put nails in the joists and place rockwool on them, tying them in place with string. This seems very DIYish, and i'm looking for a better solution - perhaps some type of insulation boarding i can nail in place.
Can anyone point me in a good direction?

The cavity walls have coil covered plasterboard on the inside and rockwool between joists currently.
I'm not sure what other information might be needed.
Thanks in advance.

Nonmonotonic

Kingspan do a foil backed rigid board in different thicknesses.

K7 or TP10 might do the business.

hopalong_ie

Hi.

You have to choice. Slab over the existing ceiling with insulated plasterboard. This will result in you loosing head space and its a tradeoff. Ideal solution is too pull down the existing ceiling and fit 50mm between the rafters assuming a 100mm rafter, you need to keep a 50mm gap between any insulation and the felt of the roof. Then you could slab over with an insulated plasterboard pref 62mm, this would give you basically current building certs level of insulation.

Personally I would rip down the ceiling, do it once do it right.