The most common roof insulation materials are polyiso, XPS, and EPS, each with different chemical compositions and material properties. Polyiso has the highest R-value at 5.7 per inch, followed by XPS with an R-value of 4.7 per inch and EPS with an R-value of 3.6 per inch. Higher R-value per inch means less material is required to achieve the desired insulating value. The thickness of the overall installation can have an impact on the overall design of the roof system. It is an industry standard for roof flashings to extend a minimum of 8-inches past the completed installation. Flashing heights are of particular importance at mechanical curbs and parapets. For a new construction installation, it is possible to raise the heights of the curbs and parapet walls to the desired height, however, in an existing building, this can be problematic.

Lightweight insulating concrete (LWIC) is an alternate type of insulation where EPS boards are encapsulated in a slurry coat of insulating concrete, then the membrane, and overburden would be placed on top of the LWIC. The R-value of the system and overall thickness are tailored to each unique project. LWIC can be advantageous for use with slope limitations because a desired slope can be achieved with the LWIC system. There are also no gaps within the installation, such as between standard insulation boards, and fasteners are not required to attach the LWIC, so there is limited thermal and air loss within the system.

While perhaps the most important criteria is insulating ability, mitigation of air infiltration is equally important. When warm outside air meets with cold inside air, condensation can form within the roof assembly, which can reduce the roof insulating value or cause microbial growth. Proper installation of the insulation is critical to mitigate heat and air flow. Where the membrane is the outermost layer, layers of rigid insulation are typically installed directly on the structural deck, and the membrane is subsequently installed on the insulation. The orientation of the boards during installation can play a significant role in energy efficiency in the assembly. The insulation should be installed in a minimum of two layers, and the joints in the top layer should be offset from the joints in the underlying layer. The edges of adjacent boards should be in contact to limit gaps between boards. Air and heat that is able to travel between gaps in the board results in energy loss. Gaps between boards can decrease insulating ability by allowing thermal loss, an increased condensation potential if air travels into the roof assembly. Air often brings moisture, which if allowed to condense within the insulation, can saturate the insulation boards. Wet insulation has an R-value of approximately zero, which is like having no insulation at all.

Photo courtesy of GAF

Staggered and offset insulation board joints.

The Energy Code specifies minimum insulation R-values, as seen in the chart at Image 5. It is important to note that next to each required R-value, ‘ci’ is denoted, which is for continuous insulation. Similar to the continuous air barrier required by code, thermal insulation is required to be continuous; the insulation from the roof should continue, without interruption to the exterior wall insulation. Continuous insulation will assist in maintaining interior temperatures but will also eliminate voids in the building envelope where condensation is likely to occur.

Coverboards

Roof durability is dependent on the entire roof assembly, including the inclusion of a coverboard. Particularly rooftops with a large amount of mechanical equipment, such as hospital facilities, tend to have more trades and increased foot traffic on the roof. Schools with rooftop amenity space such as for classrooms or outdoor cafeteria seating have overburden systems, such as pavers, which add weight to the roof system, further increasing the need for a coverboard to protect the insulation. As with any roofing assembly, high traffic areas directly on roof membranes, such as those to service mechanical equipment, should be protected with walkway pads. Walkway pads, also sold in roll form, should be installed at roof access areas, such as hatches and around equipment, where they may be accessed for service or maintenance. Walkway pads protect against abrasion and wear, but do not add compressive strength to the overall roofing system. The addition of a high-compressive-strength cover board below the roof membrane will enhance system protection, including compressive strength. Cover boards can help provide added protection against penetration by objects, including tools dropped by service contractors, wind-borne debris, and hail. Increasing the thickness of the cover board will increase its penetration resistance.

Typical coverboard materials are high-density polyiso insulation (HD polyiso) and gypsum. High-density polyiso is a high-density version of the traditional polyiso insulation. A half-inch HD polyiso coverboard weighs 0.3 psf. The advantage of using HD polyiso is that it not only adds protection to the roof assembly, but it also adds an R-value of 2.5 per half-inch. Typical gypsum coverboards, with an equivalent half-inch board, weigh 2.76 psf and contribute an R-value of 0.5 per half-inch.

Roof Installation Considerations

While the roof assembly selection, including membrane and attachment method, is important for energy efficiency and performance goals of the building, ensuring a proper installation is critical to the long-term success of the roofing assembly. After the roof assembly has been decided, the roof details must be meticulously planned to include considerations for curbs, penetrations, flashings, and review of the control layers to include water, thermal, air, and vapor. The control layers must be continuous from the roof to the exterior walls. This prevents gaps in the system that can allow for water leaks, air leaks that may lead to condensation, or thermal breaks that may create locations of energy loss.

Additionally, the time of year and interior activities must be considered. For schools, the most common time for roof replacements is summer. This can create challenges as there are compressed schedules and weather delays cannot be accounted for at the end of the schedule. For most schools, the roof replacement must be complete by the time school is back in session. This does however, create several advantages, since there may not be occupants in the school, there may be less concern with fumes from the roofing components permeating into the building interior, as well as a higher tolerance for noise generating roofing related activities. Hospitals have their own challenges as in several occupant types in the hospital, there may be occupants 24 hours a day. This limits the ability for fumes and noise generating activities. But for both hospitals and schools, these limitations make the case for designing and installing a high-performance system during new construction. High-performance roof design will increase the time between roof replacement cycles as well as increase overall durability and resilience to weather events, foot traffic, and UV degradation which wreak havoc on roof systems. Additionally, high-performance roof systems will increase energy efficiency by mitigating uncontrolled air movement and thermal loss through roof components.

Summary

When designing an assembly for medical and student populations, it is critical to consider all elements of high-performance roof design. Air control becomes essential to reduce transmission rates, facilitates greater control of interior temperatures and humidity levels, as well as manage condensation risk and the potential for mold growth, which can negatively impact indoor air quality. These details will impact the performance of the assembly and contribute to an energy efficient building and extend the useful life of the roof. Design of high-performance roofing assemblies, from the selection of the membrane, coverboard, insulation, and overall attachment method will not only impact the overall lifespan and durability of the roof assembly, but it will impact the performance, including energy efficiency of the system. High performance roof design will extend the life of the roof components, which will increase the time between replacement intervals. Construction generated odors and noise can be managed through intentional design to address the needs of students and recovering patients. Through intentional design and construction, a holistic approach can be implemented on medical and student-based campuses to protect the health, safety, and welfare of a vulnerable population.

End Notes

¹Smith RM, Rae A. “Thermal comfort of patients in hospital ward areas.” J Hyg (Lond). 1977 Feb;78(1):17-26. doi: 10.1017/s0022172400055881. PMID: 264497; PMCID: PMC2129728.
²“Optimal classroom temperature to support learning.” Regional Educational Laboratory Program. November 2018.
³ 179D Commercial Building Energy Efficiency Tax Deduction.

Kristin M. Westover, PE, LEED AP O+M, is a Technical Manager of Specialty Installations for low-slope commercial roofing systems at GAF. She specializes in cold storage roofing assemblies where she provides insight, education, and best practices as it relates to cold storage roofing. Kristin is part of the Building and Roofing Science Team where she works with designers on all types of low-slope roofing projects to review project design considerations so designers can make informed roof assembly decisions.