Energy saving is the major focus in today's search for the best value in facility management. The challenge is: how do you get good information you can trust, and literally take to the bank?
The roofing market is flooded with "green" solutions that are purported to save energy in one way or another: sustainable roofing; reflective "cool roofs"; rooftop photovoltaic systems; materials incorporating recycled materials or renewable resources; vegetative roofs; and materials that are themselves recyclable. What is too frequently overlooked is that the best way to conserve energy with a roofing system is to keep if from leaking.
The Oak Ridge National Laboratory (ORNL) Buildings Technologies Research and Integration Center (BTRIC) Web site says it best:
"The two major factors that improve the energy efficiency and durability of low-slope roofing systems are moisture tolerance and wind resistance. Moisture leads to the premature failure of roofing systems by degrading the mechanical integrity of the insulation system, corroding metal components such as fasteners and metal decks, and adding excessive weight to the roofing system. An assessment performed by ORNL has shown that if low-slope roofing systems could be kept dry, over three-fourths of a quadrillion BTUs of energy could be saved each year in the United States."
Increasing the availability of clean, abundant energy and restoring and protecting our environment are primary objectives of this independent science and technology laboratory, which serves the U.S. Department of Energy. ORNL's research makes a very persuasive argument that the waterproofing performance of roofing materials continues to play a vital role in energy saving. That is not to say that today's "green" technologies are irrelevant. But optimum energy savings can only be achieved when waterproofing integrity is given at least as much importance as the "greening" of the building.
Success in both areas can best be achieved by giving proper and thorough attention to these critical aspects of roofing design:
- Insulation Type and Quantity
- Surface Coatings and Finishes
- Vapor Retarders and Air Barriers
- Roofing Material Quality
- Type and Weight of Roof-Mounted Renewable/Sustainable Energy Systems
In addition, as our industry looks to the future, it is important that roofs be designed around site-specific microclimate conditions. Most models show that a roof properly designed for specifics of climate and building use can save more than 30 percent on energy bills.
Roof System Energy Audits
A Roof System Energy Audit is an analysis of the energy-saving potential of an entire roof assembly, based upon the life-cycle cost analysis of the previously listed assembly components. More specifically, a roof system energy audit will help you identify:
- How much and what type of insulation should be used, and how do you optimize insulation performance while maintaining compliance with all fire-related building codes?
- How relevant is reflectivity to energy savings, given your particular microclimate?
- Will your roof assembly require vapor retarders or air barriers to prevent condensation? If so, what type of system will be most effective giving the building's function and environment?
- What roofing materials will provide the optimum balance of waterproofing integrity and sustainability given the particulars of your facility's function and microclimate?
- What types of rooftop renewable/sustainable energy systems will a particular facility accommodate, and how are they likely to affect rooftop waterproofing performance?
- Which combination of roof and sustainable systems will best help you achieve your long-term goals?
Generally speaking, the thicker the insulation the more energy costs are reduced. However, there is a point where the cost of insulation catches up with the energy savings, and additional insulation leads to diminishing returns on your investment. The factors affecting your building's insulation requirements include its geographic location, its function, and the type of climate controls used.
Polyisocynurate (also known as polyiso) insulation provides the highest ratio of performance to thickness required. Building codes typically reference ASHRAE 90.1, which calls for a minimum R value of R-20 in most low-slope commercial systems; some federal projects require as high as R-50.
To provide a context for why a Roof System Energy Audit is essential, imagine how many different roof geometries, rooftop mechanical penetrations, and building operations below the roof are possible. Intuitively, it should be obvious that the insulation requirements of one roof may be very different from the requirements of another.
Further, although using the correct rooftop R value is paramount, using continuous R value throughout the roof system is a very close second. Insulation breaks at roof top penetrations, walls, and area transitions may leak a great deal of energy. These conditions need to be properly sealed. A Roof Systems Energy Audit can identify problematic areas through thermal imaging scan and visual observation.
Similarly, identical roofs in divergent climates will vary significantly in the insulation required to maintain like levels of energy saving. It is not uncommon for cold climate R value recommendations to be as high as R-50.
Vapor Retarders and Air Barriers
It is important to understand that insulation, as vital as it is to energy efficiency, is not a barrier to condensation. Air movement will variously be directed from the inside out or from the outside in, depending upon internal and exterior climate conditions. If warm humid air reaches a cooler surface at the right temperature, the air will condense into vapor.
The most common factors leading to rooftop condensation are:
- A January mean temperature of 40° F (4º C) or less
- A structure that is humidified (e.g., a pool) or where internal operations generate humidity in excess of 45% relative humidity (certain manufacturing environments)
- A structure that is enclosed and heated, with interior conditions that generate large quantities of humidity (industrial laundry)
Although the International Building Code (IBC) provides general standards for designing a condensation-free building, choosing an appropriate type and amount of insulation and combining it with a particular condensation-control system, (i.e., either a vapor retarder or an air barrier), requires a comprehensive analysis of the various roof design elements. Further, the insulation and the condensation-control system must work together to achieve optimum energy efficiency. A Roof System Energy Audit takes into account all these factors, analyzing the many options available to identify whether a condensation-control system is required to maintain lasting waterproofing integrity, and identifying the most appropriate system, should one be needed.
Surface Coatings and Finishes
In many parts of North America, maximum energy use is better quantified not at peak cooling demand times, but rather, over average yearly usage. In northern climates, the cost of a white-surfaced roof product may not return the investment it does in warmer climates. In northern climates, where there are far more heating days in the year, less reflective options such as gray mineral or aluminum coatings may actually save more money while ensuring the longest possible service life for your roof. Specifically, where cool, cloudy days significantly exceed hot, sunny ones, the medium-level reflectivity of an aluminum surface coating provides excellent reflectivity in the summer months without imposing a loss-of-heat penalty on cooler days.
Generally speaking, roofs with slope below 2:12 obtain the greatest benefit from reflective coatings and finishes. Although steeper slopes will achieve greater energy efficiency with lighter finishes, the comparative savings between a highly reflective white coating and a moderately reflective blue pigment may not merit the aesthetic sacrifice made.
Given the popularity of today's "cool roof" options, it is vital that a Roof System Energy Audit be conducted to separate the hype from the reality, so that you can properly evaluate the energy-saving contributions of various coatings and finishes. To focus exclusively on surfacing, while ignoring the far-more-critical issues of insulation and waterproofing integrity, is short-sighted.
When it comes to roofing, there is no such thing as one-solution-fits-all. The waterproofing capabilities of even the highest quality membranes and metals decline as systems age. Specifying materials with a margin of safety that allows for this normal aging process will help ensure that the waterproofing integrity of your roofs performs as warranted.
Since neither the government nor independent testing laboratories have identified one single test as the best predictor of longevity, Roof System Energy Audits typically rely upon a system's life expectancy when evaluating energy-saving potential.
To help guide your search for effective waterproofing integrity, it might be helpful to evaluate competing systems based on their performance in these specific areas, which are generally acknowledged to be the major contributors to rooftop longevity:
- For membrane systems: ASTM D 5147 testing, including tensile strength (500 lbs. plus); tear strength (900 lbs. plus); elongation; and low-temperature flexibility to -30 degrees F
- For metal systems: water penetration ASTM E 1646 (pass at 20.0 psf) and ASTM E 2140 (pass flood test); air infiltration ASTM E 1680 (20.0 psf results); wind uplift pressure ASTM 1592 , UL® 580 Class 90, and TAS 125 (Miami Dade NOA)
Keep in mind that factors of slope; building geometry (number of roof sections and complexity of design); building orientation; and the weight and positioning of rooftop equipment will all affect the service life of any roofing system. A properly conducted Roof System Energy Audit will take all such factors into account.
Renewable/Sustainable Energy Systems
Putting rooftop real estate to beneficial use through integrated rooftop technologies is an exciting prospect for most commercial building owners. Unfortunately, too often customer enthusiasm quickly fades when confronted with high initial costs. A Roof System Energy Audit can help you fairly evaluate the life-cycle return on such investments.
If you determine the return is adequate for your capital investment requirements, keep in mind that both your roofing design professional and your roofing material manufacturer need to be involved in any system that will be integrated into your roof, in order to maintain warranty protection.
Today's options for integrating sustainable solutions and roofs include:
- Photovoltaic systems that convert the infinitely renewable energy of the sun into power
- Vegetative systems that provide insulating landscapes of organic plant material and a filtration system over a rooftop waterproofing system
- Solar thermal systems that use the sun's energy to heat water, which circulates across the roof within modules, and is then routed into the building below to supplement the hot water system
The life cycle of every one of these options is typically a quarter of a century or more, so you will also want to make certain that the anticipated service life of the underlying roof meets or exceeds the anticipated service life of your rooftop renewable/sustainable energy solution. System weight is also a consideration, so involving a structural engineer to assess load capabilities is also advisable.
Reviewing building code guidelines is a good place to start the evaluation process, but only a roof industry professional can determine the precise impact that a rooftop renewable/sustainable energy system is likely to have on the waterproofing integrity of your roof. A Roof System Energy Audit takes into account all these factors to identify the optimum energy-saving design for your roof.
In the final analysis, rooftop longevity is the ultimate in sustainable design. A "green" roof that leaks negates any energy saving potential by accelerating the timetable for tear-off and replacement with all the adverse ecological impacts that implies:
- More energy consumed due to damaged insulation that can no longer perform its function
- More waste in our landfills due to tear-off and discard
- More energy consumed due to the need to manufacture replacement materials
Integrating a Roof System Energy Audit into the planning process for every roofing project, whether retrofit or new construction, will ensure that waterproofing performance and "green" innovations work hand-in-hand to optimize energy-saving performance. This same methodology can be extended to include the wall and window systems of to optimize the energy-saving performance of the total building envelope.
John L. Pierson, PE, is the engineering services manager and product manager for metal roofing systems for The Garland Company, Inc., a Cleveland-based manufacturer of high-performance roofing materials. He is a member of Garland s Speaker s Bureau and delivers seminars and AIA-accredited classes on installation techniques and roofing technology. Prior to his work with Garland, he was employed in the construction industry as a field engineer and consultant.