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Roof of the Future? Oak Ridge, Metals Group Seek the Ideal Design

Monday, April 16, 2012

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The Metal Construction Association (MCA) and Oak Ridge National Laboratory (ORNL) are embarking on the final phase of a multi-year research program to design what might be called the roof of the future.

 Phase 1 construction: PCM heat sink atop the roof deck, rigid fiberglass insulation with foil facing, above-sheathing-ventilation using raised metal subpurlins and PV laminated metal panels.
Phase 1 construction: PCM heat sink atop the roof deck, rigid fiberglass insulation with foil facing, above-sheathing-ventilation using raised metal subpurlins and PV laminated metal panels.

The association and the Department of Energy’s Oak Ridge Lab said the final phase of the “Dynamic Building Envelope Research” project will evaluate the “synergies between various metal roof assemblies,” including cool-roof coatings, for their performance in improving energy efficiency.

Components in the roof assemblies include insulation, radiant barriers, phase-change materials, and above-sheathing ventilation.

The final phase of the research also will evaluate the interaction of cool metal roofing and Si crystalline photovoltaic modules in both steep-slope and low-slope orientations, MCA said.

“This phase of the research will help us understand more impacts from the interaction between the various components of a roof assembly,” said Scott Kriner, MCA technical director.

 Phase 2 construction: Black plastic strips replaced the hase-change material blocks, while maintaining a similar geometry.
Phase 2 construction: Black plastic strips replaced the hase-change material blocks, while maintaining a similar geometry.

“Many of the experiments are designed to demonstrate how certain components can store heat or dissipate heat beneath the roof surface and minimize heat gain into the building space below the roof. Results from this final phase will allow us to optimize the design of a metal roof assembly to provide the most energy efficiency in a given climate,” Kriner said.

Full-size roof mock ups are installed on the Envelope Systems Research Apparatus and the Roof Thermal Research Apparatus (RTRA) in the Building Technologies Research and Integration Center at Oak Ridge Lab.

The multi-year research project is supported by MCA, which represents metal building component manufacturers and material suppliers.

Kaushik Biswas, research and development associate at ORNL, said data generated from the experimental roofs developed in the project “are invaluable from a modeling perspective. Validated simulation models can estimate the actual energy savings from each technology component for different climate zones and provide recommendations for optimum roof design.”

Looking to Optimize the Designs

Biswas provided the following review of the research program, initiated in 2009 at ORNL’s Building Technologies Research and Integration Center.

In phase one of the project, an experimental roof utilizing multiple energy-saving technologies was installed on the Envelope Systems Research Apparatus test facility at Oak Ridge.  The roof consisted of amorphous silicon photovoltaic (PV) laminates integrated with metal roof panels; dense fiberglass insulation with radiant barrier; above-sheathing-ventilation; and phase-change material heat sink. 

The test roof showed “tremendous potential” by reducing the heat gains and losses through the roof by up to 90% compared to a traditional asphalt shingle roof. The contributions of the individual technology components, however, were difficult to determine, Biswas said.

 The two nearly identical experimental roofs in phase 2 construction: without phase-change materials (left) and with phase-change material (right).
The two nearly identical experimental roofs in phase 2 construction: without phase-change materials (left) and with phase-change material (right).

With that in mind, a second test roof was built that was identical to the first in all respects except for the lack of phase-change material. This enabled the comparison of the roofs with and without the phase-change material.

Phase 3 of the research program is a continuation of this project, in which four steep-slope roofs are being tested to evaluate the contributions of the other components such as above-sheathing ventilation, radiant barrier, and others, and optimize the roof designs to maximize energy savings, he said.

Phase-change materials have been shown to reduce fluctuations in temperatures of building interiors by restricting heat gain during hot daytime temperatures. In cold weather, the materials help to retain daytime heat gain during nighttime hours.

In an earlier report in D+D, Kriner described the phase-change material as a wax-type substance that is contained in capsules arrayed throughout the roofing membrane. The material remains in a solid state until temperatures reach a certain point. It then melts and functions as a heat absorber, reducing heat transfer to the building space below. In colder weather, the material acts as a heat-storage device, helping to minimize heat loss from the building.

The MCA/Oak Ridge program was the subject of the 2010 D+D story, Subtraction by Addition: Multiple Parts Equal One Cool Roof System.

The dynamic roof assembly described in that report is composed of a plywood deck covered, in order, by rigid-board insulation, a membrane containing the phase-change material, an air space that provides above-sheathing ventilation, and the metal roof panels and laminated photovoltaic system. Above-sheathing ventilation has also been shown to reduce heat transfer from sunshine-heated roof surfaces.

Another D+D story, Power Packed: Project Combines Technologies in High-Tech Roof System, reported on a retrofit roofing concept developed by MCA member companies that merges existing technologies into an integrated metal roof system that can improve energy efficiency and lower energy demands in buildings. The roofing system creates an air space by adding structural subframing atop the existing roof, followed by installation of a new cool metal roof over the assembly.

   

Tagged categories: Cool roof coatings; Energy efficiency; Metal Construction Association; Metal roof coatings

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