For (1) things ljke Radenshield https://shopee.com.my/Monier-Radenshield-Do...6018.7969103304 see the diagram there, it is below the support battens. To mount foil which is lightweight, either use another set of horizontal batten to support it or just mount it to existing vertical battens.

Reflecting most of the heat back is important. Rockwool used to limit the heat transfer downwards. Rockwool is heavy stuff as the name suggests. So it can be used below radenshield but still supported by the new set of battens..density of 40kgm 60kgm 80kgm anyhow also heavy lah.

Use radenshield sheet is the lightest and more cost effective way already reduce 80-90% of the transmitted IR coz it reflect back into the roof tile.
As these foil sheet will have plenty of mounting to stick to existing strong batten the roof will not collapse lah.

Rockwool heavy I think one meter sq coverage adding on 3kg every meter sq area even at the lightest 40kg/m3 density which will be a question whether existing plaster ceiling will collapse or not.
The plaster ceiling is held on by few tiny drywall screws into the thin aluminum mounting batten only. Need further reinforcements.

As I see if that is the real pic, the roof using cement roof tiles already quiet enough (not zinc roof or plastic roof) so you don't need to put rockwool for sound insulation. Rockwool if you insist to do, the roof joists (flat part where people can walk on) need to enhance summore and link up, maybe with a new set of those "plaster ceiling aluminum batten" specifically to support the rockwool only, INDEPENDENT of existing plaster ceiling. The existing plaster ceiling will not tahan the weight of the rockwool well. Later middle of the night the paper liner of gypsum board breaks at the screw head holding it up (which is the only point load supporting the weight of the gypsum board itself, its OK for its own weight but it is not designed for additional weight) and kaboom downwards.

Noted, can proceed with both
Radenshield to reflect most of the heat out back to roof tile, basically just stick on using construction adhesive to existing roof batten (the vertical battens in the A shape of the roof). In short, easy installation but ypu may want to do the add joists before the radenshield so easier for the installer to stick the radenshield on the rood batten coz to position a ladder or stool in there based on whats available now seems a bit dangerous, not much safe area for them to work with, risk of falling thru the ceiling is there.

Rockwool above plaster ceiling with additional wooden beam (flat joists) to support weight of people walking around, and between these flat joist you can just use flat bar or the alu batten same type as the plaster ceiling folks use, join them up between joists to look like a flat ladder to support the rockwool slab weight.

All the above is passive heat reduction method. Turbine also quite passive despite the monstrous size.
Active method involve boarding up your sides (ie close off your rooftop airspace to be only yours) so youre only cooling your rooftop airspace and not the whole row of 20 houses' airspace. By making a vent to be able to support the size of a ventilation exhaust fan or two unit ventilation fan about 10in KDK it is gonna move 400 to 500cfm so the air inside is not tupperwared long enough to bake (like now). The input you can just make a passive vent above the bathroom ceiling for it to have cold input air. Why i didnt wanna suggest 12in is that the hole is big and burglar can squeeze in. 10in should be small enough for them not to be able to drop in.

This active ventilation is like you park the car in the hot sun and opening all windows. The car cools better in minutes with the window open as compared to it fully rolled up. Even better if there is a fan blowing out all the time when it is baked in the sun. The exhaust fan you can use on timer so it only work on hot hours say 1100-2000hrs. The electric usage of an exhaust fan is very minimal compared to overworking your AC.

This post has been edited by ceo684: Mar 29 2021, 05:31 PM