With hot days just ahead, many homeowners are considering adding central air conditioning. Some of us don’t want to endure New England summer heat again, while others are tired of lugging out those heavy window AC units commonly known as window rattlers, only to have to carry them back to storage in the fall. If you are considering installing central air conditioning, there are energy- and comfort-related issues you may want to consider.
The first matter to consider is placement of the AC unit and the ducts that accompany it. If the unit is going in the attic, there are year-round consequences. The unit will be less efficient in the summer because it will be working against the extreme heat of a typical attic that has insulation in the attic floor. This will cost you more money to cool.
Additionally, the thermal and air boundary (typically the second floor ceiling) has been made less effective at keeping the house warm in the winter and cool in the summer. A large area (total surface area of ducts) has been added to your ceiling insulation, and this area has an insulation value of only R-8, not R-38 as the rest of the attic floor insulation should be.
Another issue is leakage. Even though AC units and duct systems are more air-tight than they used to be, they still leak air. The worst place to have more air leaks is at the top of the house where air pressure is greatest during cold months, thereby sending heating dollars into the attic. During energy audits, when homeowners indicate that they are considering central air, I often ask them why they want to make holes in their ceiling and lower their insulation level. If, however, the roof deck is insulated and the AC system is inside conditioned space, then it is much less costly to install an AC system in an attic.