|The attic and central air system on Jackson's current project. Note the old attic fan.|
The photo is of the attic in our largest ongoing project. The air handler, plenum, and ducts for the upstairs' central air system was (and will remain) located in the attic. Between the attic and the living space below was some old and intermittent batt insulation. In south Louisiana, an attic like this, under a dark-colored asphalt shingle roof, reaches unbearable temperatures, even if it is vented. That means the central air system is pushing cold air through the really hot attic, air that gains temperature before it even reaches the rooms it is meant to cool. Which means the central air system has to run as much as 20% more than if it weren't in a hot attic. And all that entails: 20% higher bills, 20% sooner repair or replacement costs...
But here's a problem with a system like this that truly worries me: the hot, humid air in that attic stands a good chance of reaching its dew point on the machinery or ducts, condensing, then dripping into the insulation, framing lumber, or drywall of the ceiling below. The threat of damage from this condensation increases greatly if any cold, conditioned air leaks from the system into the attic, but it only takes a little drip to ruin insulation, paint, or attract termites.
Spray foam insulation, whatever other drawbacks it may have, can solve these problems by allowing the attic to stay at a temperature and relative humidity close to that inside the living areas. The central air system doesn't need to work as hard, and damp air is kept outside the building envelope by the foam.