Gas and Oil Home Heating Furnaces - Furnace not heating some rooms

We have a five bedroom house that we just moved into that our furnace is not doing a very good job of heating. Currently, two of the bedrooms, the kitchen, and living room all get heated, but the other three bedrooms do not. When I put my hand on the register it is not even warm after the heater has been on for a while. We have an attic that gives me access to most of the lengths of piping, but I wanted to find the best way to test where the heat is being lost. Is there a more efficient way than ripping off all the insulation to look for leaks in the pipes? If insulation is missing around some of the piping (which it is) would that be cause for such a huge amount of heat loss to the rooms? Is there possibly some other explanation as to why heat would be getting into some rooms and not others?

Thanks for the help,
Dan

Is the air flow in the cold rooms the same as the warm rooms?

Is this forced warm air heat or hot water heat?

Are the supply diffusers open?Previous occupants may not have been using those rooms.To save heat may have shut them.If they are open,shut all other supply diffusers to see if it increases air flow to the others,if not,it's time to go into the attic and make things 100% correct.MAKE SURE duct is still attached to clinch collars.PDF

To answer all of your questions... It is forced air heat and in the rooms where there is no heat, there is no air flow coming out either. I think I found a section of duct where one side of a joint is hot and the other is not as hot and if you go further down this duct it gets cooler. Assuming this is the joint, will duct tape do the job of sealing it or is there a better way?

Air flows inside the duct, not outside. At the point where you feel the duct being much cooler, take a set of tin snips and cut it open so you can look inside. You will find what is restricting your air flow. Your duct may also be lined and some of the lining has come loose.

Where the duct turns cold make sure dampers weren't installed.They may have loosened shutting off air.Feel around the duct.If there is a damper you should see or feel a handle about 4 inches long.The damper handle should be parallel with the duct,that is the open position.PDF