Cold Air Return Duct in Garage Ceiling (for bedroom above)
I too am surprised you haven't gotten a response.
Can you give us an idea where your location is?
I'm just a small project DIYer so don't have any expertise. But yes, insulation should help a quite a bit. I don't know if you need a vapour barrier nor which side of the insulation is should go on (may depend on location and local codes).
Can you feel the cool air within the cold air return? Insulating it will help shield it from the cold but some returns are just empty joist bays so are not protected at all from the cold.
Is it possible to move the return air vent to a different wall in the Master Bedroom so the return air duct is shorter, and perhaps not exposed to the cool garage?
A DIY Noob that knows just enough to be dangerous.