it is likeley that your baseboard system can in fact deliver enough btu but you dont have enough basebaord to heat the house in the extreeme cold ,you need to figure out how many btu you need on those cold nights and then see if you have enough linear feet of basebaord. i am not at home to look at my charts but i think a finned basebaord with a 3/4 pipe in it with water at 180 can deliver somewhere(i am guessing right now) around 5 to 600 btu per foot of baseboard..now for your poorly insulated room you might figure 50 btu per square foot of the room...so find the square feet of the room x 50 = btu per hour you need to heat the room in the extreme cold..now divide the btu by 500 and you should have the linear feet of baseboard you need

lets say the room is 12 x 15 that would be 180 square feet ..so in the extreme cold we think you need 50 btu per square foot ,that would equate to 9000 btu per hour needed, deivide this number by 500 (number of btu per foot of baseboard) equates to 18 this would be the linear feet of baseboard you need to heat that area in the extreme cold (using 50 btu per hour per square foot)