If you had an attached garage that was converted into a living space a number of years ago, it was probably done on a tight budget. These projects usually seem to become the comfortable, relaxed room where you spent most of your family time. You have discovered that year by year your utility bills have risen substantially.
Several years ago, it wasn’t cost effective to spend extra money on green building products, however since energy costs have gone up, the reverse is now true.
You may find that your converted garage of yesteryear has the least expensive windows and doors because it was considered wiser to pay a little more for the extra energy costs than it was to pay the big tab for green building products. Aluminum windows were like a heater during the summer and a refrigerator in the winter. The amount of insulation in the walls and ceiling was not taken very seriously either. There were probably many penetrations in the walls and ceiling from electrical outlets along with the leakage around the can lighting.
Over the years the utility bills have more than doubled and along with the possibility of cap and trade, it has become cost effective to purchase better and more energy efficient building products.
It will pay you to tear out all of the old energy-inefficient building materials and replace existing windows and doors with those having double pane high “E” glass. Seal all penetrations and add insulation. If your A/C unit is an older unit, it is probably an 8 or 10 SEER heating and cooling unit. You should replace it with a split system air source heat pump with a 14 to 18 SEER rating.
If this “energy eating monster” of a room has become your main “comfort” room, by up-dating it and sealing penetrations in the rest your house, your energy costs could easily drop by a whopping 60%. Even if the energy costs remain the same, the payback will take only a few years.