Tapered Insulation Can Prevent Ponding on Low-Slope Roofs

The primary and most important function of a roof membrane in a low-slope roof system is to provide weatherproofing by keeping the rainwater from entering the roof assembly. Ponding water poses the greatest risk to a roofing membrane, since it not only shortens its service life, but can lead to more serious life safety concerns when loads and deflections exceed the designed conditions. This could lead to a roof collapse. From an aesthetics standpoint, areas on roofs with a prevalence for ponding are susceptible to unsightly bacterial and algae growth as well as accumulation of dirt. Given the large footprint of low-slope roofs on typical commercial buildings, managing rainwater timely and effectively is an important design consideration in new roof design as well as roof replacements on existing buildings. In addition, the model building codes include requirements for minimum drainage slope and identify ponding instability as a design consideration for rain loads.

Tapered insulation systems are an integral part of roof system design and can help reduce or eliminate the amount of ponding water on the roof when the roof deck does not provide adequate slope to drain. The popularity of tapered insulation has grown as more designers and roofing professionals understand the importance of positive drainage in good roofing practice. Because of its wide use in low-slope roofing application, tapered polyiso insulation systems offer a number of benefits in addition to providing positive drainage: high R-value, versatility and customization to accommodate project-by-project complexity as well as ease of installation. This article highlights the key considerations for tapered insulation systems.

Slope and Drainage Requirements in Building Codes

The model building codes require that commercial roofs be sloped to achieve a positive drainage of rainwater to drains, scuppers, and gutters. The term “positive roof drainage” is defined in the 2018 International Building Code (IBC) as “the drainage condition in which consideration has been made for all loading deflection of the roof deck, and additional slope has been provided to ensure drainage of the roof within 48 hours of precipitation.” The 2018 IBC indicates a minimum design 1/4:12 units slope requirement for membrane roof systems, and minimum slope of 1/8 inch per foot for coal tar pitch roofs. New construction must comply with the minimum slope requirements in IBC Section 1507. Roof replacement or roof re-cover applications of existing low-slope roof coverings that provide positive roof drainage are exempt from the minimum prescriptive1/4:12 units slope requirement.

Roof drains are part of an approved storm drainage system and function to divert water off and away from the building. Roof drainage systems in new construction must comply with provisions in Section 1502 of the 2018 IBC and Section 1106 and 1108 of the International Plumbing Code (IPC) for primary and secondary (emergency overflow) drains or scuppers. Roof replacement and re-cover applications on existing low-slope roofs that provide positive roof drainage are exempt from requirements for secondary drains or scuppers. It is important to note that secondary drainage systems or scuppers in place on existing buildings cannot be removed unless they are replaced by secondary drains or scuppers designed and installed in accordance with the IBC.

When reviewing the options available for achieving the required slope in a roof system, designers have a number of choices. According to the National Roofing Contractors Association (NRCA) (see “The NRCA Roofing Manual: Membrane Roof Systems: 2019”) the slope can be achieved by: sloping the structural framing or deck; designing a tapered insulation system; using an insulating fill that can be sloped to drain; properly designing the location of roof drains, scuppers and gutters; or a combination of the above.

Design Considerations For Tapered Insulation Systems

Proper design and installation are critical to the effective performance of tapered polyiso insulation systems, and this is true for any product or system. Tapered polyiso is manufactured in 4-foot-by-4-foot or 4-foot-by-8-foot panels that change thicknesses over the 4-foot distance from the low edge to the high edge on the opposing sides of the panel. The standard slopes for tapered insulation are 1/8 inch, 1/4 inch and 1/2 inch per foot to accommodate specific project requirements. However, tapered insulation panels with slopes as low as 1/16 inch and other alternative slopes (3/16 inch and 3/8 inch per foot) can be specially ordered to accommodate unique field conditions. The minimum manufactured thickness of tapered polyiso insulation board at its low edge is 1/2 inch and the maximum thickness at the high edge is 4-1/2 inches.

The design of the tapered insulation system will be governed by the footprint and complexity of the roof under consideration, slope of the roof deck, presence and configuration of roof drains (primary and secondary), scuppers, gutter or drip edges. In addition, roof structures, height of parapet walls, expansion joints, curbs and through-wall flashings and any other elements that may obstruct water management also needs to be considered in the design phase. The tapered insulation system will be lowest at internal drains, scuppers, gutters and drip edges, and will slope upwards away from these features.

Keeping in mind that the primary goal of a tapered insulation system is to most effectively move water to the specified drainage points. A two-way (two directional slope) or four-way (four directional slope) system are the most common designs. A two-way tapered insulation system is commonly used on roofs where multiple drains are in straight lines. In this scenario, there is a continuous low-point between the drains and it often extends to the parapet walls. Crickets are installed in between the drains and between the building or parapet walls and the drains. (See Figure 1a.)

A four-way tapered insulation system is the most effective way to move water off the roof, and this approach is highly recommended by industry professionals. In this scenario with a drain located in the center, water is drained from the higher perimeter edges on all four sides. (See Figure 1b.) Variations of two-way and four-way systems exist to accommodate complexities in the field. In addition to two-way and four-way systems, one directional slope and three directional slope tapered systems can be used to effectively move water to gutters, drip edges and scuppers.

Keeping in mind that a tapered system is more expensive than a roof system constructed with standard flat insulation only, the tapered design is often a target for “value engineering.” Value engineering can compromise the drainage intent of the design professional, architect or roof consultant for the purpose of lowering the installed cost of the roof system. Value engineering may change the specified slope or redesign the configuration of the tapered panels. In the end, the building owner may pay for a tapered insulation system that does not effectively drain water from the roof as intended by the original design. This will likely result in higher long-term costs for roof maintenance and premature roof system failure.

A typical tapered insulation system will incorporate flat polyiso board stock (referred to as “fill panels” or “tapered fill panels”) beneath continuing, repeating tapered panels. The tapered panels can be a single panel (or “one panel repeat”) system, meaning that the taper is provided by a single repeating panel in conjunction with fill panels. (See Figure 2a.) Non-typical designs can feature up to an eight-panel (or “eight panel repeat”) system with eight tapered panels making up the sloped section prior to incorporating the first fill panels. An example of “four panel repeat” system with 1-inch and 2-inch fill panels and 1/16 inch per foot slope is provided in Figure 2b.

Finally, crickets are an integral part of a tapered insulation system and are commonly used in two-way systems. Crickets can divert water toward drains and away from curbs, perimeter walls, and roof valleys. The two factors that must be considered in the design and installation of crickets are slope and configuration. The general “rule of thumb” is that for a full diamond cricket the total width should be between 1/3 to 1/2 of the total width. The wider the design of the cricket, the more you utilize the slope in the field of the roof, which improves the drainage efficiency.

Crickets typically have diamond or half-diamond shapes. (See Figures 3a and 3b.) However, kite-shaped and snub nose crickets can also be configured to accommodate specific roof designs. To keep water from remaining on the cricket surface, the design needs to have a sufficient slope (generally, twice the slope in the adjacent field of the roof). NRCA provides guidance regarding cricket geometry (see “The NRCA Roofing Manual: Membrane Roof Systems: 2019”).

Tapered insulation systems offer a cost-effective solution to achieving positive slope and improved drainage in new roof systems and roof replacement applications. An adequate rainwater management strategy that includes both proper drainage and elimination of ponding water is critical to the long-term performance and durability of a roof system. In addition, proper design, detailing, and installation of products must be an integral part of a tapered roof system design. For more information, consult with a polyiso insulation manufacturer who provide guidance, design assistance, and technical information regarding tapered insulation systems. In addition, the Polyisocyanurate Insulation Manufacturers Association (PIMA) publishes technical bulletins to help navigate the process of designing a tapered system. PIMA’s Technical Bulletin #108 on Tapered Insulation Systems can be found at www.polyiso.org/resource/resmgr/Tech_Bulletins/tb108_Mar2017.pdf.

About the author: Marcin Pazera, Ph.D., is the Technical Director for Polyisocyanurate Insulation Manufacturers Association (PIMA). He coordinates all technical-related activities at PIMA and serves as the primary technical liaison to organizations involved in the development of building standards. For more information, visit www.polyiso.org.

Improve Commercial Roof Performance With Staggered Insulation Layers

Photo: Hunter Panels

Selecting the right components for a project can dramatically improve the performance and longevity of the overall building. In a commercial roofing project, the chosen insulation and the installation technique are critical to a building’s resilience and thermal efficiency.

From a physics standpoint, energy flows from a region of high to low potential (from warm to cold). Therefore, a significant amount of heat can leave a building through an inadequately insulated roof assembly during heating season (winter) and enter a building through an inadequately insulated roof assembly during cooling season (summer). A building with an under-insulated roof assembly may require more energy to compensate for these heat gains and losses.

The benefits of installing multiple, staggered layers of rigid board insulation have been well known for years. Industry authorities, including National Roofing Contractors Association (NRCA), Oak Ridge National Laboratory (ORNL), Canadian Roofing Contractor Association (CRCA) and International Institute of Building Enclosure Consultants (IIBEC), formerly RCI, Inc., have recognized these benefits; and contractors, designers and specifiers have followed the roofing industry’s long-standing recommendation for the installation of staggered insulation layers.

Using the optimal roof insulation product also will impact performance. Polyiso insulation offers key advantages in meeting stricter building standards and improving energy efficiency. Polyiso has a high design R-value compared to XPS, EPS, and mineral wool board. Lightweight and easy to trim, polyiso can be layered to reach the desired R-values without being cumbersome to install.

Why Are Multiple, Staggered Layers of Insulation Important?

In 2015, the International Energy Conservation Code (IECC) increased the R-value requirements for the opaque thermal envelope in many climate zones across the United States. As a practical matter, most roofs will require two or more layers of insulation to meet the local energy code requirements. In the 2018 version, the IECC was updated with specific installation requirements for continuous roof insulation. The 2018 IECC explicitly calls for continuous insulation board to be installed “in not less than 2 layers and the edge joints between each layer of insulation shall be staggered” (Section C402.2.1 Roof assembly). 

Figure 1. Multiple, staggered layers of insulation can minimize air infiltration and reduce or prevent condensation in the roof system.

Staggering the joints of continuous insulation layers offer a number of benefits:

· Increased thermal performance/reduced thermal loss: The staggered joints on multiple layers of insulation offset gaps where heat could flow between adjacent boards. The staggered approach to installing insulation reduces thermal bridging in the roof assembly. A fact sheet on roof insulation published by Johns Manville (RS-7386) notes that as much as 8 percent of the thermal efficiency of insulation can be lost through the joints and exposed fasteners of installations that use only a single layer of insulation.

· Air intrusion: When conditioned air enters the building envelope, often because of pressure gradients, it carries moisture into the roofing system. This moisture will undermine optimal performance. A peer-reviewed study on air intrusion impacts in seam-fastened mechanically attached roofing systems showed that air intrusion was minimized by nearly 60 percent when the insulation joints were staggered between multiple layers of insulation. (See “Air Intrusion Impacts in Seam-Fastened, Mechanically Attached Roofing Systems,” by By Suda Molleti, PEng; Bas Baskaran, PEng; and Pascal Beaulieu, www.iibec.org.)

Additionally, by limiting the flow of air and moisture through a roof system, staggered layers of insulation in a roof assembly can reduce and/or prevent condensation. The condensed moisture if allowed to remain and accumulate in the system can damage the substrate and potentially shorten the service life of a roof. A properly insulated roof can also prevent the onset of condensation by effectively managing the dew-point within the roof assembly. 

· Resilient roof assemblies: Staggered joints can reduce the stress put on a single insulation layer and distribute that stress more evenly over multiple, thinner insulation joints. For example, in an adhered roof system, the installation of multiple layers of insulation can minimize the potential for membrane splitting. In this system, the upper layer(s) of insulation can protect the membrane from potential physical damage caused by fasteners that are used to attach the bottom layer of insulation to the roof deck.

· Ponding water: Roof slope is often created through the use of tapered insulation systems. These systems offer an opportunity to stagger the joints by offsetting insulation layers and improve overall energy performance of a system. If the added insulation layer is tapered, the slope provided can improve drainage performance of the roof. Rainwater that does not drain and remains standing, collects dirt and debris that can damage or accelerate erosion of roof covering. Integrating tapered polyiso system with staggered joints into a roof’s design will not only improve the thermal performance but also can improve drainage and thus overall longevity of the system.

· Puncture resistance: Roof cover boards are commonly installed to provide a suitable substrate for membrane attachment as well as protect the roof assembly from puncture and foot traffic. When using products like polyiso high-density roof cover boards, the joints should also be staggered with the underlying roof insulation. This ensures the benefits discussed above are preserved in systems utilizing cover boards.

Installation Best Practices Are Keys For Success

A properly designed roof system that utilizes high-performance polyiso insulation products is a strong foundation (or cover) for energy-efficient and sustainable construction. However, the designed performance can only be achieved through proper installation. Implementing industry best practices such as the installation of multiple layers with staggered joints will optimize energy efficiency of the system and will help ensure that the roof system performs during its service life.  

To learn more about the benefits and uses of polyiso insulation,please visit the Polyisocyanurate Insulation Manufacturers Association website at www.polyiso.org.

About the author: Marcin Pazera, Ph.D., is the Technical Director for Polyisocyanurate Insulation Manufacturers Association (PIMA). He coordinates all technical-related activities at PIMA and serves as the primary technical liaison to organizations involved in the development of building standards. For more information, visit www.polyiso.org.

Environmental Product Declarations Are a Driving Force for Change

Agropur Cooperative’s new Canada Green Building Council certified and LEED accredited two-story office building in Longueuil, Quebec, has polyiso insulation on the roof. Photo: SOPREMA

With a worldwide population that continues to grow (estimated at more than nine billion by 2050), demand for natural resources is increasing at rates that threaten to stress sustainable supply. Over the last few decades, society has become increasingly concerned about the environmental impacts of human activity. The U.S. Department of Energy estimates that the built environment accounts for 41 percent of our national energy consumption and nearly as much of our greenhouse gas emissions. With an eye toward conserving resources and mitigating climate change and its effects, the building industry is on the front lines of the effort to achieve sustainability goals and create buildings that not only drop jaws, but also lower carbon footprints.

Polyiso roof and wall insulation offers high unit R-value per inch, zero ozone depletion potential, and outstanding fire performance. In this photo, polyiso roof insulation is being installed on a flat roof. Photo: Hunter Panels

Sustainability is not a one-time event, but a process that encompasses the whole life cycle of a building. To effectively ensure that resource conservation spans that whole cycle requires transparency and coordination between stakeholders starting at the beginning of the design process to assess choices based on economy, durability, utility, and sustainability. Architects and specifiers need to have a complete picture of the merits of any product that might go into a building so they can make informed decisions that include impacts from a product’s manufacturing process to its long-term applied performance.

In the United States, the Leadership in Energy and Environmental Design (LEED) standards developed by the U.S. Green Building Council have emerged as an important benchmark for rating individual building components, processes, and systems. They are designed to:

  • Promote the efficient use of energy, water, and other resources.
  • Protect occupant health and improve productivity.
  • Reduce waste, pollution, and environmental degradation.
  • Improve resiliency in the face of extreme conditions.
The new Big Ten Headquarters in Rosemont, Illinois, utilizes polyiso wall insulation. Photo: Hunter Panels

Other notable programs across the globe and throughout North America, including the Building Research Establishment Environmental Assessment Method (BREEAM), Green Globes, US Department of Energy’s Energy Star program, GreenStar, and the Living Building Challenge, employ standards that are used in concert with LEED ratings to boost performance and promote a conscious approach to resource use in building construction, operation, and maintenance.

Many manufacturers are publishing rigorous, third-party verified evaluations of the whole life cycle impacts of their products to increase transparency and allow easier comparison of alternatives. These Environmental Product Declarations (EPDs) are similar to a “nutrition label” for building products and include information on sourcing, production, and performance of the products in a standardized and independently verified format that is recognized globally and based on International Organization for Standardization (ISO) standards. This consistent and scientific method to measuring and reporting information makes it possible to consider a product’s comprehensive impact and allows to base specification choices on measurable projections.

Earlier versions of LEED allowed manufactures to make claims about a product’s sustainability in one area without disclosing deficits in another area. This led to a healthy skepticism from clients and consumers about advertised merits and prompted a move toward greater transparency and verifiability. In its most recent revision, the LEED v4 standard asks manufacturers to provide more detailed information on a material’s content and its comprehensive environmental impact before their individual products can claim sustainability designations.

Basis for Evaluation

For an EPD to have a scientific basis, the impacts need to be clearly defined and linked to important environmental concerns. To help define these impacts the U.S. Environmental Protection Agency (EPA) developed TRACI, the Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts. TRACI methodology identifies a number of important factors related to critical environmental impacts:

  • Global Warming Potential (GWP)—linked to global climate change.
  • Ozone Depletion Potential (ODP)—related to the (now closing) hole in the earth’s ozone layer caused by certain chemicals.
  • Smog Creation Potential—linked to car exhausts, power plant emissions and fumes from products that contain volatile organic compounds (VOCs).
  • Acidification Potential—linked to acid rain caused by certain smokestack emissions.
  • Eutrophication Potential—linked to excessive amounts of nitrogen in rivers and lakes causing algae blooms that consume vital oxygen in the water.

Common Standards for Comparison

The EPD process is based on a framework to ensure that these practices are conducted in a consistent and reliable manner anywhere in the world. It includes the following key steps:

  • Product Category Rule – products with similar functions are assessed in the same way using comparable measures.
  • Life Cycle Assessment – products are evaluated based on inputs in the form of resources and energy, and outputs in the form of emissions and waste for their life cycle either from “cradle-to-gate” (from raw material extraction until it reaches the “gate” of the manufacturing facility) or the more rigorous “cradle-to-grave” (goes beyond the gate to include transportation, product manufacturing, use phase and the product’s end of life).
  • EPD generation – information from this assessment is organized into the standardized format for publication, including a life cycle diagram, illustration of product components, and a summary of impacts.
  • Third-party validation – outside experts verify and evaluate the report and the research that underlies it.

The widespread adoption of EPDs is fostering change in the building products industry leading to even more ambitious sustainability goals. As a growing body of EPDs are published, they contribute to a reliable catalog of data available as a reference point to help identify markets for new products and potential areas for improvement. Manufacturers can easily evaluate which steps in their products sourcing and production could be optimized for sustainability. Comparative information can serve as motivation for product innovation, leading to better options and better outcomes for the whole industry.

About the author: Marcin Pazera, Ph.D., is the Technical Director for Polyisocyanurate Insulation Manufacturers Association (PIMA). He coordinates all technical-related activities at PIMA and serves as the primary technical liaison to organizations involved in the development of building standards. For more information, visit www.polyiso.org.

EPDs Confirm the Benefits of Polyiso

Photo: Johns Manville

EPDs from the Polyisocyanurate Insulation Manufacturers Association (PIMA) report the results of an exacting “cradle-to-grave” Life Cycle Assessment showing the merits of polyiso insulation for wall and roof applications. The findings include:

  • The energy savings potential of polyiso roof and wall insulation over a typical 60-year building life span is equal to up to 47 times the initial energy required to produce, transport, install, maintain, and eventually remove and dispose of the insulation.
  • Polyiso has a high return on embodied energy.
  • Polyiso roof and wall insulation offers high unit R-value per inch, zero ozone depletion potential, recycled content, opportunity for reuse, and outstanding fire performance.
Photo: Johns Manville

Evaluation for the third-party assessment was done by PE International and includes a cradle-to-grave life cycle assessment that covers every step in the process of creating and using polyiso products. Looking at everything from resourcing, production, transport, installation, maintenance, to eventual removal and replacement, the EPD measures impacts across a broad spectrum, including everything from how products might contribute to global warming, smog production, and ozone depletion to the energy and water use and waste disposal required at the end of its life.

Primary data from six PIMA manufacturer members was used for the underlying life cycle assessment and the EPD represents the combined weighted average production for these members.

What Is Polyiso?

Polyiso is a closed-cell, rigid foam board insulation consisting of a foam core sandwiched between two facers. In wall applications its facers, which are usually made of kraft paper-backed foil, are adhered to both sides of the foam before it is cut into sheets and packaged for shipment and the boards function both as continuous insulation—creating a thermal barrier that isn’t interrupted at every stud—and as an environmental envelope to protect the building from water, air, and heat infiltration. It is typically attached outside the wall framing and covered by an exterior finish.

It is the most widely used insulating material for above-deck commercial roof construction in North America. The boards are installed in one or more layers, depending on the insulation needs, on the steel, wood, or concrete roof deck structure and then covered with the roofing membrane.

EPD Polyiso Findings

  • High thermal efficiency. Because it is one of the most thermally efficient building insulations available in today’s marketplace, Polyiso requires less total thickness to deliver specified R-value in roof and wall assemblies, reducing overall construction costs and increasing usable building space.
  • High net return on embodied energy. A recent study comparing initial embodied energy to long-term energy savings achieved over 60 years in a typical commercial building suggests that the net energy savings potential of Polyiso wall insulation ranges between 20 and 47 times the initial embodied energy required to produce, transport, and install the Polyiso insulation.
  • Zero ozone depletion potential. All PIMA Polyiso manufacturer members produce rigid foam board with third-generation, zero ozone-depleting blowing agents. The blowing agent (pentane) used in Polyiso also is among the lowest in Global Warming Potential.
  • Recycled content. Polyiso insulation typically is manufactured using recycled material. The percentage of the recycled material by weight depends on the individual manufacturer, the thickness of the product, and the type of facer.
  • Opportunity for reuse. Although this declaration assumes the Polyiso wall insulation boards will be landfilled at the end of the wall assembly service life, it is possible to salvage and reuse the boards, either at the original site or on another construction site. Used Polyiso wall insulation may be collected and resold by several national logistics firms.

PIMA is currently updating its EPDs for polyiso wall and roof insulations, which will be available in Q1 of 2020.