ABSTRACT With an increase in the use of reflective coatings in North America, we are experiencing more and more installations of reflective roofing surfaces in cooler climates as well as warm climates. As a result, observations indicate that there are concerns with subsurface condensation where reflective surfaces are employed. This paper will explore the net effect of vapor drive on reflective roofs and will offer proper system design and product selection remedies, including insulation and the need for an installed vapor control layer. DISCUSSION The benefits of cool roofing are abundant and extend well beyond the thermal impact they can provide to a facility. It is proven that through the use of reflective coatings and membranes, the resulting surface temperature of the roof is dramatically decreased. Reduction of surface temperature and thermal shock are recognized to extend the overall useful life of a roof. Further, the additional layer of protection reduces the UV exposure to the waterproofing system and can eliminate exposure to harmful chemicals, if present. It is also recognized that lower surface temperature reduces cooling costs in the summer. In unconditioned spaces, the comfort of the inhabitants/occupants of the building is increased. How does one put a price on comfort? Finally, studies have indicated that through the use of reflective surfaces, cities can lower the urban heat island effect. This also reduces the net effect of additional cooling needed to lower conditioned temperatures because of retained heat in large urban structures. Simply put, coatings provide benefits in both warm and cool climates (although in cooler climates, the energy savings related to cooling the building may be limited). They extend the life of roof systems and reduce the energy required to cool a facility (Figure 1). However, understanding the net effect that using a reflective product may have in different climates is critical to the long-term success of any given facility. Specifically, problems can arise from cool (reflective) roofing, especially when installed on roof systems that previously had darker surfaces. These problems are observed as drips and unexplained leaks that occur when no evidence of a roof system failure can be found, especially in the spring. It is well understood that dew point 1 8 • I n t e r f a c e J u l y 2 0 1 5 Overcoming Vapor Drive Issues in Cool Roofing Figure 1 – Reflective roof surfaces can provide many benefits, but the roof must be designed and installed properly. By Joseph W. Mellott and Thomas G. Diamond, PE temperature location changes within the roof assembly throughout the year. Vapor drive and dew point temperature locations are directly related to external and internal environmental conditions. Vapor drive can become more severe in the winter with the cooler exterior roof assembly. As a result, higher amounts of moisture can condense within the roof assembly. This is a result of air leakage from the interior of the building into the roof system structure. In cases where a darker roof is installed, the dark roof will have a warmer surface than a comparable light-colored or cool roof. The dark roof will heat up the entire roof assembly and cause downward drying to occur. The cooler roof surface does not allow the roof assembly to become as warm, reducing the drying effect; and, as a result, the subsurface condensation remains and accumulates. CONSTRUCTION TYPES Acceptable and typical construction types vary across the United States. Assemblies in the Midwest and East usually consist of some type of rigid board insulation above a structural deck such as steel or concrete. The South and the Gulf Coast are similar, with the exception that a lightweight concrete pour over a form deck is as common as a steel or concrete deck. West Coast construction typically consists of a wood deck, below which fiberglass insulation with vapor retarder facers are installed. Each system has its own advantages and challenges and is selected for performance, life cycle, and economic reasons. Each type presents different thermal conditions and must be understood and handled correctly to maintain long-term performance. THE PHYSICS Several facts are omnipresent when considering thermal and moisture conditions: • Moisture is always present, in either vapor or liquid form. It is unavoidable. • Water vapor will occur on both the exterior and the interior of a roof assembly. • Warm air rises. • Water vapor, in general, is driven from warm spaces to cold spaces. • Pressure wants to be equalized. High pressure moves to low-pressure areas. W. R. MEADOWS Waterproofing Solutions For a solution based on your needs, visit wrmeadows.com or call 1-800-342-5976. H8YDR3ALA6STIC © W. R. MEADOWS, INC. 2014 follow us: W. R . M E A D O W S HYDRALASTI C 836 is a cold-applied, solvent-free, single-component waterproofing compound for horizontal surfaces, s u c h a s p o d i um d e c k s a n d planters. HYDRALASTIC 836 cures to a tough, flexible, elastic membrane that i s applicable for use on concrete sur faces , including green concrete. The product has a low VOC content and meets all North American VOC regulations. If your need is a f lexible, VOC-compl iant , wa t e r p r o o f i n g memb r a n e , choose HYDRALASTIC 836 from W. R. MEADOWS. J u l y 2 0 1 5 I n t e r f a c e • 1 9 Figure 2 – Careful consideration must be given to transitions such as this. Notice the air barrier tying into the wall. The dew point temperature is the temperature at which water vapor condenses into liquid. Within a roof assembly, water vapor condenses into liquid when it contacts a surface that is at or below this dew point temperature. Understanding these basic principles and terms of physics (and the unavoidable presence of water) is necessary to control reflectance-related condensation. There are several issues that can arise when improper construction meets physics. ISSUES Insulation installed above the roof deck often provides the best opportunity to design a high-performance roof system. Many times, the insulation is attached directly to the deck. While the deck itself can act as a reasonable air barrier, it does not always stop the migration of air and water vapor through the deck (from the interior of the building). This can occur through penetrations in the deck and where the deck meets transitions in the building, such as walls, elevation changes, and differing deck types (Figure 2). Vapor can also migrate through the vapor-retarding facer on the fiberglass batt insulation where it is not perfectly sealed against all penetrations and walls. Inconsistencies in the design with respect to the “air barrier” will allow rising air into the roof assembly, which can transport very large amounts of water vapor. Air movement itself can “transport” over ten times more water vapor than diffusion alone will. Moisture migration (vapor drive) from the interior to the exterior of the building usually occurs in winter months when the interior relative humidity is higher than the drier exterior environment. The interior air is typically warmer than the outside air, compounding the issue. Vapor drive can also be caused by the overall differential in pressure between the interior and the exterior of the building. Wind is an additional factor in the mix. When wind whips across the roof surface, it creates a pressure differential (negative and positive, depending on location). The resultant airflow through the structure can result in the deposition of moisture within the roof assembly. The combination of these factors creates a very strong moisture drive in the winter. If the roof system is not designed with an airtight air barrier, or if a vapor retarder installation is not done properly, large amounts of moisture can become trapped within the roof assembly. The moisture, in vapor form, will loop up until it meets a condensing surface. If the surface is below dew point temperature, such as the bottom of a roof membrane in a cold winter environment, the moisture in the air will condense on it. When the air and moisture loops back down into the roof assembly, it will be in the form of condensed liquid. Another factor to consider is the facility’s use of air conditioning. There are many cases where a reflective roof brought the internal temperature of the building down considerably in the summer. This created a scenario where the air conditioning within the building did not run nearly as often as before. As a result, the interior relative humidity of the building rose considerably, causing condensation within the facility that had not occurred previously. Once this moisture-laden air enters the roof assembly, it will continue to migrate upwards until it reaches the dew point temperature within the insulation at the membrane surface. This can even occur on the bottom of the roof deck in West Coast construction assemblies. By definition, once water vapor hits the dew point temperature, it condenses into liquid. In typical, nonreflective roof assemblies in summer months, some of the moisture within the assembly will be able to “dry out.” The rooftop surface temperature can rise to between 160° and 170° Fahrenheit. At that temperature, it will “reverse the moisture drive” back into the building. The high heat will then drive any moisture trapped in the roof assembly into vapor and equalize into the building. However, when the roof surface temperature is lower (around 110° Fahrenheit), the drying action is significantly reduced. Much of the trapped moisture will start to condense. The condensed moisture can result in many undesirable effects. In many cases, the “leaks” are misdiagnosed as roof leaks. In any case, the trapped moisture can cause degradation of the roofing components and eventually lead to structural damage to the deck. Further, moisture present in the insulation will lower thermal protection, since 2 0 • I n t e r f a c e J u l y 2 0 1 5 ISSUE SUBJECT SUBMISSION DEADLINE October 2015 Technology July 15, 2015 November 2015 Claddings August 14, 2015 December 2015 Extreme occupancies September 15, 2015 January 2016 Steep roofs October 15, 2015 February 2016 Building env. issues (misc.) November 13, 2015 March 20116 Design issues December 15, 2015 Publish in Interface Interface journal is seeking submissions for the following issues. Optimum article size is 2,000 to 3,000 words, containing five to ten graphics. Articles may serve commercial interests but should not promote specific products. Articles on subjects that do not fit any given theme may be submitted at any time. Submit articles or questions to Executive Editor Kristen Ammerman at 800-828-1902 or kammerman@rci-online.org. saturated insulation causes the thermal resistance of insulation (R-value) to plummet to near-zero levels. Finally, moisture in trapped spaces can lead to mold growth. HYGROTHERMAL MODELS Models were performed to analyze the temperature profiles of a roof assembly with a dark surface and a reflective surface. The dew point temperatures were then calculated to see where condensation might occur within the assembly in winter design conditions when the vapor drive is strongest from the interior to the exterior of the building. The temperatures of these “condensing zones” within the roof assembly were then monitored in summer conditions to determine how warm this portion of the roof assembly became. One model analyzed a facility in northern Minnesota with conditioned interior air space that included the following assembly: • Metal deck with the interior surface exposed to the interior of the facility • R-20-equivalent polyisocyanurate insulation board • ½-inch, high-density wood fiberboard • 2 plies of felts adhered in hot asphalt • Dark mineral-surfaced modifiedbituminous cap sheet adhered in hot asphalt A second model was then performed using the same exact assembly, only with a white reflective roof surface. Typical interior conditions were chosen to be 70° Fahrenheit and 20% relative humidity for both models. Results of the study showed that the dew point temperature of 28° Fahrenheit occurred in the middle of the roof assembly for the reflective roof surface in the winter, and near the top of the darker-surface roof assembly. The moisture present in the air will likely not condense exactly at this location, but at the first condensing surface that is colder than the dew point temperature. This likely occurs at the roof’s cover board, or just below the membrane. The moisture-laden air may not actually contact or get to this location in the majority of the roof. However, it can get to this point through any areas of the roof that contain mechanical units, pipes, duct work, or any other areas that allow for disruptions in the roof assembly. Moisture can travel through the roof assembly at these penetrations and may condense as soon as it comes into contact with a condensing surface below the dew point temperature. When these same roof assemblies were analyzed in the summer months, it was determined that the “condensing zone” for the reflective membrane reached an average temperature of approximately 80° Fahrenheit. However, the temperature of the “condensing zone” in the darkersurfaced roof assembly reached an average temperature of 150° Fahrenheit. This difference of 70° provides for a much larger drying potential for the darker roof than the reflective roof, if moisture did accumulate within the assembly. The results of the thermal models are shown in Figures 3A-D. Notice the difference in dew point temperature locations and the temperatures of these locations in summer months. It should be noted that these models are only “snapshots” of the roof assembly. To gain a better understanding of the conditions that a specific roof assembly will experience throughout the entire year, including heat gain and moisture accu- J u l y 2 0 1 5 I n t e r f a c e • 2 1 Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Blah, Why are we still debating the merits of cool roofs? Thermoplastic white roofs have proven performance in all climates. Bust the myths: vinylroofs.org/cool-roofing-myths 2 2 • I n t e r f a c e J u l y 2 0 1 5 Figures 3A-D – The results of solar effects on thermal models in Minneapolis, Minnesota, at different times of the year and with both light and dark roof surfaces. A C B D mulation within it, a more in-depth model should be performed. Doing so will allow one to determine not only if moisture accumulates within the roof assembly, but also if it will have the ability to dry back out. PROPER DESIGN AND PLANNING How can we overcome these potential issues so we can still enjoy the benefits of cool roofing? Of course the solution for new construction is much easier than in reroofing. Existing conditions may be much more expensive to fix due to reconstruction and redesign costs, which is why doing it right the first time is critical. With new construction, the following steps must be considered. Include Air Barriers Always ensure that an effective air barrier is installed above the deck. Redundancies in air barriers are very advantageous. Highperformance building enclosures cannot afford even moderate amounts of air leakage. While staggered insulation boards can act as an air barrier, a membrane installed below the insulation performs better. Continuity is key! Air barriers must be sealed around penetrations and must tie into the wall’s air barriers. Even minor gaps in the air barrier can lead to large quantities of moisture migration, as well as heat transfer into and out of the roof assembly. Include Vapor Retarders/Barriers Vapor barriers are not to be confused with air barriers. Air barriers can be: • Vapor-open • Vapor retarders • Vapor barriers The type of vapor profile needed is highly dependent on the functionality of the building, as well as the climate. Each building should be handled uniquely. Generally, warmer climates can afford (and typically prefer) a vapor retarder or a vapor-open profile. An open vapor profile will allow any moisture that finds its way into the roof assembly to eventually dry out through diffusion. If a moderate amount of moisture finds its way into the assembly via diffusion through the air barrier, it will, in theory, be able to diffuse back out in the summer months. This small amount of moisture is usually not enough to create condensation issues. (See Figure 4.) In more severe climates and in buildings where even moderate amounts of moisture are prohibited (such as museums and data centers), and in buildings where the interior relative humidity levels are extremely high (such as swimming pools or paper mills), a vapor-closed profile is needed. For metal decks, fasten a gypsum board J u l y 2 0 1 5 I n t e r f a c e • 2 3 Figure 4 – Common commercial or industrial roof system in a warm climate with low relative humidity. Conforms to ASTM E1186 Quickly detect air and water leaks in air barriers and roof membranes! FOR BUILDING ENCLOSURES Go to KTAgage.com to see the unit in operation or to make a purchase. 115 15275 KTA. 4 12-788-1300 Single and two-ply membranes Liquid applied membranes & paint Air barriers EPDM roofing sy stems Waterproofing and mor e Kit includes testing unit, standard dome, surfactant and carrying case. Optional test chambers: 12” wall to base, 48” roof seam, and inside and outside corner! to the deck and then install an air/vapor control membrane. Concrete decks require an air/vapor control membrane to be installed directly over the deck. For wood and other nailable decks, mechanically fasten a base sheet to the deck, and then install an air/vapor control membrane directly over the base sheet. Next, install the rigid insulation board above the air/vapor control membrane. The required amount of R-value is dictated by the International Energy Conservation Code, ASHRAE, or local code requirements. Hygrothermal Models Have a design professional perform a hygrothermal model of the building to see how much moisture will be present within the roof assembly and how quickly it will be able to dry out. Material type, thickness, and location can all be varied within the model to determine the optimum roof assembly. Ventilation Ventilation is critical. Ensure that the proper amount of ventilation is provided for the building. Moving moisture-laden air out of the facility or treating it with air conditioning will help reduce the risk of its finding its way into the roof assembly. Application The most critical component is proper application. No matter how well designed the roof system may be, it is dependent on proper installation. Continuity is critical with respect to the integration of the air/ vapor control membrane, as well as with the insulation, roof membrane, and coating. If an air barrier is not sealed perfectly against any and all penetrations, it presents a point where air can seep into the roof assembly. Ensure that highly skilled contractors are installing the roof and its components. Also consider having a Registered Roof Observer present during the installation to ensure that the roof system is being installed as is intended. Reroofing existing buildings can be more challenging. Adding ventilation to a building is typically not very cost-effective. Also, if insulation is already present below the roof deck, installing an air barrier/vapor retarder below it can be nearly impossible to do properly, depending on the ceiling finish. In these scenarios, adding additional insulation above the roof deck is almost always beneficial. Even a moderate amount of R-value added above the roof deck will bring the deck’s surface temperature above the dew point temperature, moving the dew point above the first condensing surface. In any case, it is recommended to perform a hygrothermal model on the building to see how the roof assembly will perform with different materials and insulation thicknesses. CONCLUSION Simply put, reflective roofing can be very beneficial. Reducing cooling-based energy demand and protecting the waterproofing substrate are valuable performance characteristics of coatings and membranes. Understanding how thermal changes to rooftop conditions can affect moisture drive is critical in building roofs to perform to and beyond their life cycle expectation. When in doubt, perform the necessary calculations with the assistance of trained professionals to protect assets and to eliminate failure. 2 4 • I n t e r f a c e J u l y 2 0 1 5 Joe Mellott is VP of Innovative Metal Company, Inc. (IMETCO). He holds multiple patents for roofrelated innovations and received the 2006 Industry S t a t e s m a n Award from the Roof Coatings Manufacturers Association (RCMA) for his work in advancing roof coatings technology. He has also served on the board of the Cool Roofs Rating Council (CRRC) and is a past president and technical chair of RCMA. Mellott has been a member of RCI, NRCA, and ARMA. A graduate of Case Western Reserve University, Joe holds a BS in polymer engineering. Joe Mellott Tom Diamond, PE, is a product engineer for The Garland Company, Inc., a manufacturer and distributor of building enclosure solutions for commercial and industrial applications. Tom’s primary focus involves designing optimum performance roof and wall systems while assuring compliance with building code. He frequently delivers seminars and AIAapproved classes on installation techniques, building enclosure design, and roofing and wall system technology. Tom Diamond, PE Pinnacle Roofing Contractors, Inc., Jacksonville, Florida, has been fined $154,000 stemming from a 24-foot fall that killed John W. Miles III. Miles—working without fall protection—fell through an unprotected skylight in a warehouse roof in November 2014, according to the citation by the Occupational Safety and Health Administration (OSHA). Pinnacle Roofing, founded in 1994, had been cited for a serious safety hazard relating to fall protection in July 2012. In the current case, Pinnacle allegedly allowed employees to work at heights greater than 6 ft. without guardrails or fall protection and had not installed protective systems on the skylights. Citations were also issued for failing to ensure the edge of the roof was marked. Roofer Fined for Skylight Death
