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.

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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.

Can share the brand of your raden shield?
I'm considering to install the Monier AL+FX series but still want to survey around and see what I can get with lower price.

1st I choose for this tile side insulation. But later decide not.

As the insulation is too thin for effectiveness. It blocks the airflow between tile and traps the inner roof hot air from going out.

So I decide to DIY insulate the ceiling. Which is what I feel more directly affects me from the room side.

The ceiling is the most heat generate for the room as the ceiling sq ft is as big as the room floor.

Use the foam spray for the insulation. Brought a few box to try on my working room. Brought from shopee.







The foam expands about an inch thick. But if you want thicker, just spray another layer. The thicker, the more heat insulated. Spray some water before and after the foam. It help to expand the foam more.



It is a fast process and dry about 4-5hr. The next day, you are able to feel the difference.

Here is the result of the temperature difference inside the room without an aircon.





I can increase the aircon temperature to 29°C and it already feels cool. And maintain at 29°C for the whole day even it really hot outside. 

The left panel is the actual room temperature near the battery. (below the cabinet)  And the aircon remote temperature setting. I never get the same reading before that.



That saves my bill a lot when work from home.

Yes. I vacuum clean the ceiling 1st.

Then spray with water before foaming.

Is the glasswool hazard to health?

Hi can you share more photo for the after installed bubble foil? Wanna know how you install the foil, whether is just take off the tile and lay on the wood, thanks

you put on top of the wood as i draw in the photo?
Because i cant see any other support under the tiles, if does not want to remove the tile and lay on

although roof tile with aluminum shield but feel no different, so better save money for aircond bills.