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An aerial shot of the student housing building on the Texas A&M campus.
Building Science

Are Hybrid Roof Assemblies Worth the Hype?

How can roofing assemblies contribute to a building's energy efficiency, resiliency, and sustainability goals? Intentional material selection will increase the robustness of the assembly including the ability to weather a storm, adequate insulation will assist in maintaining interior temperatures and help save energy, and more durable materials may last longer, resulting in less frequent replacements. Hybrid roof assemblies are the latest roofing trend aimed at contributing to these goals, but is all the hype worth it?What is a hybrid roof assembly?A hybrid roof assembly is where two roofing membranes, composed of different technologies, are used in one roof system. One such assembly is where the base layers consist of asphaltic modified bitumen, and the cap layer is a reflective single-ply membrane such as a fleece-back TPO or PVC. Each roof membrane is chosen for their strengths, and together, the system combines the best of both membranes. A hybrid system such as this has increased robustness, with effectively two plies or more of membrane.Asphaltic membranes, used as the first layer, provide redundancy and protection against punctures as it adds overall thickness to the system. Asphaltic systems, while having decades of successful roof installations, without a granular surface may be vulnerable to UV exposure, have minimal resistance to ponding water or certain chemical contaminants, and are generally darker in color options as compared to single ply surfacing colors choices. The addition of a single-ply white reflective membrane will offset these properties, including decreasing the roof surface temperatures and potentially reducing the building's heat island effect as they are commonly white or light in color. PVC and KEE membranes may also provide protection where exposure to chemicals is a concern and generally hold up well in ponding water conditions. The combination of an asphaltic base below a single-ply system increases overall system thickness and provides protection against punctures, which are primary concerns with single-ply applications.Pictured Above: EverGuard® TPO 60‑mil Fleece‑Back MembraneOlyBond 500™ AdhesiveRUBEROID® Mop Smooth MembraneMillennium Hurricane Force ® 1-Part Membrane AdhesiveDensDeck® Roof BoardMillennium Hurricane Force ® 1-Part Membrane AdhesiveEnergyGuard™ Polyiso InsulationMillennium Hurricane Force ® 1-Part Membrane AdhesiveConcrete DeckPictured Above: EverGuard® TPO 60‑mil Fleece‑Back MembraneGAF LRF Adhesive XF (Splatter)RUBEROID® HW Smooth MembraneDrill‑Tec™ Fasteners & PlatesDensDeck® Prime Gypsum BoardEnergyGuard™ Polyiso InsulationEnergyGuard™ Polyiso InsulationGAF SA Vapor Retarder XLMetal DeckWhere are hybrid roof assemblies typically utilized?Hybrid roof assemblies are a common choice for K-12 & higher education buildings, data centers, and hospitals due to their strong protection against leaks and multi-ply system redundancy. The redundancy of the two membrane layers provides a secondary protection against leaks if the single-ply membrane is breached. Additionally, the reflective single-ply membrane can result in lower rooftop temperatures. The addition of a reflective membrane over a dark-colored asphaltic membrane will greatly increase the Solar Reflectance Index (SRI) of the roof surface. SRI is an indicator of the ability of a surface to return solar energy into the atmosphere. In general, roof material surfaces with a higher SRI will be cooler than a surface with a lower SRI under the same solar energy exposure. A lower roof surface temperature can result in less heat being absorbed into the building interior during the summer months.Is a hybrid only for new construction?The advantage of a hybrid roof assembly is significant in recover scenarios where there is an existing-modified bitumen or built-up roof that is in overall fair condition and with little underlying moisture present. A single ply membrane can be installed on top of the existing roof system without an expensive and disruptive tear-off of the existing assembly. The addition of the single-ply membrane adds reflectivity to the existing darker colored membrane and increases the service life of the roof assembly due to the additional layer of UV protection. Additionally, the single-ply membrane can be installed with low VOC options that can have minimum odor and noise disturbance if construction is taking place while the building is occupied.Is the hybrid assembly hype worth it?Absolutely! The possibility to combine the best aspects of multiple roofing technologies makes a hybrid roof assembly worth the hype. It provides the best aspects of a single-ply membrane including a reflective surface for improved energy efficiency, and increased protection against chemical exposure and ponding water, while the asphaltic base increases overall system waterproofing redundancy, durability and protection. The ability to be used in both new construction and recover scenarios makes a multi-ply hybrid roof an assembly choice that is here to stay.Interested in learning more about designing school rooftops? Check out available design resources school roof design resources here. And as always, feel free to reach out to the Building & Roofing Science team with questions.This article was written by Kristin M. Westover, P.E., LEED AP O+M, Technical Manager, Specialty Installations, in partnership with Benjamin Runyan, Sr. Product Manager - Asphalt Systems.

By Kristin Westover

December 28, 2023

Featured Blogs

  • wind
    Building Science

    Prevailing Winds and Prevailing Codes: A Summary of Roof Related ASCE 7-22 Changes

    Several changes have been included in the 2022 version of ASCE 7 as they relate to the roof. You may be thinking, 'as soon as I mastered ASCE 7-16, an updated version is set to be released!'. As with any Standard, it can be expected that updates will be made to include current research or trends. While the inclusion of tornado loads and the resulting changes in the load combinations may be the most significant, there are other updates that affect roofing as well. From minor updates to basic wind speed maps, to stepped roofs, and pavers, we have compiled a summary to help you navigate the updates. Not to fret, the changes are likely to not be incorporated until the 2024 version of IBC. However, that does not preclude incorporating these changes on current and upcoming projects.Why are these Changes Significant?The goal of the calculations in ASCE 7 are to determine the uplift pressures on a roof given project constraints including building height, location, and Risk Category. The resulting uplift pressures are then compared with tested roof assemblies to determine which assembly(ies) can be installed on a roof for each unique building. The assembly selection will also dictate the fastening method, whether it be mechanically fastened or adhered, and also the number and spacing of selected attachments. Changes in the Code may affect calculated uplift pressures that may influence the available roof assemblies.What is the Update? Updated Basic Wind Speed MapsWind speeds vary rapidly and are continuously being recorded at various locations around the world. Wind speeds are recorded in miles per hour (mph) in 3-second intervals and are collected over the course of a year. Then, the maximum values of the 3-second intervals are recorded and tracked to see the occurrence throughout the year. Since wind is a random phenomenon and the speeds vary not only by day, but also by year, the maximum 3-second intervals for each year are compared over several years. Rather than simply taking an average of wind speeds over time, the average wind speeds are analyzed by determining the Mean Recurrence Interval (MRI), which is how frequently a wind speed is equaled or exceeded during any given period of time (in years). Generally, higher wind speeds have a lower recurrence interval over time, and lower wind speeds have a higher recurrence rate over time. Analyzing this data is how wind speeds for a particular location are established.Wind speed (mph) has to be translated into units of force (psf) for design purposes in accordance with ASCE-7. The resulting forces are different in various roof zones.ASCE 7-16 has published basic wind speed contour maps for each Risk Category. The wind speed maps have contour lines that show the wind speeds throughout the United States as they vary by geographical location. It is allowable to interpolate between the contour lines, or the larger value listed on the contour line may be used. Additionally, if a location-specific wind speed is desired, an online Hazard Tool developed by ASCE will give an exact wind speed based on an address or GPS coordinates. The 2022 version updated wind speeds, primarily along the coastline. Wind speeds have generally either decreased by a few miles per hour or remained unchanged from ASCE 7-16, with the exception of several cities along the Gulf of Mexico coastline that have increased wind speeds due to recent hurricanes that have made landfall.Basic wind speeds from select coastal locations in hurricane prone regions at 33 ft for Exposure Category C are shown below. Wind speed data is shown for both ASCE 7-16 and ASCE 7-22 as a comparison.What is the Update? Wind directionality factor, Kd was movedThe wind directionality factor, Kd, was removed from the Velocity Pressure Coefficient and inserted into the Components and Cladding design wind pressures equation.The directionality factor (Kd) is a load reduction factor intended to take into account a reduced probability that the maximum wind speed will exactly coincide with the weakest direction of a building. According to ASCE 7, it accounts for the probability that the wind speed will come from any one direction given a location, and the maximum wind speed will occur from the direction that produces the maximum wind pressure on the building or its components. The directionality factor has constant factors to be used for Main Force Resisting Systems or Components and Cladding, but also accounts for various shapes of structures such as arched roofs, circular domes, and chimneys or tanks.In ASCE 7-16, Kd was located in the Velocity Pressure Equation (eq. 26.10-1) where velocity pressure, qh, is evaluated at the mean roof height, h. Where qh= qz .qh=0.00256KhKztKdKeV2 Which provides a velocity pressure in pounds per square foot (psf).The resulting value of qh is then multiplied by the external pressure coefficients (GCp) and then used to determine the wind pressure coefficients for each roof zone (perimeter, corner, field, etc.)p = qh [(Gcp) - (GCpi)] (lb/ft2)In ASCE 7-22, the velocity pressure, qh, is evaluated at the mean roof height, h. Where qh= qz by the following equation (eq. 26.10-1):qh=0.00256KhKztKeV2 Which provides a velocity pressure in pounds per square foot (psf).As you can see, the directionality factor, Kd, has been removed from the equation. In calculations for both the Main Wind Force Resisting System and Components and Cladding, Kd, has been inserted into the equations. However, since we are focusing on the roof, this falls within the Components and Cladding (C&C) calculations in Chapter 30. The design wind pressures of C&C elements are calculated with the following equations:For low rise buildings with h≤ 60 ft: (eq. 30.3-1)p = qh Kd[(Gcp) - (GCpi)] (lb/ft2)For low rise buildings with h＞60 ft: (eq. 30.4-1)p = q Kd[(Gcp) - qi Kd(GCpi)] (lb/ft2)What is the Update? Velocity Pressure Coefficients, Kz and Kh updatedThe Velocity Pressure Coefficients, Kz and Kh, for Exposures B and C were updated. Kz is the velocity pressure coefficient evaluated at height z, and Kh is the velocity pressure coefficient evaluated at height z = h. The velocity pressure at mean roof height h uses Kz. The variables are inserted into equation 26.10 to determine Velocity Pressure, qz (see equations above).What is the Update? Simplified Methods for Calculating C&C RemovedThe two simplified methods, Part 2 and Part 4 were removed from Chapter 30, Wind Loads, Components and Cladding.In the 2016 version there were two simplified methods that allowed for reduced calculations:Part 1 included calculations for Low-Rise BuildingsPart 2 was for Low-Rise Building (simplified)Part 3 included calculations for buildings with h> 60ftPart 4 was for buildings 60 ft<>Part 5 is Open BuildingsPart 6 included Building Appurtenances, Rooftop Structures, and Equipment.The 2022 version has removed the two simplified methods:Part 1 for Low-Rise BuildingsPart 2 is for buildings with h> 60ftPart 3 is Open BuildingsPart 4 includes Building Appurtenances, Rooftop Structures, and Equipment.What is the Update? Section 30.12 was added to address roof paversRoof pavers are used in IRMA (Inverted Roof Membrane Assembly) and PMRA (Protected Membrane Roof Assembly) assemblies where the roofing components are installed at the structural roof deck level and then the pavers are installed as ballast on top of the roof assembly. Roof pavers vary in size, thickness, material, and spacing, in addition to installation method and pedestal size. Pavers are considered air permeable since the gaps between pavers and the space beneath the pedestals allow for partial air pressure equalization between the surfaces. Roof pavers were only addressed in the Commentary Section C30.1 of the 2016 version and are referred to as Air Permeable Cladding. However, siding, pressure equalized rain screen walls, shingles, tiles, and aggregate roof surfacing were all included in this category. In ASCE 7-16, 'because of partial air-pressure equalization provided by air-permeable claddings, the C&C pressures services from Chapter 30 can overestimate the load on cladding elements. The designer may elect to use the loads derived from Chapter 30 or those derived by an alternate method.' While equations and methods are not included in this edition, several references are included where calculation methods may be found.In the commentary of ASCE 7-22, air permeable cladding is still defined as roof or wall claddings that allow partial air pressure equalization between the exterior and interior surfaces, with the same listing of claddings to include siding, roof pavers, and vegetative modular trays. The designer may elect to calculate the net uplift pressures of the pavers with recognized literature as noted in ASCE 7016 or Section 30.12. Section 30.12 was added to include Roof Pavers for Buildings of all heights with roof slopes less than or equal to 7 degrees. The Section includes an equation (eq. 30.12-1) to calculate design net uplift pressures:p = qhKdCLnet (lb/ft2)What is the Update? Roof Zones were revised for Hip and Gable RoofsAs wind blows over roof surfaces, it creates suction, or uplift, on the roof assemblies. The amount of uplift varies by building height, location (associated wind speed), and other factors unique to each building. The uplift created on the building is not uniformly distributed, and will vary depending on factors such as roof shape. Wind uplift is highest at the corners, then perimeters, and is the least in the field, or center of the roof; these varying wind uplift locations are called roof zones.Hip Roofs are where all four sides of the roof slopes down to connect to the exterior walls at the eaves. Gable roofs have two slanted sides that form a ridge that connect to the vertical walls that extend to the ridge. Chapter 30 of both ASCE 7-16 and 7-22 include roof zone diagrams and graphics that can be used to determine the External Pressure Coefficients (GCp).Image 1: Hip roof Image 2: Gable roofImages 1 and 2: Hip roof and gable roof, images courtesy of roofingcalc.comIn ASCE 7-22, both the roof zone diagrams and the graphics to determine GCp are updated. The roof zones are simplified to have three zones, Zone 1, Zone 2, and Zone 3 (in lieu of Zone 1, Zone 2r, Zone 2e, and Zone 3), and the accompanying zone layout was modified to include the Zone changes. The External Pressure Coefficients are determined using graphics, and those also have been updated and simplified.Components and Cladding, h ≤ 60 ft, Gable Roofs Roof Slopes 7°≤Θ≤ 20° and 20°≤Θ≤ 27°Components and Cladding, h ≤ 60 ft, Gable Roofs Roof Slopes 27°≤Θ≤ 45°Hip roofs have one roof zone layout plan for all roof slopes, however, External pressure Coefficients graphs were updated for each.What is the Update? Roof Zones were revised for Stepped RoofsStepped roofs are where buildings have multiple flat roof levels, which are often seen on large hospitals, offices, and school buildings. While stepped roofs have wind uplift pressures and corresponding roof zones, it is important to note that wind on the lower roof is affected by the neighboring higher roof sections. At the intersection of the higher roof section with the lower roof section, the wind uplift pressures are lower.New diagrams have been inserted into Chapter 30 of the ASCE 7-22 version. The primary change to note is that the corner zones, zone 3, have been changed from square shapes to L shapes. This is a reflection of the standard roof zones which have been updated from square corners to L shaped corners in more recent versions of ASCE as well.ASCE 7-16, Figure 30.3-3 Components and Cladding, h ≤ 60 ftASCE 7-22, Figure 30.3-3 Components and Cladding, h ≤ 60 ftWhat are the next steps?It is anticipated that ASCE 7-22 will be adopted in the 2024 version of the IBC, so there is plenty of time to get comfortable with the updates. And as always, feel free to reach out to the Building & Roofing Science team at buildingscience@gaf.com with questions.

    By Kristin Westover

    September 07, 2022

  • Polyiso installation
    Building Science

    Designing with Polyiso

    Thermal insulation is an important part of commercial roofing assemblies. As with anything, there are ways to design with and install polyiso insulation — a better way, a best way, and many variations in-between! What may be best in terms of lowest up-front costs, may prove only good or worse over the long-term life of the building.As discussed later, important points to remember about polyiso and its use include:Due to polyiso's very low air permeability, good design and installation practices can result in low risks of condensation, even for buildings with somewhat higher than normal humidity levels.Building designers can use the label R-value, stated at a mean temperature of 75°F, for projects in most ASHRAE climate zones. However, for climate zones 6 and 7, it may be appropriate to use published values at 40°F.Electricity is four times as expensive as gas, on an equivalent Btu basis. Therefore, polyiso's greatest value is seen during summer months when air conditioning usage is at its highest.IntroductionPolyiso has become the go-to insulation for commercial roofing with about 75% market share. The reasons include:Lowest cost per R-value.High R-value per inch, meaning it doesn't require as much thickness as other types of rigid insulation to achieve the same insulation performance.Good fire resistance. Being a thermoset unlike polystyrene foams, it doesn't melt during a fire. Melting of thermoplastic foams and subsequent flowing down of molten material through the deck into a building can be hazardous for occupants and fire crews.Solvent compatibility — adhesives used in adhered systems don't adversely affect polyiso. This is not the case with polystyrene foams.Reduced condensation — When installed correctly in a properly designed system, polyiso helps to resist interior air from reaching the underside of the membrane, thereby reducing condensation risks.However, just because polyiso is specified and used doesn't mean that the ideal outcome is reached. There are installation details that need to be considered.Single-ply membrane combined with polyiso can make for a good roof, but additional factors can improve performance:The BasicsPolyiso boards have straight cut edges (the boards are not tongue and grooved). These butt joints between boards can allow for vertical air movement.Polyiso was often installed as a single layer:As shown in the figure above, a single layer of butt-jointed insulation is generally a bad idea and is no longer allowed by the International Building Code (IBC) in the 2018 and newer versions. For roofs with a single layer of butt-jointed insulation boards:There is little restriction for air flow and so, during wind events, wind uplift forces result in membrane billowing for attached systems.Membrane billowing draws interior conditioned air up to the underside of the membrane and may create a risk of condensation within the assembly.Billowing may place additional stress on membrane fasteners, a factor recognized by membrane manufacturers by the issuance of shorter guarantees or warranties for mechanically attached versus adhered membrane.Essentially unrestricted air flow up into the assembly diminishes the insulation value and lowers energy efficiency.In contrast, two layers of polyiso restrict the free flow of air into the roof assembly. Care must be taken to seal between roof penetrations and the polyiso, or air is still free to move up into the assembly as shown here:Spray foam is a good choice for achieving a seal between the polyiso and the penetrations.Two layers of polyiso with staggered joints and sealed penetrations is a good system, but two of its features still reduce its insulation efficacy. These are shown in the following magnified view of a roof assembly cross-section.This cross-section illustrates lower energy efficiency resulting from:Insulation and membrane fasteners both act as thermal bridges, conducting heat through the assembly.Gaps between insulation boards that allow thermal convection to occurWhen both the insulation and membrane are attached and fastener densities are at their highest, R-value reductions of up to 29% have been predicted.Adhered versus AttachedThe alternative to mechanical attachment is to adhere the insulation with adhesive. Typically, low rise foam is the adhesive used for both insulation and cover board installation. The original method of application was as a ribbon but for labor savings it is often sprayed in a spatter-pattern as shown in the following picture of a typical installation in progress using one of GAF's low rise foamsUsually, when polyiso is adhered, the roof membrane is also adhered. For a steel deck substrate, fasteners are typically only used for the first layer of insulation, and then the adhesive is used on the subsequent layers of insulation and membrane. While it may be possible to adhere directly to a steel deck and eliminate fasteners totally, this approach is more common with concrete decks, as shown in the schematic here:Key features of systems with adhered insulation are:Air flow up through the assembly is limited and the risk of condensation in cold climates is low. Similarly, membrane billowing is minimized due to the restricted air flow up through adhered insulation layers.Thermal bridging is minimized since only the first layer of insulation might be mechanically attached. With concrete decks, the first layer of polyiso can be adhered thereby eliminating the use of fasteners totally.Wind uplift resistance is uniformly distributed across the roof deck. A system with adhered membrane and insulation can act as a monolithic system with excellent wind uplift resistance.When combined with an adhered membrane the finished roof appearance is usually aesthetically pleasing. When applied in accordance with manufacturer's instructions, the membrane can be very flat and there will not be any insulation fasteners to telegraph through and mar the appearance.Induction WeldedInduction welded fasteners are another type of roof attachment, the largest type being the Drill-Tec™ RhinoBond® system. By definition this is a mechanical attachment method but it has many of the features of adhered systems. The technique fastens TPO and PVC membranes to the substrate below using a microprocessor controlled induction welding machine. The thermoplastic roof membrane is welded directly to specially coated fastening plates used to attach the insulation. The picture below shows such a system being used:The induction machine is placed above each plate in turn and activated for approximately 10 seconds. As the machine is moved to the next position, a weighted magnet is placed over the plate and acts to squeeze the membrane down onto the hot fastener plate causing it to weld to that plate's surface coating.For typical mechanical attachment of single-ply membranes with fasteners along the seam lines, the insulation boards are simply secured with five fasteners per 4 x 8 ft. board to keep them flat. When using a Drill-Tec™ RhinoBond® system, the combined insulation and membrane fasteners resist wind uplift forces as shown in this picture during a test of wind uplift resistance:This means that wind loads are more uniformly distributed versus a conventionally attached system.Key features of systems with induction welded membrane attachment are:Performance is very similar to adhered in terms of the distribution of loads.Thermal bridging is reduced compared to traditional mechanically attached membrane systems.There are no application temperature restrictions and so this approach can be used in place of adhesive attachment regardless of how cold it might be.Drill-Tec™ RhinoBond® systems can be cost competitive due to the speed of installation as compared with traditional bucket and roller adhesives.Dew Point CalculationsThe dew point is the temperature at which moisture vapor forms condensation. It's a function of the relative humidity and the ambient temperature. The evaluation of a roof assembly where the dew point might be reached is an important step towards designing a roof with minimized condensation risks. The topic has been covered in greater depth elsewhere, but this section summarizes the key steps. Examine the chart below (this is a simplified form of what is used by HVAC engineers).Locating 40% relative humidity in the first column and then going across to the 70°F, design dry bulb temperature shows a dew point of 45°F. This means that in an environment that is 70°F and 40% relative humidity (RH), water in the air will condense at a temperature of 45°F. A roof assembly separates the interior conditioned environment from the outside and the insulation layer in the roofing system resists heat loss or gain to/from the outside, depending on the season. Within the insulation layer the temperature has a gradient between the hot and cold side, i.e. between inside and outside.As an example, consider a building in the winter to illustrate the point. The interior is 70 °F with 40% RH, like the example on the chart above. The temperature gradually drops from the innermost part of the insulation until at the outermost part it will be at the exterior, cold temperature. The plotting of temperature through the insulation thickness is referred to as the temperature gradient of that system. Using the example, if the temperature gets to the dew point of 45°F at any point in that system then water would be expected to condense on the nearest surface. This is shown in the following diagram:Summarizing, in this example the interior air has 40% of the total water vapor that it can support. But as the air migrates up through the roof system, it gets cooler until the point where it can no longer hold onto the water vapor and condensation occurs. In the example shown above, that's at 45°F and just inside the insulation layer.As will be discussed later, dew point calculations can be used to inform the placement of a vapor retarder layer when used.When a dedicated vapor retarder layer is not used, it is good practice to ensure that the dew point is in the upper layer of polyiso insulation. This will lower the risk of interior air migrating up through the roof assembly and condensing on a surface below its dew point.Using Vapor Retarders with PolyisoVapor retarders can be used to limit humidity ingress to the roof assembly. Under certain circumstances, depending on the building use and location, condensation can occur as discussed in the previous dew point discussion. In general there are four basic building conditions that can be considered:1. Buildings with Standard Amounts of Occupancy-Generated MoistureThese are the most common situations covering for example, office, retail, and warehouse spaces. The risk of having a condensation issue is low and good roofing practices such as sealing around penetrations may likely suffice.2. Buildings with Larger Amounts of Occupancy-Generated MoistureThis category includes apartments and other multiple residency buildings, paper mills, laundries, buildings with indoor swimming pools, and the like. In fact, anything that doesn't fit into category 1 above should be evaluated to determine humidity levels. The building's air handling and ventilation systems should be carefully specified to take into account the moisture loading.3. Construction–Related MoistureMost construction practices release some amount of moisture into the building space. These can be relatively short term such as drywall installation and painting. However, some practices can release large amounts of water over a considerable time frame into the building. These include poured in place concrete floors and roof decks.4. Concrete Roof DecksThese can present a challenge for roof system designers especially in new construction. Regardless of the type of concrete, significant amounts of water remain after curing is completed. Allowing concrete to thoroughly dry is most appropriate; however, it is often reasonably impractical. Dealing with potential moisture in concrete decks is beyond the scope of this article, but guidance can be found elsewhere.It is recommended that a building science professional experienced in designs for Categories 2, 3, and 4 be involved to determine whether a vapor retarder should be used and what type.Specification of R-ValuePolyiso is manufactured to meet the ASTM C1289 Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board. ASTM C1289 specifies the thermal resistance at a mean temperature of 75°F for various product thicknesses and requires that the values at 40 and 110°F be made available upon request.Important features of manufacturer's published R-values are:The values are an average across a temperature range. The test methods need the insulation specimen to have a hot and a cold side at least 40°F apart. Most reputable test labs use a difference of 50°F for accuracy. A published value at 75°F is actually an average R-value across the range of 50 to 100°F.To take into account the diffusion of gas out of, and air into the polyiso foam, the values are based on projected "Long Term Thermal Resistance" or LTTR, obtained by rapidly aging thin slices of the foam.The Polyisocyanurate Insulation Manufacturers Association, PIMA, conducts a third-party certification program to independently validate LTTR values. This is referred to as the PIMA QualityMark™ program. The LTTR values are considered as "labelled R-values" to be used by building design professionals.Label R-values represent a 15-year time-weighted average value. They can help a design professional estimate a building's energy efficiency without having to be concerned about long-term loss of performance, which is already factored into the value.Building design professionals designing roofs for ASHRAE climate zones 6 and 7 may need to use the R-value reported for a mean temperature of 40°F.Attachment PatternsThe various fastener patterns for polyiso have been mentioned in the previous sections. However, due to the impact of fasteners on thermal bridging and wind uplift resistance, the key points are summarized here:For systems that have both mechanically attached membrane and insulation, the membrane attachment provides the wind uplift resistance. The polyiso insulation fasteners are simply there to hold the insulation flat during the roof installation and to resist long term lateral movement.Typical fastener patterns are shown here:For systems with adhered single-ply membrane and mechanically attached polyiso boards, the insulation fasteners provide the wind uplift load resistance. This is the case whether both layers of polyiso are mechanically attached or only the bottom layer (in which case the upper layer would be adhered).Manufacturers have tested the fasteners per board required to meet wind uplift resistance requirements for these combined mechanically attached and adhered systems. The number of fasteners needed depends on the board size and thickness. For common systems, the numbers are shown in the table below:For each of these combinations above, manufacturers' handbooks provide fastener patterns.The number of fasteners for these combined mechanically attached and adhered systems is very large, for example a 125,000 s.f. big box type roof could require around 50,000 fasteners, resulting in significant thermal bridging.When installing over a steel deck, to reduce thermal bridging and to make for a more robust system with reduced condensation risk, it is advisable to only attach the first layer of polyiso and to adhere all subsequent layers and the membrane.If the first layer of polyiso is attached and the rest of the system adhered, then using a 1.5" thickness for that first layer would help to bury the thermal bridging fasteners. It could also put the dividing line between first and second polyiso layers below the dew point, which is advisable.ConclusionsPolyiso is a cost-effective roof insulation and has the advantage that its permeability is low. Good design and installation practices can result in low risks of condensation even for buildings with higher than normal humidity levels.When designed correctly, mechanically attached components with two layers of polyiso having staggered and offset joints can be part of a successful roof system.Adhered insulation and membrane roof systems have advantages including reduced or eliminated thermal bridging, lowered condensation risks, and better wind uplift resistance.In cases where building use anticipates higher interior humidity levels and/or the local climate suggests higher condensation risk, then a building science professional should be consulted as to vapor retarder use and specification.Building designers and specifiers are advised to use the labelled R-values shown for a mean temperature of 75°F. For projects in ASHRAE climate zones 5 and 6, values at 40°F could be used depending on the building's geometry and local energy costs.

    By Thomas J Taylor

    March 17, 2021

  • GAF Cedar City plant
    Building Science

    How will ASCE 7-16 affect the low-slope roofing industry? A Tale of Two Buildings

    A common question being asked in the roofing industry is whether or not the 2016 version of ASCE 7 is going to increase the design wind pressures acting on a building. The answer is "yes" in many cases. So, the follow up question is "by how much?" And, that leads to the next question, "how much more capacity will roof systems be required to have when wind design follows ASCE 7-16?"This blog looks at 2 buildings of different sizes and heights in two cities in the U.S., one centrally located and one along the Gulf coast, and compares design wind pressures based on ASCE 7-10 and ASCE 7-16. By assessing the required roof-system capacity for all roof zones in conjunction with the increases in the size of roof zones, we can begin to put some real numbers to the question "by how much will ASCE 7-16 affect the low-slope roofing industry?"While analysis of two building types in two cities doesn't represent a significant study, this study offers an example of how the 2016 version of ASCE 7 is going to affect installed roofing systems over the next decade.Case StudiesDesign wind pressures (DWP) were determined for two building types in two cities. DWPs were based on varying the Risk Category and Exposure. The following table shows the building types, cities, variables, and constants.The big box store is 290' long x 169' wide x 24' tall, and the apartment building is 100' long x 40' wide x 55' tall. Wind speeds were determined using an online tool.DWPs can be determined by using proprietary third party methods (e.g., RoofNav), simplified calculation tools (e.g., RoofWindDesigner), or hand calculations following the methods within the ASCE 7 standard.For this case study, DWPs were determined using hand calculations via an Excel spreadsheet. The DWPs are slightly more refined since there was no rounding of wind speeds for ASCE 7-16. Both RoofNav and Roof Wind Designer round to the nearest 5 or 10 mph. The maps in ASCE 7-16 have been revised, not only from a wind speed perspective but from the number of wind isobars on the maps. The ASCE 7-16 maps are more refined which allows for more exact wind speed values.ASCE 7-10 Map, Risk Category II, showing a limited number of wind-speed isobars.ASCE 7-16 Map, Risk Category II, showing many additional wind-speed isobars relative to the ASCE 7-10 wind map shown in the above figure.In total, 72 different sets of DWPs were calculated. Buildings over 60' tall are designed the same in ASCE 7-16 as in ASCE 7-10; therefore, this article focuses on buildings less than 60' tall.Design Wind Pressure ResultsThe following charts provide the DWPs for each of the scenarios that were analyzed for this article.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a big box store in Kansas City, MO that is 24' tall.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a big box store in Mobile, AL that is 24' tall.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a 5-story apartment building in Kansas City, MO that is 55' tall.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a 5-story apartment building in Mobile, AL that is 55' tall.Comparing the 2010 and 2016 versionsThe primary intent of this article is to start generating data to help answer the question "how much did the DWP increase from the 2010 version to the 2016 version of ASCE 7?" Using the DWP calculated for each roof zone within each of the four scenarios, we calculated the percent increase in DWP per zone. The percent increases in DWP were calculated by dividing the ASCE 7-16 value with the corresponding ASCE 7-10 value for each roof zone. However, because there are 4 roof zones in ASCE 7-16 (and 3 roof zones in ASCE 7-10), both Roof Zones 1' and 1 were divided by the value for ASCE 7-10 Roof Zone 1 to determine the percent increase. Values in the following charts that are below 100% (shaded gray) indicate a reduction in DWP from ASCE 7-10 to 7-16.Percent increases in DWP per roof zone for a big box store in Kansas City, MO that is 24' tall, focusing on Exposure C values.Percent increases in DWP per roof zone for a big box store in Mobile, AL that is 24' tall, focusing on Exposure C values.Percent increases in DWP per roof zone for a 5-story apartment building in Kansas City, MO that is 55' tall, focusing on Exposure C values.Percent increases in DWP per roof zone for a 5-story apartment building in Mobile, AL that is 55' tall, focusing on Exposure C values.For the big box store scenarios, the percent increases in DWP per roof zone varies from 84% to 178%. The percent reductions (values less than 100%) are all in Roof Zone 1' while Roof Zones 1, 2, and 3 have increased DWP. As seen in the Figures, Roof Zone 1 percent increases are greatest while Roof Zone 3 increases are smallest.For the 5-story apartment building scenarios, the percent increases in DWP per roof zone varies from 84% to 177%. The percent increases in Roof Zone 1 are the largest. The percent reductions are all Roof Zone 1'.It is worth noting that the increases in Pressure Coefficients from ASCE 7-10 to 7-16 are, in fact, relatively indicative of the increases in DWP. For more on Pressure Coefficients and the wind design process.The case studies in this blog also show that the 2016 version of ASCE 7 imposes higher DWPs in Roof Zones 1, 2, and 3. However, given some of the substantial reductions in Roof Zone 1', it could be expected that installed roof system capacity may be lower. While that is hopeful, the roofing industry has a minimum-capacity backstop of 60 psf. This means that any calculated DWP at or below 60.0 psf (even for DWP values as low as 23 psf) will have a 60-psf-capacity roof assembly installed to meet building code requirements. The reality is many buildings are required to use a 60-psf-capacity roof system when in fact '60 psf' is more capacity than needed based on calculated DWPs.Let's take a closer look at one subset for each of the 4 scenarios.Example 1: Big Box; Central USThe big box store in KC, MO (Risk Category II, Exposure C) saw the DWP increase from 38 psf to 56 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 32 psf in Roof Zone 1'. Regardless of the increases or decreases, a 60-psf-capacity roof should be installed in both Roof Zone 1 and Roof Zone 1' per 7-16, which is unchanged relative to ASCE 7-10.Roof zones and pressures for a 24' tall big box store in Kansas City, MO based on ASCE 7-10 using Risk Category II and Exposure C.Roof zones and pressures for a 24' tall big box store in Kansas City, MO based on ASCE 7-16 using Risk Category II and Exposure C.For Roof Zones 2 and 3, the DWPs are not only increased, but the sizes of the roof zones that require higher capacity are also increased. Roof Zone 2 per ASCE 7-10 required a 75-psf-capacity roof, and a 75-psf-capacity roof will be needed per ASCE 7-16. However, approximately 9% of the roof area will need a higher capacity because of the increase in roof zone size. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Chart showing required roof capacity and the roof area (%) where each is required.Example 2: Big Box; Gulf CoastA big box store in Mobile, AL (Risk Category II, Exposure C) saw the DWP increase from 69 psf to 109 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 63 psf in Roof Zone 1'. A 75-psf-capacity roof should be installed in Roof Zone 1', and higher-capacity roofs will be required in Roof Zones 1, 2, and 3 relative to ASCE 7-10.Roof zones and pressures for a 24' tall big box store in Mobile, AL based on ASCE 7-10 using Risk Category II and Exposure C.Roof zones and pressures for a 24' tall big box store in Mobile, AL based on ASCE 7-16 using Risk Category II and Exposure C.For Roof Zones 1, 2, and 3, the DWPs are not only increased, but the size of the roof zones that require higher capacity are also increased. A larger perimeter roof zone (combining Roof Zones 1 and 2) with higher required capacity is created (relative to 7-10). Approximately 30+% of the roof area will need a higher capacity. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Chart showing required roof capacity and the roof area (%) where each is required.Example 3: 5-story; Central USThe 5-story apartment building in KC, MO (Risk Category II, Exposure C) saw the DWP increase from 45 psf to 66 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 38 psf in Roof Zone 1'.However, because the method used to determine the sizes of roof zones in ASCE 7-16 is changed, Roof Zones 1' and 1 do not exist on this building! Decreases of DWP in Roof Zone 1' are rendered meaningless, and the lack of a Roof Zones 1' and 1 means the entire roof consists of only Roof Zones 2 and 3, the perimeter and corner roof zones.Roof zones and pressures for a 55' tall apartment building in Kansas City, MO based on ASCE 7-16 (top) and ASCE 7-10 (bottom) using Risk Cat II and Exposure C.The increases in DWPs for the roof zones significantly increase the roof system capacity requirements for this scenario. Almost the entire roof area will need a higher capacity roof. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Chart showing required roof capacity and the roof area (%) where each is required.Example 4: 5-story; Gulf CoastThe 5-story apartment building in Mobile, AL (Risk Category II, Exposure C) saw the DWP increase from 81 psf to 129 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 74 psf in Roof Zone 1'. Similar to Example 3, due to the size of the roof zones relative to the size of the roof, Roof Zones 1' and 1 do not exist in this scenario.Roof zones and pressures for a 55' tall apartment building in Mobile, AL based on ASCE 7-16 (top) and ASCE 7-10 (bottom) using Risk Cat II and Exposure C.The increases in DWP for the roof zones significantly increase the roof system capacity requirements for this scenario. The entire roof area will need a higher capacity. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Performance versus PrescriptiveThe case studies in this analysis are presented using a performance method for wind design. That is, a specific DWP is determined for each roof zone and a roof system with appropriate capacity is selected for use in the associated roof zone. Another method that is often used in the roofing industry is a prescriptive method for wind design. The DWP for Roof Zone 1 (or Roof Zone 1' in certain localities) is used to select a roof system with an appropriate capacity for Roof Zone 1 (similar to a performance method), and then Roof Zones 2 and 3 use an installation method/layout that is prescriptively increased relative to Roof Zone 1 installation methods. Prescriptive installation methods for Roof Zones 2 and 3 are regularly used with mechanically attached (MA) roof systems, and for MA roofs, Roof Zone 2 increases are commonly 50% and Roof Zone 3 increases are commonly 100%.ConclusionASCE 7-16 DWPs are generally increasing relative to ASCE 7-10, but this impact is better understood by comparing the required roof system capacity for different building types and dimensions in different cities. Looking only at DWPs does not provide enough information; a comparison of required roof system capacities is needed. Not only did the DWPs change, but the methodology for determining the size of the perimeter and corner roof zones changed significantly in the new ASCE 7-16 version; the amount of roof area contained by the perimeter and corner roof zones increased. The case studies—of two buildings less than 60 ft tall in two cities—and corresponding analysis in this blog are just one additional small step to understanding how the ASCE 7-16 will affect wind design over the next decade.

    By James R Kirby

    February 21, 2021

  • Commercial building with a masonry facade
    Building Science

    The Physics of Thermal Inertia in Low-Slope Roof Design

    What are the key material properties?In a previous article the use of thermal inertia to slow down heat flux through a roof assembly was discussed. In buildings where air conditioning costs dominate and heating use is relatively low, higher thermal inertia assemblies can potentially improve energy efficiency. This is particularly the case of buildings such as offices that are only occupied during daylight hours. Thermal inertia could delay the transmission of heat into a building towards the end of the day, increasing thermal comfort and allowing facility managers to reduce cooling during the day.While the previous article provided an introduction to thermal inertia and its potential benefits, it didn't discuss the origin of the property. Also, the related but different terms, thermal inertia and thermal mass, were used somewhat interchangeably.This article describes the material properties that contribute to thermal inertia and provides a basis for material selection when increased thermal inertia is a goal.IntroductionA building enclosure with high thermal inertia will slow down the transmission of heat. This effect has traditionally been used in the Mediterranean and other temperate climates to improve thermal comfort during the summer. As the building exterior surface heated up during the early afternoon period, high thermal inertia reduced the heat transmission into the interior. This was advantageous in the days before air conditioning. Today, designing with higher thermal inertia, as described in part one of this series, could improve energy efficiency in certain situations:Buildings located in areas where energy costs are dominated by air conditioning.Buildings only used during the day and not on a 24 hour basis. Thermal inertia can push heat loads into the evening, allowing for either temperature settings to be raised or HVAC fans speeds to be reduced.For high thermal inertia within the roof assembly to have a significant effect, low to moderate rise buildings with larger footprints would be best.Thermal Inertia EffectsThere are two key characteristics of thermal inertia; the decrement factor and time delay, both of which can be readily modeled and experimentally verified. A schematic showing these is shown below:The time delay is calculated as:Φ = t[Tout(max)] - t[Tin(max)]Where t[Tout(max)] and t[Tin(max)] are the time of day when the inside and outside surface temperatures reach maximum. The decrement factor, DF, is calculated from:DF = (Tin(max) – Tin(min)) / (Tout(max) – Tout(min))The time delay and decrement factor could be used by building designers and operators to better optimize thermostat specification, HVAC use, and temperature schedules. As noted earlier, a large time delay could significantly reduce daily AC cooling demands for office buildings.Larger decrement factors can improve occupants' thermal comfort. Temperature swings are reduced, which in turn reduces HVAC cycling.Importantly, high thermal inertia will not reduce the energy reaching the interior, it only delays the transmission. This can make increased thermal inertia part of a strategy to improve energy efficiency in buildings occupied during the day but not necessarily for those occupied on a 24-hour basis.Roof Configurations and Thermal PropertiesBuilding designers and construction professionals are familiar with the property of thermal resistance, which is a measure of the extent to which a material blocks or resists heat transfer. It is expressed as R-value or U-value, which are numerical descriptions of the extent to which a material can resist heat flow.However, there is far less familiarity with the time delay of heat transfer, or the extent to which a material slows down or delays heat transfer. As will be seen, a material can have low thermal resistance but still delay heat transfer. This is the crux of the underlying properties "thermal diffusivity", "thermal mass" and "thermal inertia".Fundamental Thermo-Physical PropertiesThere are three fundamental thermo-physical properties of materials that are the components of thermal diffusivity, thermal inertia, and thermal mass. These are thermal conductivity, density, and specific heat capacity.Thermal ConductivityBuilding design professionals are very familiar with thermal conductivity, k, measured as watts per meter per Kelvin (W/(m.K)). It is normally used to characterize individual materials, for example polyiso foam, and is a measure of the heat flow through a material when a temperature gradient of 1K (i.e 1°C) is applied.A related term, the u-factor, is used to describe the thermal conductivity of a system. This could be a window, i.e. a combination of glass, air space, and frame etc.Thermal conductivity is measured when heat flow has equilibrated and it doesn't include any time delay.DensityDensity is a measure of the mass per unit volume, ρ=m/v. While this is straightforward for most materials, products such as polyiso can be harder to define. There are the facers and a small density gradient within the foam. Later, as values are listed, the facers will be ignored and the foam's density considered as an average.Specific Heat CapacitySpecific heat capacity is defined as the amount of heat required to raise the temperature of 1 kilogram of a substance by 1 kelvin, or Cp = J/(kg.K). It is worth considering the simple diagram above, showing the measurement of thermal conductivity. At the onset, heat flow through the material is delayed by its specific heat capacity. It takes energy to raise its temperature, an action required before heat can then transmit.Derived Thermo-Physical PropertiesAs noted, the fundamental properties described earlier are all measured at equilibrium and do not include any time lag or delay. For that, the following derived properties need to be considered.Thermal DiffusivityThermal diffusivity is a measure of the rate at which a temperature propagates from one point to another point in a material. It's the rate of transfer of heat from a hot side to a cold side and is calculated as:In a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk'. In a sense, thermal diffusivity is what is meant when the topic of thermal inertia is discussed casually.Thermal InertiaThermal inertia is the slowness with which the temperature of a material approaches that of its surroundings. It is a product of thermal conductivity, density, and specific heat capacity. From a building enclosure perspective, it could be considered as the rate at which the interior surface can supply heat into the interior, assuming a temperate climate.It is arguable that thermal inertia is not the best property to use to characterize a building enclosure component in terms of thermal lag.Thermal MassIt is often broadly thought that thermal mass is essentially equivalent to gravimetric mass. Conceptually, this gives rise to the view that the more massive a construction the better. However, consider two blocks - one of steel and one of concrete - each having the same gravimetric mass. The two materials have different specific heat capacities and are not equivalent in terms of thermal properties. In addition, steel has a significantly higher thermal conductivity versus concrete.Thermal mass is a property of a material that enables it to store heat and is the product of density and specific heat capacity:Importantly, thermal mass doesn't fully describe thermal lag. High thermal mass would change the decrement factor, dampening out heat transmission but is only indirectly linked to thermal delay.Roof Component Thermal PropertiesTwo deck types are predominant in North American construction of big-box type architecture; steel and concrete. Two possible idealized roof assemblies are shown below, based on the two roof deck types:Both systems have most roof assembly layers adhered. The steel deck system has a first layer of mechanically attached gypsum board to act as a substrate for the adhered first layer of polyiso.Thermal bridging is either minimal in the steel deck based system or absent in the concrete deck system (ignoring any real world penetrations).The first layer of gypsum board in the steel deck case could be used as a substrate for a vapor retarder.The membrane, adhesive layers, and steel deck are relatively thin and have no significant insulating properties. The following table shows the fundamental and derived thermo-physical properties of the other components:Polyiso and HD Polyiso specific heat values are estimates.Concrete properties are very dependent on aggregate type and moisture level. Data shown represents average values for dry concrete.Unsurprisingly, the data shows that concrete has significantly higher thermal mass and inertia versus the other materials compared in the table. As discussed in the first part of this series, insulation is best placed closest to the building exterior, and thermally massive materials placed closest to the interior. The southeast region tends to use concrete decks more than many other regions of the US, largely for improving the strength of the roof deck. This could also mean that, especially for those buildings occupied only during the day, such as offices, schools, and the like, have better opportunity for energy-efficiency benefits with well-designed roof insulation systems.ConclusionsThermal property data could be used in modeling exercises to better understand how to design and optimize energy-efficient buildings.Clearly, concrete has a far higher thermal mass and inertia than other materials commonly used in roof assemblies.Very lightweight concrete and gypsum board have very similar thermal mass, thermal inertia, and density, but have notably different properties for thermal diffusivity and conductivity.Thermal property data is fundamental to the materials used in low slope roof assemblies. However, it needs to be converted into values specific for the thickness and weight of actual roof products. That will be a topic for the next part of this series.Sources UsedVerbeke, S., Thermal inertia in dwellings. Quantifying the relative effects of building thermal mass on energy use and overheating risk in a temperate climate. PhD Thesis, University of Antwerp, 2017.Balaji, N. C., Mani, M., and Venkatarama, R. B. V., Thermal performance of building walls. 1st IBPSA Italy Conference, Building Simulation, 2013, pp. 151 – 159.

    By Thomas J Taylor

    January 13, 2021