The plenum is the space between a false ceiling and the actual ceiling that serves as a distribution area for heating or cooling systems. For unducted air returns, this is also called the return plenum and a significant amount of cooling energy is lost here.

So much attention has been placed on achieving optimum R-values and SEER ratings that almost no emphasis was given to effi ciency lost during distribution. The heat load on the roof raises plenum temperatures significantly and this directly affects the cooling effi ciency of the system since cool air is being reheated as it enter the plenum and then re-cooled.

The temperature inside this plenum went down from 93°F to 81.5°F or a reduction of 11.5°F. The supply temperature was 60°F and this translates to a 34.8% decrease in cooling load. As of this writing, the test area was limited to 1/3 of the entire roof surface. The temperature differential would have been greater if the entire roof was coated with Energy Seal reflective roof coatings.