Figure 68. Vented attic thermal processes.

Flexible Roofing Facility:  Improving attic thermal performance is fundamental to controlling residential cooling loads in hot climates.  Research shows that the influence of attics on space cooling is not only due to the change in ceiling heat flux, but often due to the conditions within the attic, and their influence on duct system heat gain and building air infiltration. (Please see Figure 68.)

The importance of ceiling heat flux has long been recognized, with insulation a proven means of controlling excessive gains.  However when ducts are present in the attic, the magnitude of heat gain to the thermal distribution system can be much greater than the ceiling heat flux.  This influence may be exacerbated by the location of the air handler within the attic space - a common practice in much of the southern US.  Typically an air handler is poorly insulated and has the greatest temperature difference at the evaporator of any location in the cooling system.  It also has the greatest negative pressure just before the fan so that some leakage into the unit is inevitable.

The Flexible Roof Facility (FRF), located in Cocoa, Florida, is designed to simultaneously evaluate five roofing systems against a control roof with black shingles and vented attic. Please see Figure 69.)  The test evaluated how roofing systems impact summer residential cooling energy use and peak demand.  In the summer of 2002, the following roofing systems were tested (Please see Table 26.)

Figure 69.  Flexible Roof Facility in summer 2002 configuration.  Cells are numbered from left to right.

Cell #	Roof Material	Ventilation	Roof Cooling Load Reduction	Overall Cooling Savings
#1	Galvalume® unfinished 5-vee metal	vented	32%	11%
#2	double roof with radiant barrier (ins roof deck)	sealed	7%	2%
#3	high reflectance ivory metal shingle	vented	38%	12%
#4	galvanized unfinished 5-vee metal	vented	22%	7%
#5	black shingles (control cell)	vented	control	control
#6	white standing seam metal	vented	7%	2%

All roof cells had R-19 insulation installed on the attic floor, except the double roof configuration (Cell #2) which had a level of R-19 open cell foam sprayed onto the bottom of the roof decking.  Measured thermal impacts included ceiling heat flux, unintended attic air leakage, and duct heat gain.

The sealed attic double roof system (Cell #2) provided the coolest attic space of all systems tested (average maximum mid-attic temperature was 81.1^oF), and therefore had the lowest estimated impact due to return air leakage and duct conduction heat gains.  However this cell also had the highest ceiling heat flux of all strategies tested, and recorded the most modest space cooling reduction (7%), relative to the control roof.

Metal roof testing was given more emphasis in 2002 due to the popularity of these products.  Researchers tested both galvanized and Galvalume^® roofs. Galvalume is a cold-rolled sheet with  a highly corrosion-resistant hot-dip metallic coating application of 55% aluminum 43.4% zinc, and 1.6% silicon. These roofs are reported to better maintain solar reflectance than galvanized roofing systems.  Average daily mid-attic maximum temperatures for the Galvalume^® and galvanized metal roof systems were roughly similar (19.6^oF and 17.3^oF cooler than the control roof, respectively).  The estimated total heat gain for these roof cells also were relatively close. 

The highly reflective ivory metal shingle roof (Cell #3) provided the coolest peak attic temperature of all the cells without roof deck insulation. Its average maximum daily mid-attic temperature was 93.3^oF (23.4^oF lower than the control dark shingle cell). While the ivory metal shingle roof's reflectance was slightly lower than the two metal roofs and white metal roof, researchers noted that the air space under the metal shingles provided additional effective thermal insulation.

Researchers also estimated the combined impact of ceiling heat flux, duct heat gain, and unintended attic air leakage from the various roof constructions. All of the alternative roofing treatments produced lower estimated cooling energy loads than the standard vented attic with dark shingles.  (Please see Figure 70.). The Galvalume® roof clearly provided a greater cooling energy use reduction than the galvanized roof.  This also was true during the 2001 study.  Nighttime attic temperatures and reverse ceiling heat flux have a significant impact on the total daily heat gain, particularly for metal roofs.

Figure 70.  2002 estimated combined impact of duct heat gain, air leakage from the attic to conditioned space, and ceiling het flux on space cooling needs on an average summer day in a 2,000 ft2 home.

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