• Reflective Roofing Research

Figure 72 Vented attic thermal processes.

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. (Figure 72)

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) is an FSEC test facility designed to evaluate five roofing systems at a time against a control roof with black shingles and vented attic (Figure 73).

5th Budget Period Experiments

The testing evaluates how roofing systems impact summer residential cooling energy use and peak demand. In the summer of 2003, the roofing systems tested are listed in Table 45. Cell numbering is from left to right beginning with the second cell in from the left.

Figure 73 Flexible Roof Facility in summer of 2003 configuration.

All had R-19 insulation installed on the attic floor except in the configuration with the sealed attic (Cell #2) which had R-19 of open cell foam sprayed onto the bottom of the roof decking. The measured thermal impacts include ceiling heat flux, unintended attic air leakage and duct heat gain.

Cell #2 had a proprietary configuration which is not reported upon in this report.

A major thrust of the testing for 2003 was comparative testing of metal roofing under long term exposure. Given the popularity of unfinished metal roofs, we tested both galvanized and Galvalume® roofs in their second year of exposure. Galvalume® roofs are reported to better maintain their higher solar reflectance than galvanized types. Average daily mid-attic maximum temperatures for the Galvalume® and galvanized metal roof systems showed significantly better performance for Galvalume® product (17.5 oF and 13.1 oF cooler than the control dark shingle respectively).

Figure 74 Estimated combined impact of duct heat gain,air leakage from the attic to conditioned space and ceiling heat flux on space cooling needs on an average summer day in a 2,000 ft 2 home.

Other than the sealed attic case, the white metal roof results in the coolest attic over the summer, with an average peak of only 94.6 oF – 22.1 o cooler than the peak in the control attic with dark shingles. The highly reflective brown metal shingle roof (Cell #3) provided the next coolest peak attic temperature. Its average maximum daily mid-attic temperature was 101.5 oF (15.2 oF lower than the control dark shingle cell). While the brown metal shingle roof’s reflectance was lower than the two metal roofs and white metal roof we observed evidence that the air space under the metal shingles provides additional effective thermal insulation.

We 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 constructions produced lower estimated cooling energy loads than the standard vented attic with dark shingles (Figure 74). The Galvalume® roof clearly provided greater reductions to cooling energy use than the galvanized roof after two summers of exposure.

One important fact from our testing is that nighttime attic temperature and reverse ceiling heat flux have a significant impact on the total daily heat gain, particularly for the metal roofs. The rank order in Table 46 shows the percentage reduction of roof/attic related heat gain and approximate overall building cooling energy savings (which reflect the overall contribution of the roof/attic to total cooling needs):

The relative reductions are consistent with the whole-house testing recently completed for FPL in Ft. Myers (Parker et al., 2001). This testing showed white metal roofing having the largest reductions, followed by darker constructions.

4th Budget Period Experiments

Figure 75 Flexible Roof Facility in summer 2002 configuration. Cells are numbered from left to right starting with the second cell in from the left.

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. (Figure 75) The test evaluated how roofing systems impact summer residential cooling energy use and peak demand. In the summer of 2002, six roofing systems were evaluated as described in Table 47.

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 was relatively close.

Figure 76 2002 estimated combined impact of duct heat gain, air leakage from the attic to conditioned space, and ceiling heat flux on space cooling needs on an average

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. (Figure 76) 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.

3rd Budget Period

Figure 77 2001 Experimental roof cell. Cells are numbered from left to right starting with the cell second in from the left

In the 2001 testing, Cell #2 with the double roof/sealed attic showed the lowest attic temperatures and narrowest temperature range. (Table 48; Figures 77 and 79) Peak attic temperatures in Cell #2 were 5 oF to 6 oF lower than this same sealed cell the year before, without the double roof. This indicates that the double roof did provide a substantial benefit. Since there is no insulation on the attic floor though, there still is a significant heat gain across the ceiling. In fact, the ceiling heat fluctuation actually is higher than the reference Cell #5. (Figure 78)

The true impact of the double roof construction of Cell #2 is most likely a combination of the benefits of a cooler attic space that reduces duct heat gain and minimizes the effects of air leakage from the attic into the house, and the drawback of the higher ceiling heat flux.

Cell #3 with its spectrally selective dark brown metal shingles, produced lower attic temperatures at night, but higher roof deck temperatures (which were most likely due to the insulating quality of the shingles which have an air space underneath them).

Roofing Experiment with Habitat for Humanity in Fort Myers, Florida

In July 2000, FSEC and Florida Power and Light instrumented six side-by-side Habitat for Humanity homes in Ft. Myers with identical floor plans, orientation, and ceiling insulation, but with different roofing systems as described in Table 49. A seventh monitored house contained an unvented attic with insulation on the underside of the roof deck rather than on the ceiling.

Each unoccupied home was monitored from July 8 through July 31, 2001 to collect building thermal and air conditioning power data. Table 50 presents the cooling performance of the roofing systems clearly showing the energy-saving benefits of reflective roofing systems in Florida, especially the tile and metal roofs with solar reflectance between 65% and 75%.

Significant findings: Reflective roofing materials represent one of the most significant energy-saving options available to homeowners and builders. These materials also reduce cooling demand during utility coincident peak periods, and are potentially one of the most effective methods for controlling demand.

• Based on comparative data from August of 2000, the maximum decking temperatures in the sealed attic home were 23 E F higher than the control home (177 E versus 154 E ). After the installation of white shingles in midsummer, the highest deck temperature from the sealed attic home measured only 7 E higher than the control in August of 2001 (161 E versus 154 E ).
• An additional month’s data was collected with the homes occupied and thermostat set points kept constant. Average cooling energy use for the homes rose by 36%, but there was no decrease in the highly reflective roofing system savings. Additional heat gained from the occupants and their appliance use increased the cooling system runtime and introduced more hot air into the air conditioning duct system.
• In 2001, the average maximum attic air temperature in the terra cotta barrel tile roof home was 15 E F hotter than the maximum ambient. After installing a radiant barrier the average difference in August was +9 E F. A similar evaluation with the light colored shingles showed that peak attic air temperatures dropped from + 29 E to +20 E F after installing a radiant barrier.
• Household interior temperature settings varied from one year to the next, making direct energy saving comparisons impossible. Still, the collected data did show that attic air temperatures were reduced by the radiant barrier. On the other hand, measured maximum plywood decking temperatures rose by 11 E to 13 E F.
• Based on previously evaluated roof buckling problems on the decking of the sealed attic home, researchers decided to install white shingles similar to those on the RWS roof. It was thought that buckling problems likely were caused by excessive heat buildup in this roofing system. White shingles replaced the dark shingles to see if this would drop the roof decking temperature spikes.