Go with the Flow: Tapered Insulation Fundamentals

Rainwater mitigation and insulating goals can both be achieved in low-slope or flat roofs with optimized, project-specific tapered polyisocyanurate insulation panel designs and systems.
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Sponsored by GAF
By Veronika Chwieroth

Learning Objectives:

  1. Assess the purpose of a tapered insulation system and how it works.
  2. Develop a basic comprehension of tapered polyisocyanurate insulation panels and how they affect roof performance: design options, crickets.
  3. Identify design intent and what key questions to answer when designing a tapered polyiso roof design.
  4. Recognize how IECC codes affect tapered polyiso insulation systems.



All images courtesy of GAF; credit: Tapered Design Group

Image of a roof with standing water.

Why is Drainage Necessary?

When someone learns that ‘tapered designer’ is an occupation, their usual reaction is asking “what does that mean?” A simple response is that a tapered designer’s role is to ensure that a roof does not become a swimming pool when it rains. A roof should not become a water feature! Architect Magazine reported that almost half of all building-related issues are due to water intrusion.1 Furthermore, 75 percent of construction defect disputes are related to roof failure.2 This is magnified in commercial roofing where a majority of buildings incorporate low-slope roofs, some completely flat. We are all familiar with the steep slopes seen in residential roofing that use gravity to handle rainwater, but when the roof decks do not provide adequate slope to the drain such as in low-slope or flat roofs, a special consideration needs to be taken to eliminate standing water by adding positive drainage with tapered insulation panels. This visual article will outline tapered design fundamentals to help architects and building designers achieve positive drainage in their low slope applications.

Figure 1: Image of an industrialized area of Secaucus, N.J.

Ensuring your building is watertight at the roof level and preventing water infiltration is only one benefit of a tapered insulation system. Other performance aspects such as R-value and compressive strengths will later be analyzed. Focus on the concentration of commercial roofs in the heavily industrialized area pictured. Whether those are storage or manufacturing facilities, office and retail spaces, even large residential apartment or condo complexes, their roofs cover significant square footage. Given the large footprint of commercial buildings, managing rainwater is the key component of tapered insulation design.

Standing water on a roof is a clear indicator that there isn’t positive drainage present to allow for water to evacuate the roof in a timely manner. The term “positive roof drainage” is defined in the 2018 International Building Code 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.”3 Even though a flat roof could be considered watertight, the potential for problems still may arise. Standing water promotes vegetation growth, and issues such as seam separation from material degradation or freezing damage increase the potential for that water to make its way into the system. A dangerous problem slow moving or ponding water also creates is added weight to the structure. To put it into perspective, one gallon of water weighs just over 8 pounds. If an inch of standing water is present on a roof, it can add thousands of pounds to the building structure and lead to life-safety concerns due to a roof collapse. Tapered insulation panels help evacuate rainwater by pointing it to drains or scuppers and creating positive drainage.

Tapered insulation systems work the same way as steep-slope shingle roofs- just with a much more gradual slope. Instead of inches per foot, the 2018 IBC requires a minimum slope of 1/4-inch per foot for membrane roof systems, and a 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, which outlines specifications for roof assemblies and rooftop structures. Recovers or roof replacements of existing low slope roofs are exempt from the minimum 1/4-inch per foot slope requirement. Understanding these constraints can allow for a more fine-tuned tapered design that achieves project-specific goals.

Tapered Polyisocyanurate

Polyisocyanurate, referred to as polyiso in short, makes up over eighty percent of the commercial roofing market share. It has the highest thermal efficiency in terms of R-value at 5.7 per inch. R-value is the capacity for an insulating material to resist heat flow. The higher the R-value, the greater the insulating value. Expanded polystyrene, or EPS, is a popular alternative, but since it has less thermal efficiency, you have to use more of it to compensate for energy losses. Polyiso has very low ozone depleting potential as well as very low global warming potential. It can use recycled content in the facer. It is free of hydrofluorocarbon, an ozone depleting chemical, and can even contribute towards Leadership in Energy and Environmental Design, or LEED, a global green building rating. With 20 polyiso manufacturing facilities nationwide, material is readily available and allows for a great variation of design flexibility to meet job requirements. There are different panel facers available such as felt or recycled and coated glass. Felt or recycled content facers are standard but do not have the reinforced strength of coated glass. Coated glass facers are made by mixing fiberglass in a slurry and gives polyiso panels added strength against job-site damage and greater resistance to moisture intrusion, all leading to longer assembly life cycles. There are also different formulations like non-halogenated which eliminate halogenated flame retardants in its chemical makeup. Halogenated flame retardants are on the Red List, a list of worst-in class chemicals and materials identified for their risk to the environment.4 This allows the range of non-halogenated polyisocyanurate panels to be categorized as Red List free and meet even greater ecological initiatives. Polyiso is a versatile material successfully used to meet code approval requirements for FM (Factory Mutual) and UL (Underwriter’s Laboratory), among others.

Tapered polyisocyanurate is available in thicknesses from 1/2 to 4.5 inches, however, smoothing the transition to a height of 0 inches cannot be reached with these panels. There are other products on the market like tapered edge strips that can be utilized to aid the transition to the roof deck. Tapered panel systems are available in slopes from 1/16-inch per foot to 1/2-inch per foot. The standard panel size is 4 feet by 4 feet, and a panel size of 4 feet by 8 feet is available if specified (and may require longer lead times to fulfill). There are also different compressive strengths that can be catered for specific applications such as amenity spaces with paver systems. Tapered polyiso is compatible with all roofing systems including built-up roofing, ballasted, and modified bitumen. It can be used with coverboards and also can be directly installed to the deck since it is the only foam plastic to pass UL 1256, a fire abatement standard for fire testing of a roof deck construction and FM 4450, qualifying tapered polyiso as a Class I assembly component when part of an insulated steel deck.5 This is because it has “low fire spread below the deck, adequate simulated wind uplift resistance, minimum heat damage potential and adequate strength” at the conclusion of FM’s fire testing.6

Figure 2: A collection of tapered panel system profiles.

Slope is the angle at which the ends of the tapered panel differ, in inches per foot. There is a thinner leading edge and a thicker back edge. A Q-panel, for example, is part of the 1/2-inch per foot tapered slope system with a 1/2-inch leading edge and ends at 2.5 inches. Remembering that 4 feet by 4 feet is the standard panel size, it gains 2 inches in height across the length of the panel. It is possible to solve and confirm the slope at hand by dividing the rise over the run, or 2 inches divided by 4 feet, equaling and confirming 1/2-inch per foot slope. A tapered insulation system includes flat fill insulation used under the tapered panels to build the sequence. This is called a tapered panel repeat- the amount of tapered panels installed before a flat fill panel is required. Base layers are flat insulation panels installed in conjunction with the tapered system that help the system reach a certain insulating property.

Tapered panel profiles are a sequence map for the contractor and roofing installer that shows what order the tapered insulation panels will be installed. Most of the panel systems use a combination of single and double letters to distinguish slope and repeat, while 1/16-inch per foot is labeled numerically and is mainly used to enhance existing slope and should not be used as the sole field slope. 1/8-inch per foot is a common slope used for reroof applications with profiles from AA - FF. We have the National Roofing Contractors Association, or NRCA standard of 1/4-inch per foot which is available in two different profiles (X-ZZ, or G-I panels) depending on the minimum thickness requirements. Three-Eighths of an inch per foot (SS, TT panels) is a slope that is favored in the west and mountain regions for swift drainage, as areas have to manage sudden, large quantities of rain. Half of an inch per foot (Q, QQ panels) is commonly used for crickets and drain sumps. Sumps are an area of exaggerated slope within a certain distance of the drain or scupper and can be designed with tapered panels, or field-fabricated as needed. A system at 1/2-inch per foot can quickly accumulate tall material heights in short distances so it is also not typically used as the field slope. There are other manufactured slopes which are uncommon such as 3/16-inch per foot, and overall slopes that can be fabricated on-site by stacking panels together to achieve a desired total slope.

The amount of panels applied before flat fill insulation is required is a repeat. The system repeat can go anywhere from one to eight panels depending on the slope being used and whether it is a standard or extended system. A lower repeat, typically a standard system, equals a lower material cost but also means more squares of material to be handled and applied to the roof. A higher system repeat is associated with extended panel systems and leads to an overall higher material cost, because the system comprises thicker panels. However, extended panel systems can reduce squares handled and applied up to forty percent. These savings are directly related to labor costs as well as adhesive costs in adhered applications, which far exceed the nominal material upcharge of the extended panels.

Figure 3: Examples of tapered design solutions.


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Originally published in October 2022