Go with the Flow: Tapered Insulation Fundamentals
Case Study: Minimum R30 Requirement with a ¼ Inch Per Foot Tapered System
Figure 9: A breakdown of a tapered insulation assembly.
Achieving R-value is confusing and it is typical to see design requests for only the tapered component of the assembly. Eliminating the layers of flat base material needed to reach project-specific insulating values from the design request allows building designers to take control of specifying the assembly. They prefer this over collaborating with a tapered designer because they may not know who to reach out to for guidance. Calculating the R-value of flat stock is easy when that information is published by panel thickness, but the same cannot be said for a tapered design. The design and layout is not something contractors, architects and building designers can estimate themselves without a dedicated team or knowledge of how to design with tapered panels. They build the minimum assembly needed to satisfy insulation requirements with flat base layer panels, ignoring that tapered panels to be added later can also add to the equation.
Working with a tapered designer can greatly impact the insulation assembly because of their regional expertise. In the case of a tapered design request for a 1/4-inch per foot system with a minimum R-value of 30, the segregated request would typically be built as shown on the left in Figure 9. The example shows an insulation assembly with two layers of 2.6-inch flat stock under the tapered system, identified as a standard two-panel repeat 1/4-inch per foot system using 2-inch thick flat fill panels.
Each 2.6-inch thick base layer has an R-value of 15 and the assembly satisfies the minimum R30 requirement before figuring in the tapered system. In actuality, the segregated request exceeds the minimum requirement after including the 1/2-inch tapered start of the X-panel. The 1/2-inch leading edge of an X-panel carries an R-value of 2.8—this is the only guaranteed thickness throughout the panel. The starting assembly height of 5.7 inches has a true minimum R-value of 32.8 and while it will satisfy code requirements since it exceeds the minimum, this assembly can add unnecessary height and expense to a tapered design.
By considering and including the tapered panel into the R-value equation, it is possible to reduce the base layer thickness to fine tune the minimum requirements. In the example assembly shown on the right of Figure 9, the base layers have been reduced to two 2.4-inch thick base layers which have an R-value of 13.4 each. This allows the assembly to reach a more exact R30 solution of R30.4 and reduce the overall start and end heights when compared to the segregated request. A plus of utilizing a tapered design professional is their knowledge of regional material availability and preference. A flat board thickness of 2.4-inches is not typical, so the tapered panel assembly can be adjusted based on local supply chains.
Figure 10: A tapered insulation assembly that reconsiders the tapered start height and marries flat panel inventory.
The insulation assembly can be optimized further by varying the tapered panel start with a thicker panel in the sequence and applying a higher R-value to the equation. This can allow the assembly to be shuffled in a way that marries inventory being shipped to the jobsite. Shifting the tapered start to the next panel in the sequence uses a Y-panel with a 1.5-inch leading edge and carries an R-value of 8.7. The base layers can be reduced to just two layers of 2-inch flat stock, where each layer has an R-value of 11.4. This assembly equals an R-value of 31.5 and exceeds the design minimum requirements, but matching inventory from base layers to flat fill eliminates varying thicknesses of flat panels that need to be handled and organized on the jobsite. Shuffling the assembly this way also can allow for the roofing professional to use existing material they have on hand versus having to order new stock to fulfill an order. As identified towards the end of this assembly, a shifted tapered panel start may increase squares handled and applied because it may introduce more flat fill at the taller ends of the system. The same methodology can be applied to extended panel tapered systems that use thicker flat fill panels in their panel repeats. Utilized in adhered installations, the thicker panels of extended systems can eliminate layers of material applied to the roof and simplify the assembly overall.
Figure 11: Graphic portraying the 2015 IECC one inch variation exception.
Drafting the best tapered insulation design starts and ends with the ability to understand how to build the proper assembly needed for the best performance- based on local code allowances. Tapered designers have regional insight to what IECC code the local authority having jurisdiction has adopted. The 2015 IECC, if implemented into local code, includes a 1-inch variance exception when trying to reach a minimum R-value.8 Simply, this rule allows for a 1-inch difference in insulation height between the lowest point and a given distance, based on the tapered slope. At 1/4-inch per foot slope, that 1-inch difference in height is met in 4 feet. In a 1/8-inch per foot system, that 1-inch difference in height is reached 8 feet from the drain or lowest point. This allows the assembly to be reduced in height even more because the minimum prescriptive R-value does not have to be designed at the lowest point.
Figure 12: A comparison between a segregated tapered insulation assembly and one that has been optimized for the region and project details.
Applying the 1-inch rule to the initial assembly request of 1/4-inch per/foot slope with a minimum R-value of 30, along with the previous lessons of marrying inventory and including the tapered panel start height into the R-value equation, allows the start height to drop from 5.7 inches in the segregated request to just 4.5 inches in the optimized assembly. The assembly start height as well as its max height have been drastically reduced by over an inch—this reduction can be crucial for projects with height limitations. In a world where cents are treated like dollars, each 10th of an inch of material has a very real cost. Reducing over an inch of material across an entire roof footprint also reduces significant added cost to the budget.
Coverboards such as gypsum have a high compressive strength and do not have any insulating property that can be applied to the insulation assembly. However, coverboards made of high-density polyisocyanurate are designed to have a high compressive strength such as 80 or 110 PSI and also have an R-value that can be applied to the overall insulation assembly. If the project resides in a region where all the aforementioned lessons in building the insulation assembly can be applied with the addition of a high-density polyiso cover board, it allows the assembly to be fine-tuned to a level that is not typically considered.
A reduction in material height of over 1-inch is critical to all parties involved in the roofing assembly, from the building owner to the architect specifying heights, to the roofing professionals tasked with installing the design. Tapered insulation systems offer a cost-effective solution to achieving positive slope and improved drainage in new roof systems and existing roof replacement applications. These panels work in conjunction with each other to create a positive slope on an otherwise flat roof. An efficient rainwater mitigation strategy that includes both proper drainage and elimination of ponding water is critical to the long-term performance and durability of a roof system. And by adding positive drainage with tapered panels, the building’s roof can be insulated to the local code and project requirements. Quality is key—proper design, detailing, and installation of products must be part of the tapered roof system design to prevent water infiltration because of ponding water that can shorten the service life of the membrane. The best protection of investment is to utilize a tapered design professional who can give regional expertise and insight to help build the best design as well as insulation assembly for the project goals at hand.
When it comes time to specify tapered polyisocyanurate insulation system in a project specification, it is important to remember three key goals.
- Do you have an overall R-value you wish to achieve in the system?
- What is the desired slope you want on your roof?
- And most importantly, what is the performance expectation of the tapered roof system vs budget to shape the best solution for the client?
Veronika Chwieroth, is a Senior Sales Design Specialist for GAF with expert regional insight for tapered insulation systems, and an alumna of Parsons School of Design.
End Notes
1Seward, Aaron. Architect Magazine, “When It Leaks It Pours,” June 2011.
2McCloskey, Christopher L., Bricker Construction Law, “Defective Work: What to do when the roof leaks?,” Summer 2011 Newsletter.
3 2018 International Building Code (IBC).
4 International Living Future Institute, LBC Red List.
5 UL 1256, 4th Edition, January 12, 2007—UL Standard for Safety Fire Test of Roof Deck Constructions.
6 FM Approval 4450, Class I Insulated Steel Roof Decks, February 1, 1989.
7 2021 International Energy Conservation Code, Chapter 4 Commercial Energy Efficiency.
8 2015 International Energy Conservation Code, Section C402.2.2 Roof Assembly, 1-inch Rule.
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The Building and Roofing Science team offers regional expert building enclosure collaboration through design, specification, and educational support for customers of GAF and Siplast, both Standard Industries companies. GAF is North America’s largest roofing manufacturer with more homes and businesses in the U.S. protected by a GAF roof than any other product. Siplast, a leader in building enclosure systems, offers a portfolio of advanced, high-performance SBS-modified bitumen, PMMA liquid-applied, PVC KEE, lightweight insulating concrete, wall air & water barrier systems, and amenity/vegetated systems. |
