I would recommend 25mm Cellotex stuck to the inside of the garage door (if not already insulated) Nice and light on the door too.
Specifically designed to eliminate thermal bridges,
The thinnest PIR insulation,
Standard board size of 1200mm x 2400mm, range of thickness
Easy to cut and shape,
Low emissivity foil facings giving improved thermal insulation,
High performance within cavity air spaces,
Long-term energy savings for building
