One of the common issues I see on energy audits and home inspections is retrofit Air Conditioning Systems that are installed in an attic space.
Usually these are older homes that never had central air and the easiest place to install the equipment is in the attic. This would be OK if the attic space was inside the thermal envelope. But most of the time there is insulation installed in the attic floor which puts all the ductwork and equipment basically outside. Ideally an attic is suppose to be the same temperature as outside. Now, this is not really realistic but with properly installed insulation and airsealing of penetrations we can come close.
The problem with these installations is I routinely find poorly run ducts that have loose connections on the trunk lines and at the boots where they connect to the ceiling. These are obvious air leakage points that will waste cool air in the summer and allow heatloss in the winter. What most people don't realize is the large return and supply registers are open during the winter time. As I like to say "be the air". As that heated air rises it goes right in those ducts that lead to the handler in the attic. Any loose connections at the boot or trunk will allow convective heatloss. The greater the temperature differential the greater the drive. Basically these can be like little vacuums sucking heat out of the home. Even if air sealing was performed properly the air handlers are metal and the insulation is usually only R4-R8 on the ducts. You will still experience heatloss through these components. Remember heat is basically energy and heat seeks cold. I have always wondered why it is found acceptable to have say R8 ductwork when the ceiling is required to have R38 or R49 (climate zones 5 & 6). You wouldn't feel right running a duct from a basement furnace along the outside of the home to get to a room would you. Of course not. But for some reason since there is a roof over the attic, builders and installers find it acceptable to do the same thing in the attic.
So what can you do about? Well, first if your building a new home you can require the builder to plan ahead to address any mechanicals that might be installed in an attic space. They should be installed inside the thermal envelope or conditioned space. If it is an existing unit you really only have a couple options. First, you can bring the ductwork and handler inside by insulating the underside of the roof. There are many ways to do this, but I prefer a closed cell spray foam. Secondly, you can block off the supply and return ducts during the winter months. One way to do this effectively is to go to a fabric store and buy foam for cushions. Cut pieces slightly larger than the ducts. Put the foam inside a plastic bag and shove it inside the duct. Then cover it with the grill. Obviously you will have to remember to remove them in the spring. This will at least prevent the convective heatloss up into the duct system. If your grill covers can close, don't be fooled. These are usually extremely leaky and really don't do much.
Terry's Tips are personal observations based on my experience and seeing the cause and effects of things. There may be many ways to address different problems and all opinions are welcome. I am always open to suggestions and certainly want you to correct me on any mistakes that might be written.