RoofViews

Building Science

Parapets Part 2: Navigating Codes

By Benjamin Meyer

January 24, 2020

Two men on a parapet looking at their phones

Part 1 of our discussion of parapets (Continuity of Control Layers) explored the many reasons continuity of water, air, thermal, and vapor control layers are necessary for long term performance.

In Part 2, we're discussing the challenges involved in navigating the range of national model codes and standards that will influence your design. Codes under discussion include the 2018 International Building Code (IBC), the 2018 International Energy Efficiency Code (IECC), and the ANSI/ASHRAE/IES Standard 90.1-2016 (ASHRAE 90.1).


The summary provided in this article is not intended to be an exhaustive list of requirements for exterior wall and roof systems in the referenced national model building and energy codes. Different versions of the referenced codes have additional and/or different requirements; these requirements may also vary by adoption and modification by the local authority having jurisdiction. It is important to refer to local codes for the applicable requirements.


The requirements for parapets generally come from the building code (IBC) and the energy code (IECC and ASHRAE 90.1). The requirements within the building and energy codes can be mandated prescriptively, as a performance threshold, or by reference through specific key standards. The performance standards are important because they don't attempt to regulate by providing exhaustive lists and itemized component requirements, like a prescriptive method. These performance requirements establish the design benchmark and then provide a methodology to demonstrate compliance with the benchmark.

The building codes and standards do not always address parapets exclusively, but many refer to "Exterior Walls" separately from "Roof Assemblies".

Summarized applicable code references for parapets.


Exterior Walls in the Building Code

The exterior wall requirements for parapets are covered in Chapter 14 of the IBC which addresses "exterior walls, wall coverings, & components." For parapets, the requirements for weather protection, water-resistive barriers (WRBs), managing vapor, and flashing apply as they do for the rest of the exterior building walls.

IBC Chapter 14: Exterior Wall applicable area highlighted in blue.


Exterior Wall Flashing

Flashing is very important and is generally repeated in both the wall (IBC 1404.4) and roof provisions of the code. The IBC includes the principle that "flashing shall be installed… to prevent moisture from entering the wall or to redirect that moisture to the exterior." This is an important starting point for parapet design where the sequencing can be a challenge among numerous wall- and roof-system contractors.

While not an exhaustive list, IBC 1404.4 includes a minimum list of areas requiring exterior wall flashing. These are summarized below:

  • Penetrations and terminations
  • Intersections with roofs, chimneys, porches, decks, balconies and similar projections
  • Built-in gutters and similar locations where moisture could enter the wall
  • Flashing with projecting flanges, installed on both sides and the ends of copings

At all of the prescriptive flashing locations listed in the IBC, the purpose is two-fold. The first is for the flashing to be installed in a way that prevents water from entering the wall system. This concept is known as "shingle fashion," or installing components of the roof, exterior wall, and parapet "such that upper layers of material are placed overlapping lower layers of material to provide for drainage via gravity and moisture control" (IBC 202). Logistically, this is best accomplished onsite by applying materials from the bottom of the building to the top, so the next progressive layer or system is then lapped correctly.

The second, and more challenging flashing requirement, is to also be installed in a manner that permits water to exit the wall system if it enters incidentally. This requires the parapet to be designed with a method and pathway for water to drain from the flashing, even from behind the cladding (think weep holes at masonry shelf angles). In addition to providing a means for drainage, the IBC also includes a drainage scenario to avoid exterior wall pockets (1404.4.1). Wall pockets or crevices are locations within a wall assembly "in which moisture can accumulate." These scenarios can be common in parapets where the exterior wall, roof, and parapet wall above might not always be in alignment. In parapets, these wall pockets should be avoided or protected with appropriate flashing for the application.

Exterior Wall Weather Protection

The Weather protection section (IBC 1402.2) requires that the exterior wall "shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly". One of the methods prescribed in this section is to include a secondary water management layer, or "water-resistive barrier" (WRB), behind the exterior cladding in the exterior wall portion of a parapet. Beyond including the WRB layer, "a means for draining water that enters the assembly to the exterior" must also be provided in the parapet wall design. There are exceptions to the secondary WRB and drainage requirements provided in the IBC for concrete and specifically tested systems, but the benefits for designed water control is applicable for all construction types.

Exterior Wall Vapor Retarders

In exterior parapet walls, protection against condensation is also required to be compliant with the vapor retarders portion (IBC 1404.3). Vapor retarder materials are separated into three classes by ASTM E96 testing (Procedure A, desiccant method):

  • Class I: 0.1 perm or less;
  • Class II: 0.1 < perm="" ≤="" 1.0="">
  • Class III: 1.0 < perm="" ≤="" 10="">

The vapor retarder classes are referenced in the IBC to identify by climate zone if a material is permitted in the assembly in a prescriptive manner (IBC 1404.3.1 and 1404.3.2). It is important to note that all materials have vapor retarding properties to some degree and may limit vapor transmission without the addition of a dedicated vapor control layer. This is also why the IBC includes the alternate performance compliance of providing a "design using accepted engineering practice for hygrothermal analysis" as described in the initial language of 1404.3.

In most cases, if a vapor control layer is needed, it is a good idea to select a vapor retarder that will allow some amount of drying from diffusion. High-humidity interior environments such as natatoriums, manufacturing facilities, and grow houses may require a vapor barrier for long-term performance. However, the decision of whether or not to add a vapor control layer to a roof assembly is normally based on risk and is best made with a building enclosure consultant. The weather protection and vapor retarding sections of the IBC apply to exterior walls, but parapets may have very different design and performance requirements than the wall assembly below the roof. That is why it is important to maintain continuity of the four control layers at this interface.

Roof Assemblies in the Building Code

The roofing portion for parapets is covered in Chapter 15 of the IBC which addresses Roof assemblies, specifically the "design, materials, construction and quality" of roofs. Regarding parapets, the roof system requirements impact the wall where terminations and transitions occur. The requirements include weather protection, flashing, coping, wind resistance design, edge securement, and specific requirements for various types of roof coverings.

IBC Chapter 15: Roof Assembly applicable area highlighted in blue.


Roof Assembly Weather Protection

The requirements for weather protection (IBC 1503) are fairly broad, requiring roof decks covered with approved roof coverings. Much more detail is covered in the additional IBC sections regarding roofing and parapets. In the roofing provisions, it is important to note that compliance with "the manufacturer's approved instructions" doesn't just affect a project's eligibility for warranty, but is also required for building code compliance.

Roof Assembly Flashing

The requirements for flashing (IBC 1503.2) are repeated in part across the wall and roof portions of the code. This repetition highlights the importance of managing water control at the transitions. The code requirements for both roofs and walls support the water control layer principles in the pen test discussed previously. The roofing chapter in the code also directly mentions the parapet walls as a critical location for both roof system transition flashing and requirements for copings. While not an exhaustive list, IBC 1503.2 includes a minimum list of areas requiring roof flashing. These are summarized below:

  • Flashing joints in copings
  • At moisture-permeable materials
  • At intersections with parapet walls
  • At other penetrations through the roof plane

Roof Assembly Coping

The roof requirements for parapet wall copings are spread across many categories. One section specific to copings (IBC 1503.3) has a limited scope, requiring materials to be limited to "noncombustible, weatherproof materials" and be installed with a "width not less than the thickness of the parapet wall". Many other requirements in the code also apply to copings in the code, such as flashing, wind design loads, and edge securement performance. More will be discussed about copings in those sections.

Roof Wind Resistance

The wind resistance for low-slope commercial roof decks and roof coverings (IBC 1504.1) is required to be designed in accordance with IBC 1609.5, which ultimately leads to utilizing ASCE 7 for determining design wind loads. There are numerous updates to ASCE 7 – 2005, 2010, or 2016 – and each has its own nuance as to how it impacts roof design loads (more here about design wind loads). Because ASCE 7 is a performance standard, it is possible to use a version with higher performance requirements because designs do not need to be the minimum allowance. Parapets are a combination of wall and roof pressures. The exact height of the parapet is not factored into the roof wind uplift calculations, but if the parapet is 3' or higher, the perimeter values can be used at the corners, lowering the uplift requirements for that portion of the roof area.

Parapets can help reduce wind uplift at the corners and perimeter


Roof Edge Securement

Securing the edges on low-slope roofs (IBC 1504.5) has a significant impact on preventing failure and allowing the roof system to resist loads as it was designed. In addition to designing the wind resistance performance for the entire building (i.e., walls, roofs, and parapets) per ASCE 7, metal roof edges are required to be tested for resistance in accordance with Test Methods RE-1, RE-2 and RE-3 of ANSI/SPRI ES-1. The referenced standard ANSI/SPRI ES-1 is a performance requirement that is specific to the strength of metal roof edges (more here about roof edge performance compliance). ES-1 covers the "baseline" flush roof edge as well as parapet coping caps. When designing, it is important to specify compliance with ES-1 in the construction documents.

Roof Coverings

The IBC provides minimum installation criteria (IBC 1507) for various roof systems, based specifically on the attributes of that roof covering. In addition to the prescriptive criteria listed within, the IBC also mandates that "Roof coverings shall be applied in accordance with the… manufacturer's installation instructions." Generally, the content of these roof covering sections address minimum substrate requirements, minimum roof slope, ballast requirements, and relative ASTM references to material standards, such as D6878 Standard Specification for Thermoplastic Polyolefin (TPO) Based Sheet Roofing.

Energy Efficiency for Parapets

Generally, within the IBC it is required that a building be "designed and constructed in accordance with the International Energy Conservation Code (IECC) 1301.1.1". The IECC has both residential and commercial provisions, and the commercial provisions apply to "all buildings except for residential buildings 3 stories or less in height." The IECC is structured in a way that provides the option of either complying with the prescriptive requirements within it or by complying with the alternate ASHRAE 90.1 energy standard.

Compliance Alternatives

The IECC has multiple compliance paths within it, including:

  1. Either following the prescriptive requirements within the IECC or ASHRAE 90.1, or
  2. Following the performance modeling requirements of ASHRAE 90.1 Appendix G.

The prescriptive options within both the IECC and the reference standard ASHRAE 90.1 primarily regulate energy use by providing lists and itemized requirements. These can be helpful when the building is straightforward and tradeoffs don't need to be made. When a building is more complex, has specific energy usage demands, or if an owner wants to demonstrate energy compliance beyond code, the performance path within ASHRAE 90.1 Appendix G is the methodology required. For example, any modeling being performed to show compliance with LEED is being performed to comply with Appendix G in ASHRAE 90.1. A growing method of compliance is whole-building design energy modeling and onsite performance testing happening in new construction. When an existing building is reroofed, the designer will most likely follow the prescriptive path to determine the amount of insulation to use.

Insulation

The insulation requirements in the table include both cavity and continuous insulation, but vary based on the framing material (IECC C301.1 & 90.1 Annex 1). Including continuous insulation in both the walls and roof systems of the parapet helps manage thermal bridging across the assemblies. The prescriptive tables in the energy codes dictate minimum R-Values in the roofs and walls based on the climate zone of the project site, the building use, and the framing materials of the wall and roof system. As described earlier in the thermal control discussion, the framing materials matter in the prescriptive requirements, especially when insulation is placed between framing members in parapet cavities.

For more complex details like a parapet, the energy code doesn't get into separate requirements for the insulation. The codes generally require that continuous insulation be depicted in the construction documents with sufficient clarity to indicate the location, extent of the work, and show sufficient detail for continuity of the thermal control layer. Per the IECC (C103.2), insulation continuity for complex conditions should be shown in the details.

Air Barrier

ASHRAE 90.1 defines a Continuous Air Barrier as a "combination of interconnected materials, assemblies, and sealed joints and components which together minimize air leakage into or out of the building envelope." It's a good definition and an accurate description of what is needed to have a completed building enclosure that minimizes air leakage (IECC C402.5 & 90.1 5.4.3.1). Actual air leakage for a building is measured by pressurizing the enclosure with a set of blowers and measuring the airflow through the blowers to determine the air leakage through the enclosure being tested, on all 6 sides. Materials and assemblies used as a part of the building's continuous air barrier are generally tested by the manufacturers of those materials and systems to comply when installed in accordance with the manufacturer's instructions for that application.

The ultimate goal of airtightness is whole-building performance. To help accomplish that goal, the energy code also specifies aspects of air barrier design (IECC C103.2 & 90.1 5.4.3.1.1) and installation (IECC C402.5.1.1 & 90.1 5.4.3.1.2) for continuity across joints, penetrations, and assemblies. Below is a brief summary of the design and installation requirements from ASHRAE 90.1:

  • Air Barrier Design
    • Components, Joints and Penetrations details
    • Extending over all surfaces, including the roof
    • Resist pressures from wind, mechanical, stack effect
  • Air Barrier Installation
    • Junctions between walls and roofs or ceilings
    • Penetrations at roofs, walls, and floors
    • Joints, seams, and connections between planes
    • In accordance with the manufacturer's instructions

Code Summary

For the various applicable codes and standards, in both roofs and walls, weather protection and flashing are important requirements at all transitions and penetrations, including parapet conditions. It is vital to specify key reference standards for wind and edge securement, in order to achieve the performance needed to keep the roof on the building as intended.

In general, the energy codes require continuity of the thermal and air control layers. Detailing the thermal control and air barriers to be continuous in the design AND field installation are critical for energy code compliance.


For more information on parapet and control layer continuity, register for the Continuing Education Center webinar, Parapet Predicaments and Roof Edge Conundrums, sponsored by GAF and presented by Jennifer Keegan, AAIA and Benjamin Meyer, AIA, LEED AP.


For more information on parapet and control layer continuity, read the Continuing Education Center article, Parapets—Continuity of Control Layers, sponsored by GAF and written by Benjamin Meyer, AIA, LEED AP.


About the Author

Benjamin Meyer, AIA, LEED AP is a Roofing & Building Science Architect with GAF. Previous experience includes: enclosure consultant principal, technical management for enclosure products, commercial design, real estate development and construction management on a range of projects that included residential, educational, offices, and DuPont industrial projects. Industry positions include: Voting Member of the ASHRAE 90.1 Envelope and Project Committees, LEED Technical Committee member, past Technical Advisor of the LEED Materials (MR) TAG, and Director of the Air Barrier Association of America (ABAA).

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A range of codes and standards are beginning to address this problem, though it's important to note that there is often a time lag between development of codes and their widespread adoption.What Is the Industry Doing About It?Long in the business of supporting high-performance building enclosures, Phius (Passive House Institute US) provides a Fastener Correction Calculator along with a way to calculate the effect of linear thermal bridges (think shelf angles, lintels, and so on). By contrast, the 2021 International Energy Conservation Code also addresses thermal bridging, but only considers framing materials to be thermal bridges, and actually pointedly ignores the effects of point loads like fasteners in its definition of continuous insulation: "insulation material that is continuous across all structural members without thermal bridges other than fasteners and service openings" (Section C202). Likewise, The National Energy Code of Canada for Buildings: 2020 addresses thermal bridging of a number of building components, but also explicitly excludes fasteners: "in calculating the overall thermal transmittance of assemblies…fasteners need not be taken into account" (Section 3.1.1.7.3). Admittedly, point thermal bridges are often excluded because it is challenging to assess them with simple simulation tools.Despite this, researchers have had a hunch for decades that thermal bridging through the multitude of fasteners often used in roofs is in fact significant enough to warrant study. Investigators at the National Bureau of Standards, Oak Ridge National Laboratory, the National Research Council Canada, and consulting firms Morrison Hershfield and Simpson Gumpertz & Heger (SGH), have conducted laboratory and computer simulation studies to analyze the effects of point thermal bridges.Why Pay Attention Now?The problem has been made worse in recent years because changes in wind speeds, design wind pressures, and roof zones as dictated by ASCE 7-16 and 7-22 (see blogs by Jim Kirby and Kristin Westover for more insight), mean that fastener patterns are becoming denser in many cases. This means that there is more metal on average, per square foot of roof, than ever before. More metal means that more heat escapes the building in winter and enters the building in summer. By making our buildings more robust against wind uplift to meet updated standards, we are in effect making them less robust against the negative effects of hot and cold weather conditions.So, how bad is this problem, and what's a roof designer to do about it? A team of researchers at SGH, Virginia Tech, and GAF set out to determine the answer, first by simplifying the problem. Our plan was to develop computer simulations to accurately anticipate the thermal bridging effects of fasteners based on their characteristics and the characteristics of the roof assemblies in which they are used. In other words, we broke the problem down into parts, so we could know how each part affects the problem as a whole. We also wanted to carefully check the assumptions underlying our computer simulation and ensure that our results matched up with what we were finding in the lab. The full paper describing our work was delivered at the 2023 IIBEC Convention and Trade Show, but here are the high points, starting with how we set up the study.First, we began with a simple 4" polyisocyanurate board (ISO), and called it Case A-I.Next, we added a high-density polyisocyanurate cover board (HD ISO), and called that Case A-II.Third, we added galvanized steel deck to the 4" polyiso, and called that Case A-III.Finally, we created the whole sandwich: HD ISO and ISO over steel deck, which was Case A-IV.Note that we did not include a roof membrane, substrate board, air barrier, or vapor retarder in these assemblies, partly to keep it simple, and partly because these components don't typically add much insulation value to a roof assembly.The cases can be considered base cases, as they do not yet contain a fastener. We needed to simulate and physically test these, so we could understand the effect that fasteners have when added to them.We also ran a set of samples, B-I through B-IV, that corresponded with cases A-I through A-IV above, but had one #12 fastener, 6" long, in the center of the 2' x 2' assembly, with a 3" diameter insulation plate. These are depicted below. The fastener penetrated the ISO and steel deck, but not the HD ISO.One visualization of the computer simulation is shown here, for Case B-IV. The stripes of color, or isotherms, show the vulnerability of the assembly at the location of the fastener.What did we find? The results might surprise you.First, it's no surprise that the fastener reduced the R-value of the 2' x 2' sample of ISO alone by 4.2% in the physical sample, and 3.4% in the computer simulation (Case B-I compared to Case A-I).When the HD ISO was added (Cases II), R-value fell by 2.2% and 2.7% for the physical experiment and computer simulation, respectively, when the fastener was added. In other words, adding the fastener still caused a drop in R-value, but that drop was considerably less than when no cover board was used. This proved what we suspected, that the HD ISO had an important protective effect against the thermal bridging caused by the fastener.Next, we found that the steel deck made a big difference as well. In the physical experiment, the air contained in the flutes of the steel deck added to the R-value of the assembly, while the computer simulation did not account for this effect. That's an item that needs to be addressed in the next phase of research. Despite this anomaly, both approaches showed the same thing: steel deck acts like a radiator, exacerbating the effect of the fastener. In the assemblies with just ISO and steel deck (Cases III), adding a fastener resulted in an R-value drop of 11.0% for the physical experiment and 4.6% for the computer simulation compared to the assembly with no fastener.Finally, the assemblies with all the components (HD ISO, ISO and steel deck, a.k.a. Cases IV) showed again that the HD ISO insulated the fastener and reduced its negative impact on the R-value of the overall assembly. The physical experiment had a 6.1% drop (down from 11% with no cover board!) and the computer simulation a 4.2% drop (down from 4.6% with no cover board) in R-value when the fastener was added.What Does This Study Tell Us?The morals of the study just described are these: Roof fasteners have a measurable impact on the R-value of roof insulation. High-density polyisocyanurate cover boards go a long way toward minimizing the thermal impacts of roof fasteners. Steel deck, due to its high conductivity, acts as a radiator, amplifying the thermal bridging effect of fasteners.What Should We Do About It?As for figuring out what to do about it, this study and others first need to be extended to the real world, and that means making assumptions about parameters like the siting of the building, the roof fastener densities required, and the roof assembly type.Several groups have made this leap from looking at point thermal bridges to what they mean for a roof's overall performance. The following example was explored in a paper by Taylor, Willits, Hartwig and Kirby, presented at the RCI, Inc. Building Envelope Technology Symposium in 2018. In that paper, the authors extended computer simulation results from a 2015 paper by Olson, Saldanha, and Hsu to a set of actual roofing scenarios. They found that the installation method has a big impact on the in-service R-value of the roof.They assumed a 15,000-square-foot roof, fastener patterns and densities based on a wind uplift requirement of 120 pounds per square foot, and a design R-value of R-30. In this example, a traditional mechanically attached roof had an in-service R-value of only R-25, which is a 17% loss compared to the design R-value.An induction-welded roof was a slight improvement over the mechanically attached assembly, with an in-service value of only R-26.5 (a 12% loss compared to the design R-value).Adhering instead of fastening the top layer of polyiso resulted in an in-service R-value of R-28.7 (a 4% loss compared to the design R-value).Finally, in their study, an HD polyiso board was used as a mechanically fastened substrate board on top of the steel deck, allowing both layers of continuous polyiso insulation and the roof membrane to be adhered. Doing so resulted in an in-service R-value of R-29.5, representing only a 1.5% loss compared to the design R-value.To operationalize these findings in your own roofing design projects, consider the following approaches: Consider eliminating roof fasteners altogether, or burying them beneath one or more layers of insulation. Multiple studies have shown that placing fastener heads and plates beneath a cover board, or, better yet, beneath one or two layers of staggered insulation, such as GAF's EnergyGuard™ Polyiso Insulation, can dampen the thermal bridging effects of fasteners. Adhering all or some of the layers of a roof assembly minimizes unwanted thermal outcomes. Consider using an insulating cover board, such as GAF's EnergyGuard™ HD or EnergyGuard™ HD Plus Polyiso cover board. Installing an adhered cover board in general is good roofing practice for a host of reasons: they provide enhanced longevity and system performance by protecting roof membranes and insulation from hail damage; they allow for enhanced wind uplift and improved aesthetics; and they offer additional R-value and mitigate thermal bridging as shown in our recent study. Consider using an induction-welded system that minimizes the number of total roof fasteners by dictating an even spacing of insulation fasteners. The special plates of these fasteners are then welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for additional membrane fasteners. Consider beefing up the R-value of the roof insulation. If fasteners diminish the actual thermal performance of roof insulation, building owners are not getting the benefit of the design R-value. Extra insulation beyond the code minimum can be specified to make up the difference.Where Do We Go From Here?Some work remains to be done before we have a computer simulation that more closely aligns with physical experiments on identical assemblies. But, the two methods in our recent study aligned within a range of 0.8 to 6.7%, which indicates that we are making progress. With ever-better modeling methods, designers should soon be able to predict the impact of fasteners rather than ignoring it and hoping for the best.Once we, as a roofing industry, have these detailed computer simulation tools in place, we can include the findings from these tools in codes and standards. These can be used by those who don't have the time or resources to model roof assemblies using a lab or sophisticated modeling software. With easy-to-use resources quantifying thermal bridging through roof fasteners, roof designers will no longer be putting building owners at risk of wasting energy, or, even worse, of experiencing condensation problems due to under-insulated roof assemblies. Designers will have a much better picture of exactly what the building owner is getting when they specify a roof that includes fasteners, and which of the measures detailed above they might take into consideration to avoid any negative consequences.This research discussed in this blog was conducted with a grant from the RCI-IIBEC Foundation and was presented at IIBEC's 2023 Annual Trade Show and Convention in Houston on March 6. Contact IIBEC at https://iibec.org/ or GAF at BuildingScience@GAF.com for more information.

By Authors Elizabeth Grant

November 17, 2023

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