I have a return air duct that travels through the outside wall of my home and gets so cold that when it runs under the wood floor turning to the furnace, the floor is also ice cold. I am sure this is making my heater work too hard.

I have heard that blocking the return air can hurt the system (I do know which room this comes from and I have returns in each room). But would this be better than frigid return air in the heater?

I have also seen that you can insulate the ducts. Does this involve tearing out the drywall to add insulation behind the duct? And will this really help for all of the work that is involved in doing that? Is there a way to add insulation without destruction?

Thanks.

Do NOT block the return air unless you install a new return air of the same size or bigger somewhere else in the room or home. Blocking it can cause major problems.

Usually you have to tear out the wall to get to the duct. The easy way out is to install a new return air in a different location and eliminate the other.