Garage ceiling insulation

[philkalico]

I have a double garage with a room above which always seems cold.
The garage ceiling consisits of plasterboard on 6inch joists with 20mm chipboard flooring to the room.The space between plasterboard and chipboard is filled wth fibreglass insulation. The room has 170mm of fibreglass in the roof void above.
I am proposing to fix 50mm Celotex Ga4000 to the underside of the garage ceiling in order to cut down cold air flow up into the room above.
Can anyone advise if this will make a credible differnce with regard to keeping the room warmer. Will the resultant reduction in U value be significant.

Thanks

 

[freddiemercurystwin]

Yes it should make a noticeable difference.

 

[mointainwalker]

150 mm fibre-wool = U 0.27
added 50 mm Celotex = U 0.17

garage ceiling in order to cut down cold air flow up into the room above.

How is this penetrating into the room above ?