In older homes, return air ducts are typically just a cavity in the wall, which is not energy efficient.
We recently had a party at my home. With more people in the home, and children running in and out of the door to the backyard, I turned my thermostat down to a cooler temperature. As the air conditioning was running, my wife that we had a leak. Condensation had built up on an air vent. Cold air passing through the hot attic was the culprit. I have been slowly and surely updating my ducts. Mostly this involves adding new insulation over the ducts. Once the ducts are no longer exposed to the heat of the attic, condensation decreases. This made me realize that one bit of advice which I give during home inspections was not being followed by myself: improving my return air vent. In older homes, return air ducts are not well made.
What might be happening to your return air vent? There are two basic problems: they are not well situated with only having one duct for a home;and they are simply cavities in the wall framing. Air flow through your home is important for air quality. In many homes, there will be only one return air vent situated near in a hallway for the bedrooms. This pulls air from this part of the house, while the opposite end of the home remains stagnant. The solution is to have vents in different parts of the home. The problem with the wall cavity revolves around fire safety and energy efficiency. If you have electrical wiring running through the duct, and if an electrical fire is caused by that wiring, the return duct becomes the perfect conduit to spread that fire through the home (particularly with the air feeding the fire). The other aspect is that the framing and walls can heat up through thermal bridging. This will heat the air returning to the air conditioning system, making the air conditioner work harder.
Many of these older return air ducts are quite large. In fact, I can stand in them. In my home that is not the case. The cavity is 11″x26″, so I cannot squeeze into it easily. However, the corners and other joints should be sealed for the best results. I could have opened up my return plenum in the attic, and maybe dropped down some duct tubes which may have fit. This was a poor solution in my situation, because I needed enough air for my HVAC system, and I would have decreased that amount. I decided upon using foam insulation boards with a radiant barrier on one side. The eight feet long boards happened to be the right height for my duct. To reduce joints on the face, I cut the boards to the proper size, and then did a back cut which did not go through the face to allow me to bend the board into the return air duct. This meant that I did have the corner joints to seal. For the upper section of the duct, I cut boards to fit snugly between the joints on each side holding the first boards in place. Since I could fit into the lower portion of the duct, I taped the lower joints. Maybe not the most elegant solution, but it works. Sorry the pictures just were not coming out well. The best description I can give would be this: imagine that you have four pieces of cardboard. You can slide them into the inside of a frame, like a box kite. The pieces fit, but to make it snug, where the cardboard pieces push against the kite frame, you install a brace inside the cardboard pieces to force them against the frame. My extra piece of foam board was the brace.
Most of these older return air ducts are large enough to go into the cavity. A few are like mine, so my solution may work for you. Most of these return air ducts do have wire running in them, which can be placed behind the boards. Thermal bridging may not be a major issue, but every little step helps improve your air conditioning system, which in turn leads to less energy usage.