Figure 1: Denver International Airport.

ABSTRACT 4. The selection process for the new roof membrane, As a result of a moderate hailstorm in June of 2001, recover board, and roof coating; and the original eight-year-old reinforced PVC single-ply 5. Design parameters of the project. membrane on the flat roofs over the Jeppesen Terminal and passenger bridge at the Denver International Airport For this project, the fully-adhered, 90-mil EPDM roof (DIA) experienced moderate to extensive hail damage. was determined to be the least intrusive and the best The subject roofs are adjacent to the impressive and system to provide adequate resistance to hail and renowned Bird Air tent roofs, which were not damaged by mechanical damage. The designers and owners expect the hail. After careful consideration of many different the new EPDM roof to provide excellent service in a hail roof systems, the roof membrane was replaced with a hazard zone and achieve a useful life that far exceeds the non-reinforced, fully-adhered, 90-mil ethylene propylene norm. diene monomer (EPDM) that was painted with a white elastomeric roof coating. The evaluation, design, and Client: City and County of Denver construction of the new roof presented the owner, design Architect: DMJM H+N, Denver, CO team, and contractor with some unique challenges and Roof consultant: CyberCon Engineering, Inc., experiences that are shared in this case study. The top­ Centennial, CO ics that will be discussed are: Roofing contractor: Earl F. Douglass Roofing, 1. Logistics of reroofing an aviation facility and the Commerce City, CO security challenges of post 9/11/2001, Product supplier, roofing membrane: Firestone 2. Emergency measures that were taken to prevent Building Products Corp. further damage to the roof and building compo­ Product supplier, coverboard: Georgia Pacific nents prior to and during construction, Product supplier, premanufactured coping: Peterson 3. Results of the evaluation of the roof systems and Aluminum existing conditions,

16 • INTERFACE S EPTEMBER 2005 INTRODUCTION • Vapor diffusion and degradation of This article will discuss the results of polyisocyanurate. CyberCon’s initial investigation of the roofs • Thermal profile of black EPDM at the DIA, design considerations for reroof­ membrane versus a high albedo ing, and some of the challenges that were coating. encountered during the reroofing process. • Failure of butyl-based glued seams This paper will also discuss the characteris­ on an elastomeric membrane. tics, physical properties, and some of the pitfalls of the roofing products used and the In the final analysis, it was determined rationale for selecting an elastomeric ther­ that a fully-adhered EPDM membrane, moset roofing membrane and dense, water coated white, installed over a glass-faced resistant coverboard for reroofing. For most gypsum coverboard, mechanically fastened discerning readers, the information provid­ over the existing polystyrene and polyiso­ ed will be a quick refresher on polymer cyanurate rigid insulation, would be the chemistry, mechanics of materials, mois­ best reroof option for the DIA. The new roof ture vapor transmission, thermal proper­ has passed severe hail and wind tests and ties, and sound roofing practice. For the has excellent weathering characteristics. It roof consultant in training, it is hoped that is anticipated that the roof coating will the information provided will inspire further weather away at approximately 1 mil per study of the topics discussed. year and therefore have to be reapplied in DIA is a commercial air carrier facility approximately 12 to 15 years. The service­ 23 miles northeast of the metropolitan able life of the roof should extend well Denver area, on 34,000 acres of the high beyond 30 years. The life cycle cost was mountain desert prairie of Colorado. The deemed to be comparable to other mem­ airport has six active runways and handles brane systems, despite the anticipated cost approximately 104,000 enplanements each of recoating the roof. The rationale for day. The terminal and concourse facilities selecting an EPDM roof system will also be represent a total of 5.45 million square feet discussed in this article. with a total of 94 passenger-loading gates. The airport structures consist of a main ter­ DEFINITIONS minal and three remote concourse buildings Absolute Humidity – A measure of the (A, B, and C), which are connected via an actual amount of water vapor contained in underground automated transportation a unit volume of air; distinct from “relative system. Concourse A is connected to the humidity,” which is the ratio of air’s North terminal via a pedestrian bridge. absolute humidity to the air’s water vapor An international airport presents chal­ holding capacity.9 lenges that are unique due to its 24/7 oper­ Diffusion – The process whereby water ations, ongoing public activity, security, vapor or gases migrate through permeable and the sensitive nature of the airplanes. membranes or partitions by osmosis. Gases Special considerations are necessary for always migrate from regions of high concen­ reroofing to address noise, odors, and risks trations to regions of low concentrations to the operations of the facility. until equilibrium is reached. Our findings and conclusions from the Elastomer – A macromolecular material investigation of the original roof systems that, at room temperature, returns rapidly may also be of interest to the reader. The to approximately its initial dimensions and following will be discussed in more detail in shape after substantial deformation by a the body of this article: weak stress and release of the stress.4 • Hail damage to weathered reinforce­ Glass Transition Temperature – The ment PVC membranes. temperature at which a polymer becomes • PVC degradation as a function of its brittle, and above a certain point, the poly­ environment and raw materials. mer is deemed prone to failure.2 • Designing resistance to mechanical Plasticizer – An important component damage of a membrane. in the formulation of PVC membranes to • Long-term serviceability of a PVC give them flexibility to withstand elongation, membrane. strain, and thermally-induced stresses nor­ • Slip sheet damage below mechani­ mally experienced by a roof system. Most cally fastened single-ply mem­ plasticizers are esters of phthalic acid. The branes. molecular weight and compounds vary from • Premature plasticizer migration. one manufacturer to the next. The plasticiz­ • Heat degradation of polystyrene. er content in a new membrane is around

S EPTEMBER 2005 I NTERFACE • 17 PROBLEMS WITH THE EXISTING ROOF SYSTEMS

SYSTEM NO. EXISTING CONDITIONS: OBSERVATIONS: 1. Low-slope Deck: Structural concrete (sloped at 1/4"/ft.) • Extensive hail damage and mechanical damage areas of Insulation: 4" of extruded polystyrene (XEPS) from icicles falling off of tent roof terminal, east Slip Sheet: Manninglass felt • Plasticizer loss and west of Membrane: 60-mil reinforced mechanically-attached PVC • Damaged slip sheet the tent roof • Thermal degradation of PVC • Isolated heat degradation of polystyrene • Minor corrosion of fasteners • Consolidation of insulation in high-traffic areas 2. Low-slope Deck: Structural concrete (flat) • Extensive hail damage roofs on North Insulation: Tapered XEPS 4.5" start, (sloped at approximately • Plasticizer loss Terminal and 1/4"/ft.), total maximum thickness 16" • Damaged slip sheet Customs areas Slip Sheet: Manninglass felt • Thermal degradation of PVC Membrane: 60-mil reinforced mechanically-attached PVC • Isolated heat degradation of polystyrene 3. Mechanical Deck: Metal (sloped at 1/4"/ft.) • Minor hail damage and elevator Barrier Board: 5/8" Type X gypsum • Plasticizer loss penthouses at Insulation: 2 layers of XEPS, 1" and 3" for a total thickness of 4" • Damaged slip sheet terminal Slip Sheet: Manninglass felt • Thermal degradation of PVC Membrane: 60-mil mechanically-attached PVC 4. Steep-slope Deck: Structural concrete (sloped at 3"/ft.) • Extensive hail damage areas of Insulation: 2.6" polyisocyanurate • Plasticizer loss Terminal East Membrane: 60-mil mechanically-attached PVC • Thermal degradation of PVC and West • Minor corrosion of fasteners • Minor facer delamination 5. Passenger Deck: Metal (sloped 0" to 3"/ft.) with fireproofing on bottom • Extensive hail damage bridge (slope side • Plasticizer loss 0" to 1"/ft.) Barrier Board: 5/8" Type X gypsum • Thermal degradation of PVC and barrel roof Insulation: 2.6" of polyisocyanurate over Customs Slip Sheet: Manninglass (0" to 3"/ft.) Membrane: 60-mil mechanically-attached PVC 6. Airport Deck: Structural Concrete (sloped at 1/4"/ft.) • Severe buckles and deformation of Operations Insulation: 2.6" polysiocyanurate mechanically attached polyisocyanurate Building (AOB) Membrane: Fully-adhered .060" un-reinforced EPDM • Severe facer delamination above 10th • Degradation of insulation cell structure (1/4" to floor offices 1/2" of the top layer turned brown and friable). • Extensive failure of adhesive seams • Corrosion of fasteners 7. AOB Deck: Metal (sloped at 1/4"/ft.) fireproofing on bottom side • Deterioration of lap sealant Penthouse Insulation: 2.6" polysiocyanurate, mechanically attached The rigid insulation was deemed salvageable Membrane: Fully-adhered .060" unreinforced EPDM 8. AOB 6th floor Deck: Structural concrete (flat) • Buckles and deformation of polyisocyanurate offices Insulation: Tapered polyisocyanurate (4" start, sloped at • Isolated facer delamination approximately 1/4"/ft.) • Degradation of insulation cell structure (1/4" to Membrane: Fully-adhered .060" unreinforced EPDM 1/2" of the top layer turned brown and friable). • Failure of adhesive seams • Minor corrosion of fasteners Table 1: Problems with existing roof systems.

36%. A loss of 10 to 12 percent can result in Thermoplastic – A material that BACKGROUND premature failure of the roof system. Low becomes plastic or viscous when heated Construction of DIA molecular weight plasticizers are more and semi-rigid when cooled. Asphalt and The construction of the terminal and volatile than those that have higher molec­ PVC are classic thermoplastic materials. Airport Operations Building (AOB) was ular weight.1 completed in 1991 and the passenger

18 • INTERFACE S EPTEMBER 2005 ed white to prevent thermal degra­ have greatly improved the dimensional sta­ dation of the polystyrene insulation bility of the original non-reinforced prod­ below. ucts. The membrane can be manufactured Frequent inspections were per­ white or tinted in various colors with pig­ formed to identify new cracks and ments, which is an advantage when solar to effect repairs to prevent water reflectivity or an architectural statement is infiltration through the existing desired. The heat welding of the seams, single-ply membrane. Fortunate­ using hot air welders, is also fairly consis­ ly, the re-roof project was per­ tent and reliable. formed during a drought, which PVC copolymers in their normal state was recorded as the driest period are rigid, brittle, and easily shatter under in the previous 50 years. stress. Plasticizers, fillers, pigments, pro­ The original roof system cessing oils, biocides, and stabilizers are specified and installed on the compounded with the PVC resins to impart vast majority of the buildings at the desired physical properties to the mem­ DIA was an off-white, 60-mil, brane. Plasticizers are used to give the sheet Figure 2: polyester scrim reinforced, flexibility and suppleness over a wide range Hail-damaged membrane. mechanically-fastened PVC single-ply mem­ of temperatures. Stabilizer packages are brane, which was produced using the cal­ introduced to provide resistance to thermal bridge was completed in 1993. The roofs on endaring process. PVC is a thermoplastic and photochemical degradation. the terminal, AOB, and passenger bridge material, which means that it becomes soft­ Plasticizers begin to migrate out of the were replaced in 2002. Eight different roof er (or more plastic) with heat and semi rigid sheet and the chemical bonds begin to systems were identified. The following is a with cold. PVC is known for its resistance to change as soon as the membrane leaves the brief description of the roof system compo­ acids, alkalis, and many other chemicals, production line. This phenomenon, known nents and the type of problems noted. as well as its self-extinguishing properties as weathering, is normally a very slow and when subjected to fire. It is soluble in cer­ gradual process, which under favorable DISCUSSION tain solvents, such as tetrahydrofuran, conditions, allows a PVC membrane to pro­ Thermoplastic PVC Membranes which is used in PVC cements.3 vide a useful life commensurate with other As early as 1999, only eight years after One of the main reasons PVC was used conventional roof systems. The glass transi­ the original roof was installed, the PVC on the airport is that it was perceived that tion temperature and specific gravity are roofs on DIA began showing signs of degra­ the membrane might be exposed to jet fuel. increased and shrinkage of the membrane dation and premature plasticizer loss. It was also one of the few membranes on the occurs as a result of thermal degradation Melting of the extruded polystyrene rigid market that had a UL fire rating for all roof and plasticizer loss. Exposure to heat, insulation was also noted, particularly in slopes encountered ultraviolet radiation, and various absorbent super insulated areas with southern expo­ at the airport. The sure to the sun. In June of 2001, a moder­ membrane, with its ate hailstorm hit the airport, causing exten­ heat-welded and sive damage to the PVC membrane on the solvent seams, was terminal. The membrane did not shatter also touted to be (unlike some of the first-generation PVC superior to other roof systems); however, concentric cracks conventional roof were noted in the membrane at points of systems, despite hail impacts. In most cases, the cracks ran the lack of a long- completely through the membrane, render­ term performance ing the roof system unsuitable as a water­ history. proofing system. In some instances, the Much has cracks emanated from the underside of the been learned single-ply membrane, but did not manifest about thermo­ Figure 3: on the top or exposed side of the membrane. plastic PVC sin- Patches The damage noted was more extensive gle-ply mem­ from hail on the darker discolored sheets used to con­ branes since damage. struct the PVC membrane, (which appar­ they were introduced in the ently had different run numbers), and in U.S. from Europe in the late 1970s. The compounds areas that were exposed to more solar radi­ non-reinforced sheet is no longer produced will accelerate the degradation of PVC. ation. New cracks developed in the mem­ or marketed due to the shattering phenom­ Thermal degradation is manifested by a brane at hail impacts that were not notice­ enon and hail damage that occurred with brown color, as the membrane goes through able during the first visual inspection. The the product. All of the sheet goods market­ dehydrochloronation of the polymer.2 visible cracks were repaired immediately ed and sold in the U.S. today (except for Plasticizer loss occurs by migration, evapo­ with EPDM primer and EPDM peel-and­ flashing materials) are reinforced with poly­ ration, washout, hydrolosis, and exposure stick flashing tape. The repairs were paint­ ester or fiberglass mats or scrims, which to microbes.1

20 • INTERFACE S EPTEMBER 2005 Polystyrene rigid insulation (which was damage to reinforced thermoplastic mem­ since cracks may develop but not penetrate used extensively on the airport) has an branes can also be attributed to the same completely through the membrane. affinity for PVC plasticizers; therefore, the changes in physical properties; however, Impact resistance of the membrane can two roof components must be adequately the mat or scrim prevents a weathered be improved by providing a denser sub­ divorced for the duration of the roof’s useful membrane from shattering into thousands strate below the membrane, which resists life. PVC membranes should also be ade­ of smaller pieces. Isolated cracks through deformation and elongation, thus minimiz­ quately divorced from asphalt-contaminat­ the membrane, generally in the form of con­ ing the strain energy imparted to the mem­ ed surfaces for the same reason. centric circles, are usually noted after a hail brane. Bridged or unsupported areas are As PVC weathers, it loses some of its event or impact, which compromises the still vulnerable to damage. elasticity and flexibility. It has been demon­ watertight integrity of the membrane. The The long-term serviceability of a PVC strated that new PVC membranes will gen­ kinetic energy of a missile is converted to roof membrane is a function of its dimen­ erally produce a tensile load of 4-6 pounds strain energy in the membrane, compressive sional stability and flexibility throughout per inch when subjected to a 100°F temper­ yield stress in the substrate, disintegration the temperature range it will experience ature drop. As the membrane weathers and of the missile, and momentum after impact. during its anticipated serviceable life. The becomes more brittle, the thermal load pro­ Hailstorms are usually preceded by a stability of these physical properties is a file can increase fourfold to 20 plus pounds sudden drop in temperature, which, as dis­ function of the raw materials, manufactur­ per inch.1 It has been suggested that the cussed above, causes the membrane to ing process, and environment to which the shrinkage and thermal load profile of a PVC shrink, resulting in thermally induced loads roof system is exposed. membrane can be affected by the manufac­ in the material. At the moment of impact, turing process and raw materials used in the membrane begins to deform, and, as Polystyrene Rigid Insulation the products.2 The extent of the effects from with any material “under load,” develops The vast majority of insulation used on manufacturing is beyond the scope of this additional tensile stresses on the underside the original roof at the airport was extruded article. and compressive stresses on the top side of polystyrene, which ranged in thickness The shattering phenomenon that the membrane. If the strain exceeds the from 4" (R-20 on structurally sloped areas) occurred with the first generation PVC non- elastic limit of the material at the time of to 14" (R-20 to R-70 on the flat decks with reinforced membranes can be attributed to impact, cracks begin to emanate from the tapered insulation). Most of the insulation plasticizer loss, shrinkage, lack of flexibility, bottom side toward the top surface. An board was deemed salvageable, which rep­ and increased thermal loads during sudden underside view of the membrane is needed resented a sizeable investment by the drops in temperature. Hail and impact to ascertain the full extent of hail damage, owner, the city, and the county of Denver.

S EPTEMBER 2005 I NTERFACE • 21 extruded polystyrene rigid or exceeded 100˚F, some severe deformation insulation boards had melted in the underlying polystyrene rigid insula­ or deformed from heat, particu­ tion occurred in isolated areas (next to larly where the slip sheet was intersecting clerestory windows). The sur­ damaged or missing, and in face temperature of the black EPDM mem­ areas next to the reflective brane was measured at 180˚F at approxi­ clerestory windows (refer to mately 1:30 p.m. on a hot summer day. The Figure 1 for view of window membrane in these areas, next to the reflec­ areas). To melt extruded poly­ tive windows, received much more solar styrene, the temperature needs radiation than other areas of the roof. As to reach 165˚F or greater, soon as the high-albedo, white roof coating according to the manufacturer’s was installed, the surface temperature published literature. dropped significantly to approximately The ambient air temperature 105˚F, which prevented any further damage directly above the membrane was to the top layer of the polystyrene rigid noted to be 95˚F on a hot summer insulation board. day. The melting indicates that the temperature between the Polyisocyanurate Rigid Insulation Figure 4: Melted XEPS. membrane and the polystyrene Polyisocyanurate rigid insulation (PRI) exceeded the air temperature by 60˚F to with a permeable organic facer was used on The new roof system had to be compatible 70˚F, which is incongruous with static ther­ some areas of the airport. When the roof with the existing polystyrene rigid insula­ mo profiles of building envelopes that have was installed, hydrochlorofluorocarbons tion. A new, ballasted, single-ply membrane been presented in the past. The slip sheet (HCFCs) were used as blowing agents for over a coverboard would have been an eco­ protected the polystyrene somewhat, but the plastic foam insulation boards. Various nomical choice on any other project. The did not prevent deformation of the insula­ roof systems, using this product, were owner’s choice to eliminate any roof system tion board. Insulation boards that were encountered: with loose rocks, which could become mis­ severely deformed or melted were replaced • Mechanically-fastened reinforced siles during high winds, was respected and with like kind. PVC membrane over a single uni­ other reroof options were considered. A single layer of 5/8" glass-faced gyp­ form layer of PRI over a structurally Extruded polystyrene is a plastic foam, sum board with a primed surface was sloped concrete deck. closed-cell rigid insulation with a fluorocar­ installed over the existing polystyrene rigid • Mechanically-fastened reinforced bon blowing agent. The polymer that makes insulation (secured with mechanical fasten­ PVC membrane over a single uni­ up the cell walls is relatively impermeable to ers and Galvalume™ metal plates) to pro­ form layer of PRI over a structurally water vapor, which renders the product vide a dense, hail-resistant substrate for the sloped metal deck. useful in areas that may become exposed to new membrane and divorce the polystyrene • Fully-adhered non-reinforced EPDM moisture, (i.e., foundation walls, roofs, wall from the elevated temperatures that were membrane over tapered PRI boards, cavities, protected membrane roof assem­ anticipated with the new black EPDM mem­ mechanically fastened over a flat blies, etc). It has a fairly stable aged R-value brane. A couple of isolated areas, near concrete deck. of 5.0 per inch. clerestory reflective windows, experienced Extruded polystyrene is considered a some heat degrada­ sustainable roofing product and can be sal­ tion and had to be vaged for subsequent membrane replace­ repaired (Figure 4). ments, under favorable conditions. The The new black product has a published service tempera­ EPDM membrane ture range of -100 to 165˚F; therefore, it was painted white to must be protected from temperatures above lower the surface this range. This can be accomplished with temperature of the slip sheets, coverboards, and reflective roof during the hot membranes or coatings. Polystyrene will summer months, disintegrate when exposed to various sol­ which will prevent the vents, including membrane adhesives, cold polystyrene from process cements, and other petroleum- melting. During re­ based chemicals. A limited number of adhe­ roofing (after the new sives are suitable for laminating polystyrene black 90-mil EPDM boards to one another and to other prod­ membrane was in­ ucts. Solvent-free, rubberized emulsion stalled, and prior to the adhesives have been developed that are application of the white compatible with polystyrene. elastomeric coating), When the existing membrane was and when the ambient Figure 5: removed, it was noted that some of the air temperature reached Damaged oxidized polyisocyanurate.

22 • INTERFACE S EPTEMBER 2005 • Fully-adhered, non-reinforced EPDM Polyisocyanurate foam is a cellular plas­ product becomes dimensionally unstable: membrane over a single uniform tic insulation that entraps a gaseous blow­ (5 to 20% dimensional changes in each layer of PRI, mechanically fastened ing agent within microscopic cells of a poly­ direction and 15 to 60 percent change in over a structurally-sloped concrete mer matrix. The cells of the foam are pri­ volume can occur).1 deck. marily closed, and due to the low thermal Over wet substrates, weathering, mois­ • Fully-adhered non-reinforced EPDM conductivity of the entrapped gas, the foam ture gain, and dimensional changes in poly­ membrane over a single uniform is an effective insulator. The cell walls are isocyanurate foam insulation products tend layer of PRI, mechanically fastened permeable and, with time, the blowing to be more pronounced with dark roofs, due over a metal deck. agent(s) can escape from the cells while to the elevated temperatures. The designer

ambient gases such as N2, O2, and H2O or installer of a roof system should properly On the concrete deck areas, several vapor diffuse relatively quickly through the evaluate the presence of moisture. Non­ problems (moderate to severe) were noted cell walls and permeate the foam. The destructive moisture evaluations are highly that were not evident on the metal deck resulting change in the gas composition of recommended for recovers where trapped areas. The organic facer was deteriorated, the foam over time, and the corresponding moisture is probable. Cementitious decks the membrane was delaminated from the decrease in its insulating capability, are should be checked for moisture content if a facer, the insulation board was buckled in a referred to as “aging” of the foam.8 vapor retarder is not used or specified. On convex fashion, the top 1/4" layer of the Studies have shown that in the pres­ new construction, vapor retarders are rec­ board was discolored and friable, the insu­ ence of heat and moisture, the aging ommended over concrete decks to prevent lation fasteners and plates were corroded, process of plastic foam insulation products the residual moisture from affecting the roof and the glued EPDM seams were delami­ is accelerated and the physical properties – system components. Studies are being done nated (refer to Figure 5). Each one of these thermal conductivity, compressive strength, on the drying effects of loose-laid, pressure- problems can be attributed to the effects of density, coefficient of linear expansion, and equalized venting single-ply roof systems. moisture. Residual water from the poured­ vapor permeability – are affected substan­ The long-term viability is still being in-place concrete was the probable source tially above 120˚F (49˚C). Under dry condi­ researched.11 of the initial moisture, causing the EPDM tions, polyisocyanurate foam insulation is Water vapor always diffuses from seams to fail within a relatively short time, dimensionally stable (<5% change in dimen­ regions of high absolute humidity to regions which precipitated the damage to the insu­ sion and <15% by volume) up to 220˚F of low absolute humidity. The greater the lation board and fasteners. More water (104˚C). At temperatures above 120˚F (49˚C) difference in absolute humidity across a entered the roof system via open seams, and a relative humidity above 90%, the permeable structure, the faster the rate of which exacerbated the deleteri­ ous effects of moisture on the roofing components. There was evidence that patches and repairs were made when leaks occurred, but the residual moisture was probably not removed. The facer is a critical and integral part of polyisocyanu­ rate rigid insulation products. It provides dimensional stability and fire resistance to the insu­ lation board and a means to adhere a roof membrane to an otherwise unstable and friable material. If organic facers become wet, the fibers expand, allowing the dimensionally unstable plastic foam board to expand or contract, resulting in “bowing” or “cupping” of the board. When the cellulosic facer dries, it shrinks, imparting stress on the board. If the roof system goes through enough wetting and drying cycles, the structural integrity of the facer becomes compromised and a fully-adhered membrane will eventually become disbonded. Figure 6: Psychrometric chart, courtesy of RIEI.

24 • INTERFACE S EPTEMBER 2005 YEAR OF INTRO: TECHNOLOGY IMPROVEMENTS

1985 - 1986 Butyl-based splice adhesive replaces water- and heat- sensitive Neoprene-based adhesives.

1985 - 1986 EPDM-based wall flashings replace heat- and ozone-sensitive Neoprene-based flashings.

1987 - 1988 Tape laminates replace adhesive seams at roof edges and battens.

1988 - 1989 Metal battens and screw fasteners replace treated wood nailers and nails at base tie-ins.

1991 - 1992 Reinforced perimeter fastening strips introduced as an alternative to metal battens for base tie-ins.

1992 - 1993 Seam tape with high solids primer replaces seam adhesive.

Table 2 diffusion. Thickness of a material also af­ would not explain the brown discoloration fects the permeability or rate of diffusion. In of the top surface of the insulation board most building situations, warm air tends to (refer to Figure 5), which is usually indica­ have a higher absolute humidity than cool­ tive of photochemical oxidation. The sol­ er air. This gives rise to the adage, “water vents in the bonding adhesive, in concert vapor goes from hot to cold.” This is not with moisture and heat, may have reacted necessarily true for buildings with unusual­ with the polymer matrix of the insulation ly high or low interior humidity or buildings board. Further study is needed. This dem­ with wet or moisture-laden components. onstrates the need for a vapor retarder over Water vapor migration is usually not a con­ freshly-poured (moisture-laden) concrete cern until the gaseous molecules reach the decks. dewpoint temperature and condense into liquid water.10 Elastomeric EPDM Single-ply Membranes Foam insulation products can become EPDM is an elastomeric thermoset poly­ wet by vapor diffusion followed by conden­ mer synthesized from ethylene propylene sation. Water accumulates in the foam and a small proportion of a diene monomer when vapor drive acts in concert with ther­ with rubber-like or elastic properties. The mal gradients and the vapor is restricted on black membrane, which has carbon black the cold side of the building envelope. as a UV inhibitor, has excellent weathering Moisture accumulation is reversible, which characteristics. EPDM is not resistant to means foam insulation can eventually dry petroleum oils and gasoline.3 The mem­ out, if allowed to, by vapor diffusion. brane will swell and soften when exposed to On the airport project, evidence shows these chemicals. Other than silicone, EPDM that the permeability of the EPDM mem­ has the best service temperature range of brane was not sufficient to allow adequate any elastomeric membrane on the market amounts of water vapor to escape to the (-65 to 300˚F). A non-reinforced membrane atmosphere. The polyisocyanurate rigid has a tensile strength of 1400 lbf/in2 and an insulation and facer at the DIA project expe­ ultimate elongation of 300%. EPDM also rienced numerous cycles of wetting and performs very well under heat and exposure drying as a result of the residual moisture to ozone. EPDM membranes have a proven trapped between the deck and black EPDM track record. Due to improved product tech­ membrane. The black membrane reached nology and superior details, problems with temperatures of 180˚F in the summer, shrinkage and failed seams have greatly which increased the thermal gradient, va­ diminished since the product was first por pressure, and therefore, the vapor dif­ introduced to the roofing market more than fusion rate, thereby accelerating the degra­ 25 years ago.12 dation of the foam insulation and facer on It was decided that a black, 90-mil, non- the top side of the boards. This, however, reinforced EPDM membrane would be

S EPTEMBER 2005 I NTERFACE • 25 ings from heat and UV exposure, ROOF REPLACEMENT PROS AND CONS extending service life. OPTIONS • Greatly reduces the temperature gradient of the roof, which lowers Built-up roof Rejected because of asphalt fumes, loose rock, and potential the relative humidity and vapor for melting existing polystyrene. drive within the roof system compo­ nents. SBS modified, hot- Rejected because of asphalt or solvent fumes and potential • Provides an aesthetic appearance to or cold-applied for melting existing polystyrene. the roof.

APP modified, torch Rejected because of risk of open flames and the negative It is anticipated that the coating will or cold-applied effect of solvent fumes on polystyrene. weather at the rate of 1 mil per year. This gradual chalking process is what keeps the PVC Rejected because of past experience at airport. roof white and highly reflective for the life of the coating. It is anticipated that the roof TPO Rejected because of limited performance history. may need to be recoated at least once dur­ ing its useful life. The cost of the coating is EPDM ballasted Rejected because of loose rock or pavers. offset by the savings achieved from the extension of the useful life and reduced EPDM mechanically Rejected because of dynamic forces on membrane and energy consumption during the summer to attached fasteners and noise from the extensive use of fasteners. condition the air in the building. The peel strength and adhesion of EPDM fully-adhered Accepted because of durability, low life cycle cost, excellent water-based acrylic roof coatings have resistance to wind and hail, and minimal intrusion on improved greatly since primers or “pre­ facility operations. washes” were introduced. Practically all manufacturers of roof coatings and EPDM Table 3 membranes offer white coating specification and warranties. The prewash was lightly sprayed on the surface of the finished EPDM roof and then rinsed off with water. installed over a layer of 5/8" glass-faced hard board. The fully-adhered EPDM over After the membrane dried, the water-based gypsum board, which was mechanically fas­ 5/8" glass-faced gypsum board has passed coating was applied with rollers. To avoid tened over the existing salvaged polystyrene the most severe hail tests and has received overspray, a spray apparatus was not used. rigid insulation. The only drawback per­ the highest ratings from United Laborator­ Water-based acrylic coatings perform ceived with the EPDM roof on the airport ies (UL), the National Institute of Standards best on roofs with adequate slope and was its black color, which gets hot during and Technology (NIST), and Factory Mutual drainage. Ponding water will cause the coat­ the summer months from solar radiation. (FM) for hail resistance. A 2" hail warranty ing to swell and peel away from the mem­ The temperatures would get hot enough to was issued by the roofing manufacturer. brane. When exposed to standing water for cause deformation of the existing poly­ The membrane installation was rather prolonged periods, moisture will be styrene insulation board. innocuous, but some precautions were absorbed into the coating, causing dimen­ White EPDM, which has a titanium needed to avoid adhesive fumes from being sional changes. This induces shear stress at dioxide UV inhibitor, was rejected because drawn into the building through the fresh the adhesive bond. If the water absorbed the product does not have the same weath­ air intakes of the mechanical air handling into the coating freezes, the coating will ering properties as black EPDM, which uses system. Plywood chutes eight feet high were break down and eventually delaminate from carbon black. It was decided to coat the roof built around the fresh air intakes and were the substrate. with approximately 15 mils of a high albedo very effective. white acrylic elastomeric roof coating, after ROOF SYSTEM SELECTION PROCESS washing and priming the membrane. Prior High Albedo White Roof Coating Owner’s Concerns to coating, seams of the EPDM sheets were A white roof coating was specified and As one can imagine, the facility manage­ mated with 3"-wide butyl seam tape and a installed to lower the surface temperature ment and engineering staff had several con­ high-solids, butyl-based primer. The seams of the roof in order to prevent the poly­ cerns and expectations with regard to the were then covered with a 5"-wide, butyl- styrene rigid insulation from heat degrada­ new roof systems. With the exception of one backed, self-curing EPDM flashing tape. tion. The coating also provides the following roof area (System No. 7), the original roofs This seaming method provided redundancy benefits: did not perform as anticipated and had to and ensured that any small pinholes – par­ • Encapsulates all seams and edges of be replaced prematurely due to hail damage ticularly at tee joints – were sealed water­ the membrane and flashing materi­ and degradation of the physical properties tight for the serviceable life of the roof mem­ als, thereby providing additional of the polyisocyanurate rigid insulation and brane. redundancy in the waterproofing PVC roof membranes. The manufacturer’s The hail and puncture resistance are system. roof guarantee did not cover hail damage, excellent with EPDM when installed over a • Protects the membrane and flash­ and the manufacturer would not participate

26 • INTERFACE S EPTEMBER 2005 in replacing the roof because of the change PROJECT CHALLENGES ber. Over 4,000,000 square feet of roofing in the membrane’s physical properties and The first and foremost challenge was had to be installed in six months. likelihood of further hail damage. Because selecting a roof system that would be as Return air intakes were located near the of the premature degradation of the PVC unintrusive and risk free as possible for the roof level, which presented a potential prob­ membrane, foot traffic on the roofs had to facility operations and customers of the air­ lem with offensive solvent fumes being be limited to times when the outdoor ambi­ port. The transfer of materials on and off of drawn into the air handling units and dis­ ent temperature was 40°F or above. The the roof had to be minimized and the roof­ persing them into the occupied building. design team had to come up with a roof that ing products had to be rather innocuous. Plywood chutes were constructed around would address the following issues: By salvaging the majority of the existing the intakes and anchored with tie-backs to • Heat Aging: The high R-values insulation board, the owner enjoyed cost resist wind pressures. This precaution was (super insulated roof at apex of savings as well as reducing the risk of deemed effective, and no complaints were tapered insulation system) and the debris blowing onto the tarmac. received with regard to the solvent fumes reflective glass on the clerestory Security issues were paramount, espe­ (refer to Figure 7). windows places the membrane in an cially after the September 11, 2001, terror­ environment with abnormally high ist attack on the U.S. The contractor had to CONCLUSIONS heat and solar radiation. The new provide a sufficient number of laborers who Weathered reinforced PVCs are subject membrane must have good heat could pass the stringent screening methods to hail damage. In hail zone areas, design­ aging properties. imposed by the government. Even moving ers should consider a hard coverboard that tools onto the jobsite was cumbersome. offers more resistance to impact deforma­ • Sustainable: The new roof system Personnel on the roof had to be badged and tion and lowers the strain energy imparted must be compatible with the exist­ escorted by qualified persons. to the membrane. ing polystyrene insulation boards. A Staging was limited and difficult. The Polystyrene rigid insulation is subject to large investment had already been airport officials required that staging plans heat degradation, even when a light-colored made with the thermal insulation be submitted and approved weeks in membrane and slip sheet are provided, par­ and tapered system. advance. Most of the material and debris ticularly when exposed to extraordinary handling was done at night. Underground solar radiation that occurs with reflective • Low Meltpoint of XEPS: The tem­ tunnels and buildings precluded crane clerestory windows. perature of the membrane must setups to specific areas. remain well below the melt point of Special cranes with long the polystyrene insulation board. booms were utilized to move materials on and • Hail Resistance: The new mem­ off of the ten-story brane must be proven to withstand Airport Operations hail. Building. Debris, flammable • Facility Operations: The roof goods, and loose mate­ replacement process must be as rials could not be non-intrusive as possible, and the stockpiled on the roof. finished roof must not utilize loose- Products and materi­ laid components that could become als that were not missiles in the event of a tornado or properly bundled high winds. Removing the poly­ together had to be styrene rigid insulation also posed a removed from the high risk for the tarmac operations, jobsite at the end of particularly on windy days. each day. Some parapet • Maintenance: The roof system wall sections were should be easy to repair and require curved. New sections Figure 7: Return air chutes. little maintenance. of coping had to be custom fabricated • Temperature Limitations: The and pre-finished. Each curved section had Fiberglass slip sheets are subject to adhesives and products must be to be measured with a template and damage on mechanically-fastened, single- workable during all four seasons of shipped to the metal supplier for fabrica­ ply roof systems due to fluttering of the the year, due to the long duration of tion. Over 100 different coping sections had membrane on windy days. Slip sheet dis­ the project. to be custom made. placement will result in premature plasti­ The roof had to be installed during dry cizer migration of PVC over polystyrene. Options Presented to Owner and warm weather because of the water- High thermal R-values and solar reflec­ Several roof replacement options were based roof coating, which is subject to dam­ tion from windows accelerate weathering of presented to the airport staff. The pros and age from freezing prior to curing. The estab­ a PVC roof membrane, resulting in prema­ cons were discussed for each roof type. lished schedule was from April to Novem- ture plasticizer loss and degradation.

28 • INTERFACE S EPTEMBER 2005 Manufacturing processes, compounds, vent moisture migration and condensation and environmental exposure can have a within an insulated roof assembly. major affect on long-term flexibility and ser­ Construction traffic can cause scarring viceability of PVCs. of the roof membrane that will cause latent Generally, polyisocyanurate rigid insu­ problems post construction. lation products perform very well under The product quality and long-term ser­ extreme heat or temperatures often experi­ viceability of roof membranes differ greatly enced in a roof assembly, even when among manufacturers. In the selection installed below dark or black membranes process of a roof system, there is no substi­ which can achieve surface temperatures of tute for time-proven performance. 180oF. When moisture is introduced and To be successful, difficult projects becomes trapped in the insulation from require extraordinary planning, synergies membrane leaks, vapor diffusion, or resid­ from all parties, cooperation, and constant ual water in cementitious roof decks, the open lines of communication. weathering and degradation of polyisocya­ nurate rigid insulation are intensified. In REFERENCES and FURTHER READING such instances, the physical properties and 1. Dr. H.O. Laaly, The Science and dimensional stability of polyisocyanurate Technology of Traditional and Mo­ rigid insulation are eventually compro­ dern Roof Systems, Volume 1. Los mised, and the sustainability of the product Angeles: Laaly Scientific Publishing, is negated. 1992. Concrete decks on new construction are 2. Ralph Paroli, Brian Whelan, and apt to contain large amounts of residual Thomas Smith, “Shattering of moisture that can have a deleterious effect Unreinforced PVC Roof Membranes: on some plastic foam insulation products, Problem Phenomenon, Causes and organic facers, and adhesive seams of elas­ Preventions,” Proceedings of NRCA/ tomeric membranes. Designers and in­ NIST Tenth Conference on Roofing stallers should consider vapor retarders Technology, April 22-23, 1993, pp. over moisture-laden concrete decks to pre­ 93-107, NRCA, Rosemont, IL. 3. Walter Rossiter Jr. and Robert Math­ Proceedings of the SPI 34th Annual ey, National Bureau of Standards, Polyurethane Technical/Marketing “Elastomeric Roofing,” Roofing/ Conference, October 1992. Siding/Insulation, Harcourt Brace 9. “Moisture Vapor Transmission,” The Jovanovich, Inc., September 1979. Society of the Plastics Industry, Inc., 4. American Society for Testing and Polyurethane Foam Contractors Materials, “Standard Definitions of Division, Washington, DC 1994. Terms Relating to Rubber, ASTM 10. ASTM E-96, “Standard Test Method Designation D-1566-77a,” 1977 for Water Vapor Transmission of Annual Book of ASTM Standards, Materials,” ASTM, Philadelphia. Part 37, Philadelphia, Pennsylvania, 11. Warren R. French, “Post Installation 1977. Field Evaluation of a Pressure- 5. American Society for Testing and Equalized Single-ply Roofing System Materials, ASTM C-755; “Standard to Determine Drying Effects on a Recommended Practice for Selection Moist Cementitious Roof Deck,” of Vapor Barriers for Thermal Interface, Roof Consultants Insti­ Insulations,” ASTM C-755, Philadel­ tute, August 1999. phia. 12. James L. Hoff, “EPDM Roof System 6. N.V. Schwartz, M. Bomberg, and Performance: An update of Histori­ M.K. Kumaran, “Water Vapor Trans­ cal Warranty Service Costs,” Inter­ mission and Moisture Accumu­ face, Roof Consultants Institute, lations in Polyurethane and Polyiso­ September 2003. cyanurate Foams,” Water Vapor 13. Jim D. Koontz and Vickie Crenshaw, Transmission Through Building “Simulated Hail Damage and Impact Materials and Systems; Mechanisms Resistance Test Proceedures for and Measurements, ASTM, Phila­ Roof Coverings and Membranes,” delphia, 1989. Interface, Roof Consultants Insti­ 7. American Society for Testing and tute, May 2001. Materials, Standard Specification for Poly(Vinyl Chloride) Sheet Roofing, This article, originally entitled, “Reroof ASTM D-4434, Philadelphia. Project Case Study: Denver International 8. R.R. Zarr and T. Nguyen, “Effects of Airport Terminal Flat Roofs,” was presented Humidity and Elevated Temperature as a part of the 19th RCI International on Density and Thermal Conductivity Convention & Trade Show in April 2004 in of a Rigid Polyisocyanurate Foam,” Reno, Nevada.

Nick A. Lovato

Nick Lovato is president of CyberCon Engineering Inc., a roof­ ing and waterproofing consulting firm founded in 1991. He graduated from the University of New Mexico with a B.S. in civil engineering and has 25 years of professional experience in roofing, including technical sales support, roof consulting, and construction management. Mr. Lovato has been a mem­ ber of the Roof Consultants Institute since 1992 and is a past director of Region V. Nick resides in Littleton, Colorado with his wife Candace.