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

As unique as humans are, the same can be said for the built environment we work in. Each building has its own story, shape, function, and there is no one-size-fits-all solution when designing with tapered panels. A possible design for a roof with internal drains is to point the tapered panels in a way to create a single valley between the drains, connecting them. A two-way slope is straightforward to install and preferred by contractors for its minimal cutting waste as more full panels can be utilized and laid out in straight lines. However, this design can allow for water to sit in the valley and against the roof perimeter without being able to evacuate in a timely manner. There is a design element that can be added to increase drainage: crickets. Crickets are built with cut tapered panels and are an overlay to the tapered system. Their purpose is to help divert water to the drain by stretching out the flat valley they cover into separate, smaller, lower-sloping valleys at the cricket’s edges.

While there is direct flow to the drains from the tapered system, there is indirect flow from the crickets. Crickets are necessary to aid in rainwater mitigation but can also introduce issues like standing water at their edges if utilized incorrectly. In the case of offset drains seen in Figure 4, the upper design solution is direct but requires more miter cut ridges and valleys. By stretching out the valleys and lining up the drains, the lower solution simplifies the amount of cutting. Crickets are added because they are needed to help push water around the expanded valley. The perimeter shape of the roof and drain placement dictate the outcome of the tapered design, especially when designing for a constant perimeter height. This is when the tapered system has the same max height around the entire roof perimeter, but this cannot always be achieved if the drains are not centrally located and aligned. It is important to remember the performance expectations when considering a tapered insulation system design. The application method, drainage expectation, and budget are critical when finding the right balance for clients.

The following animation helps to visualize how tapered panels are arranged in a few simple designs to point rainwater towards drains and evacuate water off of a roof, including the aforementioned designs above.

Video courtesy of GAF; credit: Tapered Design Group

Tapered panel designs with animated rainwater simulating water evacuation.

The Necessity of Crickets

Crickets, or saddles, are a necessary design feature and can work efficiently—if they are designed correctly. Crickets, or saddles, are an overlay to a tapered insulation system or to a flat-insulated structurally sloped system by diverting water to the drains.

Figure 4: Diagram of a cricket and common terminology.

Crickets are a design feature that can work efficiently if they are designed correctly. Crickets are assembled from cut tapered panels, so they have their own panel slope to refer to. The unwritten rule is that the cricket panel slope should be twice of the roof field or tapered slope. If the tapered system is 1/4-inch per foot then the crickets would need to be 1/2-inch per foot. Cricket valley slope is the critical aspect that determines how effective a cricket will be. If the cricket design does not create enough of a slope along the outer edges it can introduce standing water. In most cases, the wider the cricket, the greater the cricket valley slope will be, aiding in moving water off of the roof.

The NRCA states that the total cricket width should be between one-quarter and one-third of the overall length, erring on the wider side to ensure proper water diversion. This works for a majority of cricket applications, but it is still possible for crickets to be inefficient if the designer does not consider the cricket valley slope in regards to the tapered or roof field slope. Wider is always better if the space permits. Working with a professional tapered designer ensures that crickets are adequately sized. Panel designers are also the first people who can offer design recommendations and alternatives.

Building owners can protect their investment even if the roof only requires crickets to push water to drains, with the roof field slope built into the deck. Slope can be already designed into the roof structure using sloped wooden trusses or metal beams installed at a specific angle to achieve a desired roof slope. These projects see the most on-site changes decided by the roofing installer at the time of installation, whether their decision is to save on labor or minimize cutting by reducing the size of the crickets. These field reductions are problematic when the provided plan is ignored because planning cannot be done in advance to prevent the potential of standing water due to undersized crickets. Changes to the design such as reducing cricket width should be communicated with the tapered insulation designer to confirm if the reduction in size will still yield an efficient cricket that drains rainwater off of the roof.

Figure 5: Undersized crickets that leave standing water and accumulated dirt because they are inefficient for their application.

To help visualize these cricket issues, an image of a cricket is provided that has been reduced in size so drastically that it does not resemble a cricket nor function as it needs to. The cricket valley slope in this example is close to 0 inches per foot, evident by the standing water seen on the roof. Undersized crickets do not divert water quick enough to the drains and allow dirt to gather, encouraging algae and bacteria growth. While membrane manufacturers may allow for a ponding exclusion in their material warranties because they believe the membrane will stand the test of time, allowing for undersized crickets can prematurely shorten the service life of the membrane and not only cause performance issues, but aesthetic issues as well if lower levels with collected dirt can be seen from above.

Designing for Slope and Designing for Shape

Architects and building design professionals have to consider a plethora of details to put together not only the roofing assembly, but various other components in the scope of work. For the roof to function with a tapered insulation system, the most important factors to consider are the required slope by code or project details, and the placement of the drains and scuppers. These details shape the overall design and will highlight areas of concern where insulation heights can get tall, such as door or window thresholds. Historically, roofs were just meant to shelter its inhabitants from the elements, but now the underutilized potential of roof surfaces is being realized. The sky's the limit for large installations in the world of commercial roofing, such as solar and vegetative roofing, or amenity decks. The impacts on the sustainability goals of a building include: increased energy efficiency, rainwater retention, energy generation, biohabitat restoration, food production, reduced urban heat island effect, and outdoor space.

Figure 6: A project rendering showing multiple levels of roofing.

In most cases, drafting and supplying just the top-level roof plan in the drawing set is not enough. A roof plan may only show that specific level at that specific height and not lower or higher levels that are considered roofs such as entrance canopies, terraces and balconies, and bulkhead roofs such as stairs and elevators. It is common for roof plans to be shown on the floor plan level that they exist on. It is best to submit floor plans for each floor as well as the roof plans for projects with multiple levels.

A rendering is a great way for other roofing professionals to quickly identify where roof areas are. Designers may include slope lines on roof plans as a hint that those areas will need a roofing assembly, but their ability to convey what their performance expectations are may be limited. Details outlining deck type and insulation assembly should be provided for each roof surface to help roofing professionals understand the project parameters. A project manual should also be included to further specify performance and installation directions. This information has a direct influence on not only the tapered design but the insulation assembly as well. To assist building design professionals, the following examples will walk through how a tapered solution is applied to a roof plan.

 

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Originally published in October 2022

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