Thermoboard or Spray Foam Insulation? max 25mm which is best? I need to apply some insulation to a concrete ceiling the -topside...

I need to apply some insulation to a concrete ceiling the -topside man exposed to open nouns, hence its cold in winter on the inside!. I own limited headroom so can with the sole purpose go to max of 45mm inc plasterboard. Which preference is best? The room will be heated and correctly ventilated, so I a short time ago need to stop the cold nouns against the ceiling causing the cold, as the walls are insulated.
A friend offered me a used hot tub....
Answers:    Spray foam requires studding or furring out space. The Thermoboard can be directly applied to ceiling if smmoth plenty, thus preserving headroom. Also spray foam is more expensive & requires pro application. The other is DIY.