Trying to insulate underhouse garage a bit
My kid's bedroom is directly above the 1-car garage in our 1950's ranch. It tends to be very cold. There is some insulation in the ceiling but it's minimal. Since the plaster ceiling in the garage is sort of crumbly I can't get someone to blow in cellulose without risking the ceiling falling out...so a "real" fix will have to wait another year or two. At that point the ceiling will be replaced and I'll probably just hire someone to spray a thick layer of foam before re-drywalling.
However, it has occurred to me that I could drill holes at the edge of the ceiling on the external wall, poke the straw of a can of max expanding foam in there, and try to get some foam up onto the band joist to try to seal off some of the air infiltration. The bottom edge of that wall in the bedroom can get condensation if a blanket is left against it at night in the wintertime.
Question: is this likely to be at all successful, or not worth the trouble?