The Phase-change Revolution: Optimizing metal roofing and wall systems with advancing technology

by Catherine Howlett | April 1, 2013 11:57 am

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by David A. Brown, PE
The custodian at Asheboro Church of God (North Carolina) had a well-worn routine during summer months. Every Saturday, he would turn on the church’s air-conditioning (AC) at 3 p.m. to ensure the space was cool and comfortable for morning service the next day. The 1115-m² (12,000-sf) building needed an 18-hour cool-down time.

In a time where sensitivity to environmental impacts is growing and utility bills are rising, this is unacceptable. Buildings demand reliable, high-thermal performance envelope solutions that:

• drive energy-efficiency;
• prevent heat loss;
• eliminate thermal bridging; and
• control solar gain over its operating lifetime.

These performance criteria are key factors in minimizing energy running costs and reducing carbon emissions, while maximizing property value and lease-out opportunity.

When evaluating a solution for the Asheboro church’s re-roofing application, the construction team was faced with these performance objectives. In June 2011, after investigating new innovations in sustainable technology, the team opted for phase-change materials (PCMs).

A ‘roof hugger’ technique—a sub-purlin system that fits on existing metal panels, supports new panels, and can be configured to add insulation—was used for metal-over-metal re-roofing. The new panels were seamlessly integrated with sheets of a plant-based compound that absorbs heat when temperatures exceed the desired level and releases it when the temperature cools. Like other phase-change materials, it helps regulate the interior climate.

The changes made to the church’s appearance were notable, but the most interesting improvement centered on a substantial reduction in energy consumption. Now, instead of an 18-hour cool-down period, the custodian can turn on the AC at 6 a.m. on Sunday to achieve the desired temperature for the day’s service. The one-week delay in HVAC activation was immediately recognizable in the church’s power consumption. Peak summer internal temperatures were reduced from 35 to 26 C (95 to 79 F). Overall, monthly bills dropped significantly. In two months, the church had reduced its power bill for this portion of the facility by more than $1500 (roughly 50 percent) over previous bills for the same timeframe. Church leaders report the structure is more comfortable and the energy bills still remain low even though the structure’s weekly usage has increased.

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The phase change difference
While metal roofs and metal-framed structures are valued for strength, durability, aesthetics, reflectivity, and sustainability, their low thermal mass causes them to rely on insulating and alternate mass materials to achieve maximum energy efficiency.

Traditional insulation works as a simple barrier, or resistor, slowing heat transfer. This is a good start, but technology has gone beyond insulation to develop metal roofing and/or wall applications integrated with phase-change material technology, which absorbs and releases excess heat as needed.

Phase-change materials capitalize on nature’s basic law, heat always moves toward cold. For example, if an ice cube is put in a glass of room-temperature water, it absorbs the latent heat from the water as it liquefies, making the water cooler. If the glass of water is put back in the freezer, it gives off latent heat energy as it solidifies back into ice.

Rather than ice cubes, building technology has created phase-change compounds that liquefy and solidify at specific temperatures. The point at which a substance changes phase can be set at room temperature so the substance absorbs energy above the phase-change temperature and releases energy when the temperature drops below the set point. This process reduces temperature fluctuations, making buildings more energy-efficient.

In the Asheboro church’s case, PCMs were further optimized by being installed in sheets integrated with metal panels and insulation in a complete metal roofing system. Rather than just blocking heat transfer, PCMs—which have heat-storing capacities up to 90 times of concrete—absorb heat and release it when the air temperature drops. The materials changes from a solid to a gel, and then back again.

Phase-change materials can absorb and release energy at a predetermined temperature. Using energy that would otherwise be wasted, the systems absorb heat:

• entering through insulation (in a cooled building); and
• trying to escape through insulation (in a heated one).

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Integration with metal roofing
Marrying a metal building envelope with phase-change materials makes sense. Metal roofing and wall systems promote energy efficiency with inherent reflective qualities allowing buildings to stay cooler and use less electricity for mechanical systems. This means a decrease in power generation and a reduction of pollutants.

Following the fundamental property of materials to absorb heat when they melt and to release it when they solidify, PCMs absorb and release heat at pre-set temperatures. While materials exhibit this behavior, some go through this phase change at or near room temperature, absorbing and releasing heat in the process.

When both elements (metal envelope and PCMs) are brought together, and the integrated system is placed into the structure of a building, the phase-change material is able to absorb heat during the day and then release it at night. This makes the entire energy cycle more efficient. Fewer kilowatt hours are used to heat and cool buildings while the phase-change material intelligently captures and releases otherwise-wasted energy.

When integrated into a metal wall or roofing system, PCMs have been shown to provide energy savings up to 30 percent in heating and up to 50 percent in cooling, therefore reducing indoor temperature fluctuation. Integrated systems also improve occupant comfort and efficiency by:

• reducing the need for mechanical heating and cooling;
• decreasing greenhouse gas (GHG) emissions;
• lowering overall energy use; and
• shifting energy usage away from peak demand.

A roofing or wall assembly with metal panels, insulation, and PCMs creates an ideal application for reasons including installation ease and design aesthetics. Architects who do not want the energy-efficient system to alter the overall design aesthetics or standard construction practices and materials have options beyond adding more insulation or photovoltaic (PV) systems. Highly energy-efficient roofs and walls can be designed with thinner assemblies, negating the need for thicker, bulkier exterior envelopes. Additionally, these systems can be installed at much lower cost than solar power systems and offer better payback without incentives or subsidies.

An integrated metal roofing or wall system with phase-change materials is easy to install and can be integrated in both new construction and retrofit applications. In the assembly of a roofing system, PCM is placed over the decking and under the insulation. For new or retrofit applications, PCM can be stapled, screwed, or glued to metal or wood studs.

Unlike insulation or vapor barriers, PCM does not require a continuous and unbroken plane to be effective. The more space covered by the material, the better the results—performance is based on thermal mass as opposed to thermal resistance. However, since gaps will not negate the material’s effectiveness, it is less dependent on skilled installers than other building envelope enhancements.

PCM in a wall assembly, either new or retrofit, is placed between the wallboard and the insulation within the framing members.

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Passing the test
An increasing amount of testing has been done to measure the efficiency of phase-change materials in building envelope construction. In Arizona, two identical sheds measuring 4.8 x 3.6 x 2.4 m (16 x 12 x 8 ft) were constructed to test the effectiveness of PCM. In both sheds, the wood stud walls were insulated to an R-value of 19, while the roofs were insulated to R-30. The interiors were finished with gypsum board. The 4:12-steep roofs received composition shingles. This ratio is determined by the vertical rise divided by the horizontal span.

To round out preparation for testing, the buildings were conditioned with high-efficiency heat pumps controlled by digital programmable thermostats set at 22.7 C (73 F) for heat mode and 25 C (77 F) for cooling. In the control shed, no phase-change materials were installed. In the other, PCM was added to the walls and roof.

HVAC run-times were greater for the control building and less for the PCM-lined building. The implications of shorter run-times include:

• lower cost for energy;
• lower levels of carbon emissions;
• smaller HVAC units needed;
• less stress on the power grid during peak times;
• more consistent indoor temperature; and
• greater tenant comfort.

While architects and specifiers have the opportunity to include phase-change materials in metal walls, roofs, and buildings, there are potential applications in other areas of industry and manufacturing as well, which include:

• transporting combustibles and hot or cold perishables—such as food, pharmaceuticals, and
• organ or blood tissue—by maintaining temperature in the containers;
• electronics and telecommunication shelters to protect temperature-sensitive equipment
• greenhouses requiring AC and/or heat to protect plants;
• laboratories needing precise control of cooling and heating for chemical reactions; and
• increased solar efficiency in crystalline silicon PV applications.

As mentioned, a building can experience energy savings up to 30 percent in heating and up to 50 percent in cooling through PCMs. In warmer climates, the interior temperature becomes consistently comfortable so AC can often be eliminated. In all cases, energy savings from phase-change materials enable smaller, more efficient heating and cooling units to be installed. Direct savings are realized by reduced electric and gas utility bills.

Third-party testing has proven these statistics. The energy-efficient metal system used in the Asheboro church was inspired by testing performed by the U.S. Department of Energy’s (DOE’s) Oak Ridge National Laboratories (ORNL) and funded by the Metal Construction Association (MCA). During the third quarter of 2009, three experimental attics using different retrofit roofing technologies were constructed at the ORNL Natural Exposure Testing Facility.

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The first test attic represents the traditional way of roof retrofit, where the old roofing materials are removed for disposal in landfills and replaced with a new roof cover. The two remaining attics utilized roof-over-the-roof technologies. Both employ metal roofing panels capable of being installed directly over the existing roofs without requiring the old material’s removal. These metal panels contain a cool-roof coating to minimize solar heat gains.

In the final test attic, roof-integrated PV laminate and PCM were also used. The test data demonstrated a roof-over-the-roof can be an effective way of not only refurbishing the aged surface, but also improving energy performance of existing roofs. During the winter and spring of 2009 and 2010, the PV/PCM attic showed a
30 percent reduction in the heating load compared to the conventional shingle attic. On average, the maximum daytime temperatures were about 15 percent lower in the PV/PCM attic compared to the shingle attic. This difference was higher in the late spring and summer, with a 50 percent reduction in cooling load and a 75 percent reduction in nighttime heat loss.

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The best conditions for PCM
The interior temperature of a PCM building is more stable, providing a comfortable environment. Evening out the temperature of a building lessens extreme temperature spikes. This alone improves a building’s energy efficiency. The cool-down and heat-up times are greatly altered. Phase-change materials integrated with metal roofing and wall panel systems are most effective in buildings with low thermal mass, where energy costs are high and in climates with large internal and external temperature gradients. While financial justification can be made in nearly all building types and climate zones, return on investment (ROI) is maximized when outside ambient temperatures fluctuate higher or lower than a building’s internal operating temperature.

Bio-based phase-chase materials significantly reduce the temperature variant so the mechanical equipment can work less to maintain the building’s desired internal operating temperature. In climate-controlled buildings, the results are lower energy consumption and lower operating costs. In non-climate controlled buildings, the result is greater comfort because internal temperatures are moderated when there are large daily swings in outside ambient temperature.

Sustainability factors
In addition to energy savings and comfort, a PCM system holds a range of sustainability benefits. It can be made from natural products such as highly refined soybean and palm oils. The material behind bio-based phase-change technology is completely biodegradable and recyclable. If a building is demolished, the PCM can be extracted and applied toward another project. Buildings incorporating metal roofing and/or walls assemblies integrated with phase-change material are eligible for the following credits under the Leadership in Energy and Environmental Design (LEED) program:

• Sustainable Sites (SS) Credit 7.1, Heat Island Effect–Non-roof: one point for covering 50 percent of hardscape with reflective roofing;
• SS Credit 7.2, Heat Island Effect–Roof: one point for roofing with high reflectivity and high emissivity;
• Energy and Atmosphere (EA) Credit 1, Optimize Energy Performance: up to 19 points for a 48 percent energy budget reduction under ASHRAE 90.1, Energy Standard for Buildings Except
• Low-rise Residential Buildings (reflective roofing and PCMs contribute to this reduction);
• EA Credit 2, Onsite Renewable Energy: one to seven points for onsite renewable energy (e.g. PV), one point for producing one percent of required energy, and seven points for 13 percent of required energy;
• Materials and Resources (MR) Credit 2, Construction Waste Management: one point for recycling/salvaging 50 percent of site waste, and two points for 75 percent (metal serves as the primary contributor);
• MR Credit 4, Recycled Content: one point if entire building contains 10 percent of recycled content and two points if it contains 20 percent or more (metal is primary contributor); and
• MR Credit 5, Regional Materials: one point if 10 percent of products are manufactured or harvested within 805 km (500 mi), two points if 20 percent or more of products are manufactured or harvested within 805 km of the project site.

Conclusion
When phase-change materials are used on a project, the temperature will be more comfortable for the building’s inhabitants. By integrating these products into roof and wall systems, a building can maintain a consistent and comfortable temperature, while delivering durability to the design.

David A. Brown, PE, is the product development director for Fabral—supplier of metal products for building envelopes in architectural, commercial, post-frame, industrial, transportation, and agricultural applications. He has spent the past 28 years serving technical and sales and marketing needs in the metal wall and roof construction industry. Brown is an industry member of Construction Specifications Institute (CSI) and achieved CSI’s Construction Documents Technology (CDT) certification. He can be reached via e-mail at dbrown@euramax.com.

Source URL: https://www.constructionspecifier.com/the-phase-change-revolution-optimizing-metal-roofing-and-wall-systems-with-advancing-technology/

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