I wish that I had my camera on a recent home inspection to show you how badly some of our homes are insulated. This home was in the process of being remodeled, so the interior wall coverings had been removed to expose the wood frame and the sheathing for the exterior wall. There was no insulation in these cavities created by the studs. The home was built in the sixties, and there was a construction paper in place, but this set up does allow heat and air to move through the wall.
In order to make our air conditioning units more effective, we have to control what air is being conditioned, and where that air is going. The ultimate solution is to fill those spaces with insulation, and you will find companies which specialize in this process. It involves cutting holes in your walls to access the cavity to blow in insulation. Then the holes have to be repaired so you do not see a hole every sixteen inches. A homeowner may be able to rent this type of blower, and you can definitely by the insulation, but making so many wall repairs may not look good. My solution for the do it yourselfer would be to cut the holes at the top of the wall to blow the insulation down, then placing crown molding in the room to cover those holes. The one problem with this method could be that fireblocks were put into place, which would mean that some spaces would not be insulated. (You would have to run a wire down your holes to see if the space goes all the way down to the floor.
For most of us this may not be the most affordable option, so let us look at steps we can take which helps us accomplish a tight seal for our home. First look at your window frames on the interior where they meet the sheetrock (or other internal wall covering). During my home inspections, I find many spaces between these two elements. My own home had such a gap. One winter I felt a cold breeze from the window area, when I looked up I saw the gap. This was a simple fix. I bought a silicone caulk for a window. Around windows and doors, you may find gaps too. There is a foam insulation which has one side with an adhesive, so it can be stuck into place. The only problem that I have found with this material is you need to purchase the right size for your door or window. Too big will prevent you from closing the opening, while too small does not work.
A product that I just recently came across was foam insulation for outlet covers. At first, I thought this is just a gimmick; however, when I thought about air flow through a wall, I changed my mind. Think of a brick veneer on a home. At the base there will be weepholes that allow moisture to drain out from the wall (there is a one inch gap between the brick and the sheathing attached to the framing. Air could flow into these holes, and it could make its way to that uninsulated cavity. Your light switch or plug outlet is an opening where that air could flow through. For that reason, I decided to install these foam pieces on my outlet covers. I admit that you may not experience significant heat loss through a cover, but the tighter the home, the less work for your air conditioning system.
There are some spots you may not consider when looking at energy loss. Here in Texas, fireplaces are not commonly used, but I find dampers open during my home inspections. You are sending conditioned air right out of the house. Doors or windows which do not properly close leave gaps too. Rehanging a door or adjusting a window may be needed. Cracks in your exterior wall veneer. These cracks allow rain to come into the walls causing damage, but otherwise you face the same idea as the weepholes mentioned above. There are many styles of caulk or other means to seal these cracks. Along the lines of cracks are expansion joints or wall joints. I find that after five years the caulk in expansion joints need to be redone. I have even found wall joints and expansion joints that never had caulk.
Just take a walk around your home to consider each opening in the wall from the obvious (doors and windows) to the not so obvious (outlet covers). You may find quite a few spots to work on. Lastly you should turn your attention to the attic. You should have at least six inches of insulation in your attic over all living quarters. During my home inspections, I try to crawl through most of the attic to see what is going on. I frequently find that insulation is great around the attic opening, but further away it is non-existent. My favorite insulation right now are the batts which come in a plastic sheath. These are the easiest for a homeowner to handle. If you have little insulation, go with a high R-number. These numbers represent how easy it is for heat to pass through it, so the higher the number indicates that heat has a harder time. What I did in my home had to do with cost. I installed an R-32, because that is what I could afford at the time. Later, I bought batts of R-13 to place over the previous ones. This gives me an R-45 which is pretty good.
Before you move onto any other projects, insulating your home should be the first priority. A new air conditioning unit can be great, but it still would be useless if you have not dealt with tightening the seal of your home.