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Energy Efficiency: The Envelope Please
Whether the goal is to cut energy costs, gain LEED points, reduce occupant complaints or address the issue of climate change, the building envelope is a prime opportunity to improve the energy efficiency of buildings. But steps to reduce energy losses through the envelope must be taken carefully to avoid creating problems.
Overwhelmingly, industry consultants agree that today’s watchword in energy efficiency is ‘air.’ More than ever before, facility executives are striving to take greater control of their air. Like any task, however, there are right ways and wrong ways of going about it. First, the way not to do it.
“In a large percentage of buildings we’re involved with today, the building owners need them to be tighter — that is, more airtight,” says Curt Liscum, senior consultant at Benchmark, Inc.
How Tight Is Too Tight?
The trend is pushing the envelope — quite literally. While taking control of airflow within a building is a positive trend and one that helps cut energy spend, facility executives should be cautioned not to take the trend too far.
Buildings have become so airtight, Liscum says, that it can cause problems with the envelope.
“It’s to the degree that the mechanical guys get a bit worried about it,” he says.
One problem in over-pressurizing a building comes when structures use mechanically fastened, single-ply roofs. Such positive pressure within a building can cause the roof to billow up and, if the problem is persistent enough, cause failure of the assembly, usually where the roof ties to the wall.
But when correctly done, managing airflow within a building can yield great savings for the energy spend. Effectively employing air-barrier systems and helping curtail leakage can help facility executives achieve the energy savings they desire.
“Air leakage can be 40 percent of energy loss,” says Eric Seaverson, manager of The Restoration Group at StructureTec.
Developing an integrated system is paramount. Consultants recommend that an air-barrier system be tied to the roofing system as well.
JoAnn Brookes, project architect for HOK, says the air barrier system between the wall system and roofing needs to be continuous.
“Often there’s a break right there — between the roof and walls — that lets cold air in,” she says.
Seaverson has seen this first hand, saying that one building he was working on complained of air loss.
“When we went up into the plenum, there were big holes up there that were letting cold air right into the building,” Seaverson says.
System continuity also applies to wall system insulation. Savvy facility executives understand thermal bridging, but to have an effectively operating structure, Seaverson says that even the insulation should be continuous.
“Pull the insulation out from between the studs, and place it on the outside of the studs,” he says. “If you have metal studs on your structure, they’ll still act to transfer heat if the insulation is only packed between them.”
Improved window technologies are helping make the building envelope more efficient.
Seaverson mentions “warm edge” window technology that helps reduce thermal bridges within windows.
Multipaned windows must be held at a certain distance within the window frame by spacers. Initially, manufacturers used aluminum spacers but, like metal studs, the aluminum acted as a thermal bridge, transferring heat and collecting condensation.
Modern warm-edge windows replace the metal with a spacer that uses a combination of sealant and desiccant in a thermoplastic compound. The spacer usually incorporates a thin, fluted metal shim of aluminum or stainless steel. Another approach uses an insulating silicone foam spacer with a desiccant and a high-strength adhesive at its edges to bond to glass.
Window and glazing technology are much better than were historically available, says Donald Fournier, chair of the Building Research Council of the School of Architecture and a program manager at the University of Illinois, Champaign-Urbana campus.
“Incorporating appropriating glazing into the envelope design is more complex but has the opportunity to have glazing be beneficial rather than a minus,” Fournier says.
Misconceptions and Overlooked Strategies
One hallmark of green buildings is the use of integrated design. This strategy, which relies on a team approach to design, brings together building-envelope and mechanical-system designers, who plan one system with the others in mind. This approach is one way to prevent problems. But even well-designed systems must be operated properly. Modern buildings add a layer of complexity; the automated systems they use usually are not programmable by laypersons, and can use competing, confusing logic. This can lead to problems.
According to Brookes, facility executives might not fully understand how modern, complex building systems operate. When they tinker with the systems, even if their intentions are good, things can go amiss. In high-performing buildings especially, inadvertently changing ideal building performance setpoints can wreak havoc.
Brookes cited one company in a previously balanced building which did just that — adjusted HVAC setpoints — and ended up with an energy-robbing cold plenum.
And Liscum marvels when building owners seem happy that they’ve met building codes.
“Code is the minimum acceptable standard,” he says. “And we could do more. Right now we have very marginal energy codes.”
One way to improve energy efficiency is by integrating the building envelope with the mechanical systems, making sure the two are matched.
Seaverson related a story of a building he investigated that was extraordinarily dry and over-pressurized. As a result of the dryness, the facility executive had to use a humidification system to ensure the comfort of the office workers. When the building was rebalanced, the humidifiers — which automatically turned on when the ambient humidity dropped below a certain percentage — ceased to fire back up. Balancing the building envelope eliminated the need for humidifiers.
“So they saved energy right there, not having to use a part of their mechanical system,” he says. “When you fill all the holes in the envelope, your mechanical systems don’t have to be as big.”
Just how do facility executives avoid pitfalls when trying to maximize building envelope performance? Consider these best practices for selection, design, and installation.
1. Find Holes and Plug Them
“From an energy standpoint, the one key is consistency,” says Liscum. “Make sure the envelope is continuous and consistent.”
Too often, he says, gaps are left around windows, between bats, around electrical outlets, and so on. Insulation products need to be uniformly used across the envelope.
“We often hear people say, ‘Oh, but that’s just a little hole,’” says Liscum. “But little holes can cause you big problems.”
2. Save Money and Energy by Re-using Materials
Seaverson says that effectively re-using materials is one way of saving money and being green. “We were working with an owner who decided they wanted a green roof,” he says. “It turns out that we could reuse the majority of the roof insulation.”
Seaverson’s team performed an infrared scan to make sure the insulation was dry, then re-used most of it.
3. Consider Longevity
“It’s not very green to be replacing things every five years,” Seaverson says. “Select durable products.”
Additionally, keep in mind that some elements could be costly if problems develop. Vegetated roofs are one such example. The membrane beneath a vegetated roof is always wet and, should something go wrong, access to the problem can be very difficult. That puts a premium on working with proven systems, suppliers, designers and contractors.
4. Commission During The Construction Process
Consultants report that they’re seeing problems in buildings as new as five or 10 years old. The reasons for this can be many. For example, contractors might have been rushed to put up a building or simply didn’t follow specs.
To combat this trend, industry experts recommend commissioning during the construction process, not just after the building has been built.
5. Monitor the Building
“Meter and submeter the building,” says William Rose, a council member of the National Institute of Building Sciences and a professor at University of Illinois, Champaign-Urbana campus. “Use that information to analyze building energy-use performance according to outdoor temperature and relative humidity, occupant use and equipment operation.”
He also recommends setting a target value for energy use in a building to establish a baseline of performance data. When you make a change, he says, monitor the result.
6. Reduce Loads
If mechanical equipment is to be replaced, then a substantial investment in building envelope recommissioning should always come first, Rose says. Loads should always be reduced, and replacement equipment should always be smaller.
7. Understand the Building
Brookes stresses that architects, designers and building owners all need to communicate. Designing a high-performing building exterior requires expectations to be addressed up front.
“As architects, we need to teach the owner about how the building works,” she says. “It’s then up to the facility managers to take it to the next level.”
Options for an Efficient Exterior
The initial design of a new building is the best time to consider exterior and envelope options that may contribute to overall energy efficiency. Many of these design elements are not exactly new, but if facility executives or their designers haven’t worked with them in the past, they might as well be. What follows is a brief guide to energy efficient exteriors strategies, and resources that can help facility executives learn how, when and why to use them.
Cool roofs Roofs that reflect the sun’s energy and re-emit any heat that is absorbed are called cool roofs because they help keep the building cool during hot summer months. This is especially true during the hot hours of the afternoon when air conditioners have to work the hardest and energy is the most expensive. Cool roofs come in a variety of materials, from white single-ply to both unpainted and painted metal. For more information see “Keeping Cool,” article, or visit the Cool Roof Rating Council.
Green or vegetated roofs Garden roofs have been around for centuries, but only recently have they come of age as a viable option for commercial facilities. Green roofs are considered an energy efficient option because the thick planting medium acts as a layer of insulation, keeping the building cooler during the summer and warmer during the winter. For more information on green roofs, visit Green Roofs for Healthy Cities.
Insulated metal panels Commonly used for walls, insulated metal panels (IMPs) use a skin of steel or aluminum with a layer of polyurethane or other insulating material. Depending upon their construction, the prefabricated panels have an R-value between 14 and 30. IMPs are used on all types of commercial facilities and are available in a variety of colors, styles and sizes. For more information, visit the Metal Construction Association or The Metal Initiative.
Efficient windows A wide range of options is available, including different types of frame material, a variety of coatings, gas–infill between lites of glass, and, for existing buildings, window films and interior storm windows. Windows are rated on a variety of metrics, including U-Factor, solar heat gain coefficient (SHGC), visible transmittance (VT), air leakage (AL), and condensation resistance (CR). Many state and local energy codes include prescriptive requirements for one or more of these measures, so it’s important for facility executives to educate themselves regarding their local codes, and then find window products that meet those criteria. For more information, visit the National Fenestration Ratings Council.
— Greg Zimmerman, Executive Editor
Loren Snyder, a contributing editor for Building Operating Management, is a writer who specializes in facility issues. He was formerly managing editor of Building Operating Management.