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Energy-Efficient Envelopes for Metal Building Construction

Mark Robins, Senior Editor, Posted 10/27/2011

The basic function of any building's energy-efficient envelope is to separate the outside environment from the building's interior. The design of metal buildings makes them very compatible with energy-efficient envelopes allowing for a fast and economical blend of energy-efficient applications that starts with the erection process.

An energy-efficient envelope is the foundation upon which all other systems are designed to fit. "In new construction, energy concerns or improvements can be made by adding insulation in both roof and wall assemblies," says Jim Bush, vice president of sales and marketing, ATAS International Inc., Allentown, Pa. "In a retrofit situation, additional insulation can easily be added during the renovation process."

Two primary energy-efficient envelope functions are air infiltration and thermal performance. "Infiltration directly relates to how effective the building envelope will be in providing air and water tightness in the roof, walls and other envelope components such as windows, doors and flashings," says Brad Rowe, national marketing manager, Thermal Design Inc., Madison, Neb. "Before one can have good control of energy efficiency, there must be functioning barriers in the envelope for air, water and heat."

Energy-efficient envelopes produce a controlled environment enabling occupant comfort and energy cost reduction. "To be a true energyefficient envelope, it must minimize air infiltration which can have an adverse affect on indoor air quality and rob the building of valuable conditioned air which in turn increases energy costs," says Mark Engebretson, director of marketing and business development, Therm-All Inc., North Olmsted, Ohio. "Since the insulation in the roof and walls in metal building solutions is often exposed, the acoustical performance and aesthetics of these assemblies plays an important role as well. Roofs and walls are not the only components considered part of the envelope, though they typically represent the largest surface areas. Doors, windows, skylights and foundation are the other components that must be considered carefully to complete an efficient design. Another component that has been identified more recently as playing an important role, especially in the southern climates, is cool roofing."

The effectiveness of all installed envelope components and the seal minimizing air infiltration at each joint, seam and penetration is critical. "Air infiltration is probably the biggest problem of building envelopes regardless of how much insulation is installed," Rowe says. "If the wall leaks between or through the components due to pressures from wind, mechanical systems or vapor differences, then energy efficiency will be difficult to achieve, maintain and manage. The overall effectiveness is highly dependent on using quality materials installed with quality workmanship. Caulking that shrinks, tape that loses adhesion, staples that rust, and weather seals that shrink, deform and disintegrate over time are just a few examples of envelope component failures that reverse the initial effectiveness."

Controlling interior space conditions at the least expensive energy cost is the main goal of an energy-efficient envelope. It's important to select the optimum levels of thermal performance based upon building use, conditioning requirements and the systems used in the enclosed space. Rowe believes optimal performance levels can be effectively achieved and maintained by maximizing insulation thickness, using correct vapor retarder applications and installing effective air barriers. "With proper building envelope optimization, all other building systems can then function properly without massive over sizing due to functional uncertainties," he says. "Improving the building envelope design and optimizing the installed insulation performance will return more value to the building owner than any other building material going into the project."

An air barrier

Air barriers prevent airflow between the inside and outside of buildings. Air passage due to differential pressure is the primary cause of heat loss in insulated buildings. Air barriers enhance energy efficiency and prevent moisture problems, which help improve indoor air quality and protect longterm structural integrity. A study from the National Institute of Standards and Technology shows that air barrier systems in non-residential buildings can reduce air leakage by up to 83 percent and energy consumption by up to 40 percent.

There are two broad types of air barriers: vapor impermeable air barriers and vapor permeable air barriers. "Both types can be either sheet membranes- mechanically-attached or self-adhered-or fluid-applied," says Jane Wu, product marketing manager, Grace Construction Products, Cambridge, Mass. "For any air barrier to be effective in preventing air leakage, it must be continuous. Using high-quality flashing systems at all openings, and following manufacturer's application instructions and details are crucial to ensure a continuous air barrier throughout the building envelope.

"Perm-A-Barrier VPS is a self-adhered vapor permeable air barrier membrane consisting of a breathable carrier film with a specially designed adhesive. It protects against the damaging effects of air and water ingress on building structures by creating a solid barrier against air infiltration and exfiltration, which minimizes associated energy loss and condensation problems. Perm-A-Barrier Aluminum Wall Membrane is a self-adhered impermeable air barrier sheet. It is specifically designed for up to 12 months of UV exposure, allowing for extended job-site scheduling flexibility and reduced concern for membrane damage and replacement, resulting from exposure."

Foam insulation's impact

Foam insulation used in metal building solutions is typically rigid foam insulation incorporated into the envelope via individual rigid board panels, field-applied sprays, or factory-integrated with the cladding, such as insulated metal panels. This insulation retards heat transfer, either into or out of the building. "Foam insulation can be highly effective in this regard because it has a very high heat flow resistance per inch of material," says Robert A. Zabcik P.E., LEED AP, director of research and development/Green Building Initiative, NCI Group Inc., Houston. "If utilized with facings or metal skins, it can also be a very effective air barrier preventing outside air from entering the building. Although all buildings need some air infiltration for indoor air quality, bringing this air at a controlled rate through the building's HVAC system instead of unintentionally through the building envelope is a primary focus of newer energy codes."

Prescriptive model energy codes are now recognizing and requiring continuous insulation in metal walls in most climate zones. "Rigid foam insulation such as THERMAX with its R-6.5 at 1-inch and wide range of facer configurations, delivers not only the required R-value, but with proper detailing, can work as part of an air barrier system," says Doug Todd, market manager at Dow Building Solutions, Midland, Mich. "Available in thicknesses up to 4 1/4 inches and up to 30 feet long, THERMAX provides the long-term, stable R-value that is needed over the life of the building, and meets fire, building and energy codes.

"Rigid insulation also offers a number of performance advantages over other types of insulation. Because it doesn't sag or compress, and is usually placed over the metal wall girts/roof purlins to form a continuous layer of insulation, it maintains its R-value over the long-term. By properly detailing the joints of THERMAX insulation and at interfaces such as ceilings and floors, designers can specify an easy-to-construct, field-tested air barrier that many newer energy codes require."

Fiberglass cavity insulation

Fiberglass insulation is one of the most popular and cost-effective, energy-efficient envelope materials for metal building solutions. Fiberglass insulation has very desirable attributes such as acoustic absorption, sound transmission reduction, inorganic fibers, lightweight compaction for shipping economy, ease of handling and overall low installed costs.

For many years taking a single layer of fiberglass with a vapor retarder laminated to it, and compressing it between the roof/wall sheets and the secondary framing was a very common application. However, "the recognition that there was a tremendous loss of thermal performance through compression has brought about changes in how fiberglass is being installed," says Engebretson. "Because metal building systems provide a natural cavity with the roof purlins and wall girts, these cavities are being utilized to add additional insulation and improve the overall thermal performance. In conjunction with standing seam roofs utilizing a foam thermal spacer block at the purlins, as well as the availability of taller standing seam roof clips, the improvements have been vast. There are now fiberglass alternatives to the most stringent energy codes that can provide a bright, clean finished appearance."

Fiberglass insulation has been designed specifically for metal building systems. Fiberglass manufacturers in the North American Insulation Manufacturers Association working in conjunction with National Insulation Association have developed important standards for the manufacture and lamination process of fiberglass. NAIMA 202-96 and NIA "Certified Faced Insulation" help ensure the thermal performance to both the contractor and building owner.

In spite of its many advantages, fiberglass insulation has received some negative publicity in the last few years for "the way it is commonly compressed in metal building assemblies, leading to a loss in in-place R-value," Zabcik says. "That's unfortunate because it is a very inexpensive way to insulate a building. Yes, there is a loss in R-value but it does not go to zero like some would have you believe. Additionally, most fiberglass insulation manufacturers have been actively developing and testing new proprietary systems that are highly effective and proven performers. Many of them integrate continuous air barriers as well. Our advice would be to spend some time familiarizing yourself with ASHRAE requirements and researching the performance metrics for each fiberglass system rather than eliminating fiberglass from contention."

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Pushing the envelope

Energy-efficient envelope product manufacturers are quickly improving and releasing their products to accommodate a growing and competitive market. "We are likely going to see the greatest impact from new processes and ancillary products that augment our metal roof and wall panels along with our complete building envelopes," predicts Zabcik. "In the next few years, improved performance in foam insulations and incorporated blowing agents, as well as new details and installation processes developed with energy efficiency in mind, will become more prevalent. Innovative approaches incorporating building integrated photovoltaics, phase change materials and above sheathing ventilation could also be factors in the coming years.

"Our main advice would be to not focus on one or two individual performance metrics, but to consider every potential solution in the context of how it affects the entire building. Again, whole building modeling can be a big help in this respect, but even the most optimized solution will flounder if it is not built correctly. Good installation techniques and quality control, both in the factory and the field, is critical to achieving the desired performance level. That will never change."

The metal construction industry must be aware of new regulations regarding energy-efficient envelopes to fully optimize their performance. "Designers should keep abreast of model energy codes changes and recognize that these only specify the minimum," says Todd. "Moving forward, the codes will increasingly require more energy-efficient approaches. For example, the new ASHRAE 90.1-2010 standard will cut energy use by 30 percent from 2004 requirements. It recommends continuous insulation as a requirement in most climate zones and has increased R-value requirements for metal buildings nationwide. The model codes are also moving to mandatory air barrier requirements. Fortunately for the design community, there are prescriptive methods to detail the insulation, such as THERMAX insulation, as an air barrier system and to meet the air leakage and CI requirements in a single solution."

There are new computer software programs assisting in the overall design of not only the envelope, but the entire building performance. ASHRAE, IECC and the DOE are working together to better educate designers and code officials, and are offering more tools to assist in this process. "[Metal Building Manufacturers Association] is working together with suppliers to help create more energy-efficient envelope designs as well as assembly testing," says Engebretson. "There has been improved and increased usage of infrared equipment and other technologies to locate weak areas of the envelope for both heat loss and air infiltration. This is leading to better construction techniques."

Rowe believes to maximize the full potential of energy-efficient envelopes now and in the future, "all parties from insulation manufacturers to the installers, including designers, must accept their responsibilities to achieve the installed performance of the products. So specify it, demand it, order it, install it, inspect it and reject it if it does not meet the specifications and expectations, otherwise it will not perform as expected. The building owners deserve nothing less than the performance they are led to believe they will get."

Interested in learning even more about energy-efficient envelopes for metal building solutions? The National Institute of Building Sciences, under guidance from the Federal Envelope Advisory Committee, has developed a comprehensive federal guide for exterior envelope design and construction for institutional/office buildings titled the "Building Envelope Design Guide." This guide provides information and resources regarding energy efficiency, green building construction guidelines, daylighting, blast and wind safety, flood resistance and indoor air quality.

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Envelope Influences

Building function and climate location have a big influence on energy-efficiency envelopes.

"Envelope energy efficiency is more important for a heated office building with high human occupancy and air conditioned, than for a warehouse with minimal human occupancy that is heated only and maintained at a lower inside temperature," says Mark Engebretson, director of marketing and business development, Therm-All Inc., North Olmsted, Ohio. "How many hours of the day and days of the week the building is operational is another design consideration. The shape and building orientation can be important for capturing daylighting from windows and skylights. The larger the building, typically the more surface area is exposed to the outside environment which in turn creates the need for a more efficient building envelope to reduce energy costs."

Roof color has an influence on energy-efficient envelopes. "Factory-coated metal cool roofs are excellent candidates to consider, but sometimes, the best answer may very well be a darker color that can help create warmth in a primarily heated environment," says Robert A. Zabcik P.E., LEED AP, director of research and development/Green Building Initiative, NCI Group Inc., Houston.

"As far as building sizing goes, minimizing the exterior surface-area-to-interior-volume ratio is a good idea, but that may not be an option aesthetically or from a site layout perspective. Nor does that strategy do anything to address internal heat and humidity loads. So how does somebody know what the optimal mixture of solutions is? Whole building modeling is probably the best option available. It allows a designer to pick the most efficient design with confidence because it considers the performance metrics of multiple solutions. That's why whole building modeling requirements are being introduced into high-performance building programs like LEED and standards like ASHRAE 189.1 and IgCC."

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