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...other problems that need be dealt with. There is heat gain to the thermal distribution system when ducts are located in the attic. Also, leaky supply ducts can cause negative pressures within the house when the air handler is on. The negative pressures can cause hot air from the attic to be drawn into the conditioned space through gaps around recessed light fixtures of other bypasses, adding further to the home's cooling load (see Figure 1).
[FIGURE 1 OMITTED]
With metal roofing systems becoming more popular for new construction and re-roofing, the Florida Solar Energy Center (FSEC) wanted to learn more about these roofing systems, and about their effect on attic temperature. And we were curious to find out how well metal roofing systems performed compared to traditional black-shingle roofing systems. During the summer of 2002, we performed testing on four finished and unfinished metal roofing systems and two roofing systems using traditional dark asphalt shingles. Our research shows that controlling attic air temperatures can be just as important as reducing ceiling heat flux during times of peak cooling loads (see "Not So Extreme Attic Example," p. 14). We found that the metal roofing systems we tested generally perform well, although the performance of some of them degrades over time.
Side-by-Side Roof Testing
The experiments took place at our Flexible Roof Facility (FRF) in Cocoa, Florida. The FRF is a 24 ft x 48 ft frame building constructed with its long axis oriented east-west (see photo above). The roof and attic are partitioned to allow simultaneous testing of multiple roof configurations. The attic is sectioned into six 6-ft-wide test cells that are thermally separated by partitions. The partitions between the individual cells are well sealed to prevent air flow between cells and are insulated to R-20 using 3 inches of isocyanurate insulation. The gable roof has a 5/12 pitch and 3/4-inch plywood decking. With the exception of cell 2, R-19 unsurfaced batt insulation is installed in a consistent fashion between the attic trusses in the test bays. The attic is separated from the conditioned interior by 1/2-inch gypsum board. The interior of the FRF is a single air-conditioned space.
The facility allows reconfiguration with different roofing products, and we've used it to examine different levels of ventilation and installation configurations for the roofing. We've also compared reflective roofing, radiant barriers, and sealed attic construction. Our recent testing addressed several questions. (Note that 1:300 ventilation is 1 [ft.sup.2] of attic ventilation, or net free vent area, per 300 [ft.sup.2] of attic floor area.)
* What is the performance (ceiling flux and attic air temperatures) of a standard black asphalt shingle roof with 1:300 ventilation (the control cell)?
* How does the Galvalume metal roof compare in thermal performance with a galvanized metal roof?
* How does an ivory metal shingle roof perform compared with the roof with a lower solar reflectance that was installed the previous summer?
* How does an innovative double roof construction with an insulated roof deck, radiant barrier, and no attic ventilation perform...
NOTE: All illustrations and photos
have been removed from this article.
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