Martik wrote:

Bdrm A: 12x12' 2 exterior 2x4 walls, one with a 5' patio door double pane,
vinyl,

Bdrm B: 11x10' 1 exterior 2x4 walls with a 5' patio door double pane, vinyl

Both A and B floors are over unheated garage but ceiling has R28

Bdrm C: 14x13' 2 exterior 2x4 walls, one with a 72"x34" low-e double pane,
vinyl, floor is over heated bsmt

I assume that the 2x4 walls are insulated since the rest of the house seems
to have R12

Attic is R40

Bdrm A is much colder than the others. The only reason I can think of is the
larger exterior wall space of A but then C also has a large space. Any other
ideas? Maybe part of the wall is not insulated. How can I test this?

Thanks

No way to know without information on how it's being heated. Forced hot
air? Hydronic baseboard? Electric baseboard? Radiant of some type?
Number of FHA vents? Length of baseboard? Zoning / thermostats?

Could be many things, particularly an unbalanced system as noted by
others.