Internal ceiling insulation

[Nogsy]

Hi, recently moved into house were the kitchen is directly over the garage. The kitchen is heated, has cavity wall insulation, has a flat roof (which I think is insulated.
However the floor feels cold as it is essentially an exterior wall?
I am thinking that attaching some polystyrene sheeting directly to ceiling of garage below would help the insulation. I don’t want to use a false ceiling. It is only a garage so doesn’t need to be aesthetically pleasing.
Any suggestions/advice would be very welcome
Cheers.

 

[motorbiking]

Why can't you remove the plasterboard in the garage and put rock wool loft roll between the joists and refit the plasterboard? You really don't want polystyrene in a garage that is not encapsulated in a fire resistant surface. Rock wool will be cheaper and is also highly fire resistant.

If this is a recent build it should have been insulated like this anyway. So I would check first.

 

[Nogsy]

Thanks for reply. Have only just began consider a solution. Aren’t some of the insulating sheets fire resistant? Trying to find a workable solution which would save removing the plasterboard or using false ceiling etc.

 

[Deleted member 174758]

Aren’t some of the insulating sheets fire resistant?

Mineral wool is fire resistant and has been used for fireproof packings (e.g. around fire doors, blocking voids, etc) for decades. Mineral wool can be purchased as "battings" (in semi-rigid sheet form), but even that is designed to be contained within a wall or ceiling. The vast majority of PIR (foam) sheet insulation is not fireproof, as Grenfell illustrated

Trying to find a workable solution which would save removing the plasterboard or using false ceiling etc.

Plasterboard is fairly cheap, so replacing it after insulating between the joists would be a slightly messy if relatively cheap option providing you could dispose of the waste materials at a reasonable cost (plasterboard is classed as toxic to the environment and costs extra to dispose of)

If you have the space you could always overboard the ceiling with either cement fibreboard (which is also an insulator, although nowhere near as good as PIR) or insulated plasterboard (which has PIR insulation glued to the back of the sheets). The question has to be, "How substantial is your existing roof/ceiling in the garage?"

 

[Nogsy]

Many thanks for replies.
Looks like, best to removed ceiling boards and put in rock wool then replace. Could be messy.
Cheers