Unvented Attic, Summer Conditions
When a builder decides to put an air conditioner or duct work in a hot attic, he has determined that you will overpay your utility bill forever. Air conditioning systems don't make cold air. They lower the air coming back to the unit approximately 22 degrees.
Typical air conditioning system:
When the air in the house is 74 degrees, the thermostat turns the unit on. The fan sucks air through the return ducts to the air handler where the air blows over the cold coil. Humidity is removed from the warm moist air as the air temperature drops, The now cooler air should leave the air handler 22 degrees lower or at 52 degrees.
What happens when we put the a/c air handler and duct work in the hot attic?:
Attics regularly exceed 135 degrees in the summer. The 74 degree air from the house passes through a hot return duct located in a hot attic. The 74 degree air rises to 78 degrees under the influence of the hot return ducts. The now 78 degree air enters the air handler. Because the air handler is in a 135 degree attic it can only reduce the 78 degree air 19 degrees to 59 degrees. The 59 degree air passes through supply ducts in the 135 degree attic which increase the 59 degree air to 61 degrees.
Wasted energy and money:
The net result is an attic a/c system that is operating at less than 59% of it's designed efficiency. That means for every $100 of a/c you pay to your utility company you receive $59 of a/c delivered into your home. 41% of the energy is wasted dollars, wasted carbon-dioxide destroying the ozone and wasted energy.
As the pictures show above, spray the underside of the roof deck with 5-6" of open spray foam insulation to bring the duct work within the insulation envelope.
Advantage Foam Insulators