Go with the Flow: Tapered Insulation Fundamentals
Case Study: Generating a Tapered Insulation Layout
Figure 7: Roof plan with design intent and overlaid with a true 1/4-inch per foot tapered design.
Tapered insulation does not function like light-weight insulating concrete where slopes can be shaped on-site and fit as needed. There are real design limits in the direction and angle that tapered panels can be arranged. Converging ridges and valleys are designed in 45- and 90-degree angles in regards to the low point (drain or scupper) that they are pointed to. The example roof plan shows 1/4-inch per foot as the desired slope for the whole roof design. Lines have been drawn in to represent how the design should function, with a central ridge and sides pointed to the drains. This design functions in theory, however, tapered panels of a similar slope cannot be arranged in a custom point-to-point fashion as shown.
The same roof plan has been overlaid with a true 1/4-inch per foot tapered insulation design, where all sides are the same slope as the roof plan designer had intended. The difference in design is easily identifiable when valley lines are compared from intent to reality. This is the only way that panels of a similar slope can be arranged without creating gaps between panels. Heights at the perimeter have been marked to show how tall the high points are when the system gains an inch in height for every four feet of distance away from the drain. Before designers suggest reducing the tapered field slope, different design solutions should be applied to test if a different approach can solve issues such as height restrictions.
Figure 8: The same roof plan with a two-way design and cricket overlay compared to a split-system tapered design where the varying slopes exist on the same plane.
Utilizing a two-way design with crickets as an overlay can potentially reduce overall heights by stopping the max field slope height at the design’s ridge. However, the original intended design still cannot be achieved because other issues become apparent. The outer corners of the roof will have tall material heights because crickets are recommended to be designed with double the field slope. These 1/2-inch per foot crickets gain two inches in height across four feet and add significant height to the overall system. They also introduce issues such as water ponding behind the existing mechanical unit. Reducing the cricket to clear the mechanical unit may also reduce the cricket valley slope and render the cricket inefficient. Overall, the simplified design of a two-way slope is not worth the added labor and material height to the system when the cricket overlay is over half of the design, as is seen in the roof plan overlay.
Rules are meant to be broken! An option that tends to create unnecessary fear of complexity is a split-system design where two differing slopes exist together in the same plane. The greater slope is not an overlay to the lesser slope, such as in the previous design solution of a two-way slope with crickets. Here, tapered panels can be arranged and cut in angles that are not 45 or 90 degrees, as is limited with the miter cuts needed to design with panels of a similar slope. One-fourths of an inch per foot and 1/8-inch per foot exist together in the same plane and create a tapered design solution where direct drainage exists with two differing slopes. Every four feet of distance in the 1/4-inch per foot system is matched at 8 feet in the perpendicular direction of the 1/8-inch per foot system, and does not require large crickets with indirect drainage. The lesser 1/8-inch per foot slope is applied to function like the cricket in the two-way design and like the field slope in the roof designer’s original intent. And at half the intended slope, the overall material height has been reduced from over 17 inches, to 11 inches, to just 9 inches. Popular with recovers of existing roofs with height limitations, this design option can alleviate many job-specific issues like door and window thresholds and difficult mechanical unit placement.
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
