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

The Importance of Energy and Insulation

The name says it all: tapered insulation system. Thus far, the discussion has been about the features of tapered polyisocyanurate and designability that can be achieved when using various design methods. But a critical component of polyisocyanurate is its insulating property and how that plays into the roofing assembly. One of the fundamentals of an efficient building envelope is an energy-efficient roofing system. A significant amount of energy can escape a building through inefficient or underdesigned roof assemblies. Consideration has to be taken to design to minimum or exceed minimum energy code requirements for the region the building exists in, and what the function of the space is below that roof assembly. An occupied space needs to remain comfortable year-round, while an unoccupied space may not require a high-insulating value to save on costs. Not only do tapered panels point rainwater off of the roof, a tapered insulation system also needs to insulate a roof as outlined by project details to prevent requiring even more energy output to compensate for energy loss.

Local energy codes are requiring higher and higher insulating values, or R-values, as requirements change to make our built environment more efficient in terms of energy use. Heating and cooling, as well as retaining that energy within the building enclosure, start and stop with a proper roof assembly. The American Society of Heating, Refrigerating and Air-Conditioning Engineers, or ASHRAE, has updated their regional zone map for the first time in 25 years to account for changes primarily in the counties of northeast states like Pennsylvania and New York to warmer regions. These regions dictate the amount of minimum insulation needed in an assembly for new construction as referenced in the adopted International Energy Conservation Code, or IECC, for the city or county that the building exists in. It takes time for cities to adopt and implement changes and there are still areas of the United States using IECC 2009 as the basis of design. Metropolitan areas such as New York City tend to be at the forefront of implementing R-value changes and requirements to meet energy initiative demands.

IECC code states that in order for insulation to count towards the roof assembly's total R-value, it must be entirely above the roof deck—any insulation above ceiling tile is not included.7 Tapered systems on the roof surface go up in height from the lowest point, and the minimum height of insulation at the drain or scupper is the only constant guaranteed at every area regardless of the roof design. This is different from an average R-value, which is just that, the average value between the lowest point in the system and the tallest point. This can be a dangerous approach if the roof has a complex shape, creating tall areas of insulation as the distance from the drain increases. Large distances between drains and difficult placement in regards to the roof perimeter can skew the average and render a tapered design to be inefficient. It is likely that areas of the tapered design could be far below minimum insulation requirements. The local authority having jurisdiction may not accept average R-value as the basis of design, while some jurisdictions allow average values for existing roofing applications. Taking the time to confirm insulating expectations will alleviate performance issues down the road


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Originally published in October 2022