Thermal bridging occurs when a more conductive, or poorly insulating, material allows an easy pathway for heat flow across a thermal barrier. ASHRAE 90.1 defines a thermal bridge as an element that has higher thermal conductivity than the surrounding materials, which creates a path of least resistance for heat transfer. IECC 2024 has requirements for mitigating thermal bridging in Section 402.7.

Thermal bridging due to studs is the reason that continuous insulation has a higher effective R-value than insulation that is tucked into the space between framing members. In addition to thermal bridging caused by structural members, thermal bridging also occurs at breaks in the insulation itself. Additionally, the dew point within the building enclosure can be managed more easily with continuous insulation. When building designs split insulation between inside the cavity and outboard of the structure, there are risks for condensation due to the reduced effective R-value of the in-cavity insulation due to thermal bridges of the structure.

As building construction evolves and performance levels of materials increase, it would seem that tiny breaches through the insulating layers of an assembly would matter less and less.⁷ However, the reverse is actually true: the tighter and better insulated the building, the bigger the difference from all of the weak points in its thermal enclosure. Fasteners and other introduced materials can counteract the ability of insulation to slow and reduce heat flow by acting as a short circuit. Attachment methods used to secure material can create unintended thermal bridges.

Major codes, including the 2024 IECC, address thermal bridging by accounting for framing materials and linear penetrations which act as bridges. Nevertheless, there is a growing body of research pinpointing the impacts of fasteners and attachment methods. Investigators at the National Bureau of Standards, Oak Ridge National Laboratory, the National Research Council Canada, and consulting firms Morrison Hershfield and Simpson Gumpertz & Heger (SGH), have conducted laboratory and computer simulation studies to analyze the effects of point thermal bridges.

A team of researchers at SGH, Virginia Tech, and GAF set out to determine exactly how much of an impact thermal bridges caused by fasteners had on the effective R-value of a roof. They started by simplifying the problem. The team developed computer simulations to accurately anticipate the thermal bridging effects of fasteners based on their characteristics and the characteristics of the roof assemblies in which they are used. The simulation broke the problem of thermal loss down into parts, so that the researchers could know how each part affects the problem as a whole. Researchers also wanted to carefully check the assumptions underlying computer simulations to ensure that results matched up with what the team was finding in the lab. The full paper describing this research work was delivered at the 2023 IIBEC Convention and Trade Show.

The project discovered that one #12 fastener reduced the R-value of a 2' x 2' x 4” thick sample of polyiso board (ISO) alone by 4.2 percent in the physical sample, and 3.4 percent in the computer simulation. The research team found that fasteners in roofs with a steel deck made a big difference in thermal performance as well. Both physical and computer approaches showed the impact of thermal bridging, wherein the steel deck acts like a radiator, exacerbating the effect of the fastener. In the assembly with just ISO and steel deck, adding a fastener resulted in an R-value drop of 11.0 percent for the physical experiment and 4.6 percent for the computer simulation, when compared to the assembly with no fastener. The research also showed that high-density polyisocyanurate cover boards go a long way toward minimizing the thermal impacts of roof fasteners. When a cover board was added, the physical experiment showed a 6.1 percent drop (down from 11 percent with no cover board) and the computer simulation a 4.2 percent drop (down from 4.6 percent with no cover board) in R-value with the addition of a fastener.

Overall, the team concluded that roof fasteners have a measurable impact on the R-value of roof insulation, steel decks amplify this effect, and insulating cover boards mitigate this effect.

These impacts will continue to have an increasing impact on the effective R-value of roof assemblies. Changes in wind speeds, design wind pressures, and roof zones as dictated by ASCE 7-16 and 7-22 have exacerbated thermal bridging potential due to fasteners. Under these provisions, fastener patterns are becoming denser in many cases. This means that there is the potential for more metal penetrating the roof on average per square foot of roof area than ever before. More metal means that more heat can radiate from the building in winter and into the building in summer.

To avoid miscalculating the thermal penalties of thermal bridges, Phius (Passive House Institute US) provides a Fastener Correction Calculator⁸ along with a way to calculate the effect of linear thermal bridges. A range of codes and voluntary standards are beginning to address this problem, though it is important to note that there is a time lag between development of codes and their widespread adoption.

As more materials and additional requirements are added to enclosures, the design professional needs to recognize when materials and assemblies need to change to achieve higher energy performance and avoid performance failures. “In a broad sense, as energy efficiency is improved in building enclosures, moisture risks can increase from decreased heat flow across the assemblies. As we improve energy efficiency, we may also be increasing moisture risks in building enclosures. And the increased risk may be more complex than the historical designs and more tightly coupled to the building’s HVAC operations, structural elements, and occupant-use conditions.”⁹

Maintaining a Quality Design

After careful research and selection of materials and design, it is imperative to plan an approach to construction and detailing that prevents changes to the construction documents. A change forced in the field can result in lower realized building performance than originally intended. There are some considerations that should be taken into account to prevent such late changes.

The what?

There are four critical components to a robust detail: the materials selected, the installation methods, a simplification of the detail itself where possible, and thorough communication among design and installation professionals on the previous three components.

Four quick checks can help design professionals ensure that these requirements are met. They are:

1. Materials: are the selected materials compatible? Will they adhere to each other where needed?
2. Installation Methods: how are the control layers installed and interfaced? What is the order of installation of the different materials? Can the installation be easily completed on the timeline needed?
3. Simplify: can fewer materials be used? Can the installation be done in fewer steps or by fewer trades to avoid confusion in the field?
4. Communication: is the whole team engaged in conversation to create a multidisciplinary approach? Are details drawn and specifications written clearly to communicate both the materials and the intent of the design professional?

The Why?

The relative importance of each control layer is based upon the magnitude of impact it has on the performance of the building enclosure. As discussed above, the liquid water control layer must be addressed first. Uncontrolled liquid water in a building is extremely deleterious and failures render other control layers less consequential. Second in importance is the air control layer. This is due to its interrelationship with heat and moisture. The thermal control layer is next, as it manages where condensation of moisture vapor could potentially occur. Finally, the water vapor control layer should be considered, taking into account that materials used for the other layers may already be serving this function.

Identifying and maintaining continuity of the four key control layers is important in the design phase. Detailing and identification of the control layers in the drawings is critical to ensuring that the design intent is implemented in the field. This can require the design and specification callouts to be very specific. If the sequencing of components or trades in the field impacts the intended continuity or performance of the control layer in the design, it needs to be addressed before construction starts to prevent rework.

Any of these barriers’ effectiveness can be greatly reduced by discontinuities, even small ones. These openings or flaws can be caused by poor design, damage during construction, improper installation including sealing and flashing, mechanical forces, aging, and other forms of degradation. Thankfully, there are ways to avoid these discontinuities and set up a project for success.

The How!

A specification requiring continuity of control layers, communication amongst trades, verification of material compatibility, and quality control provides the critical key to success.

Designers can include a few fundamental items in their specifications to help ensure the control layers will be installed properly and that the interface details are not missed. First, the specification should set up communication amongst all parties throughout a project to put potential problems under a continual, multidisciplinary spotlight that helps a team to catch them as early as possible, even during the design stage. Items such as defining the complete scope of work for the contractors installing the different control layer assemblies and specifying the party responsible for installing flashing and transitions between systems assigns responsibility and allows for easier scheduling and quality assurance in the field. Clarification on any questions amongst parties should be addressed during a pre-construction meeting at which all parties involved are required to be in attendance.

Second, it is good practice to require shop drawing submittals that include details at the many interfaces around the building enclosure. This will confirm that all participants understand how the materials are to be installed and who will install them. This will also highlight areas that may have been missed in specifications as to which trade is responsible for which part of a specific detail. Critical detail locations are often difficult to illustrate on two-dimensional drawings. They can require exploded diagrams, isometric drawings, and sequenced information to better communicate the design intent. On this note, designers need to define any items labeled “by others” on shop drawings during the review phase. All “others” should be changed to the specific contractor’s name and/or trade.

Next, the materials selected for each part of the system need to be vetted. Questions such as whether the materials will adhere to each other, if they are chemically compatible, and if they can be installed in the order required, should be answered before construction starts. This information, along with support to assist in any detailing questions, should be sought from the manufacturer’s representatives of the chosen products. If substitutions are made, compatibility and adhesion of the components need to be reconfirmed. Additionally, architects can specify mockups in the project documents and even require performance testing of the mock-up to confirm interfaces are functioning as intended. This helps to test and validate the constructability of the design and anticipate issues that may arise during construction prior to the start of work. Mock-ups can also help installation crews by being a reference to the agreed-upon methods and providing orientation for any new crew members on the project. Having a physical example allows for confidence that, when the roof and other assemblies are installed, the team can feel assured that it will perform according to the design intent.

Finally, quality control and operational maintenance are best practices to ensure long-term performance of the building enclosure. It is important to have quality assurance and quality control occur throughout the project and not be confined to a final walk-through in the field. Fixing issues or missed details at the “almost done” stage is very hard when faced with sequencing realities. Operational inspection and maintenance plans can also be required to be submitted for all major systems in the building enclosure.

In Summary

Here is a starting checklist for what is needed in construction documents:

• Show a continuous air barrier.
• Provide details.
• Specify the attachment method of the roof assembly to minimize thermal bridges, such as fasteners.
• What else do you include on your list?

Designers aim to realize a building that is able to satisfy both aesthetics and building science performance criteria. From the designer’s perspective, one of the biggest challenges is meeting this design vision in a way that embraces all relevant performance mandates. While it may be tempting to release responsibility for the way buildings function to contractors, manufacturers, or “others,” building designers are ultimately responsible for satisfying building science performance mandates that result in durable, functional, and beautiful buildings.”¹⁰

END NOTES

¹ Kesik, Ted J., Ph.D., P.Eng., MASHRAE. “Building Science Concepts.” Whole Building Design Guide. December 4, 2019. https://www.wbdg.org/resources/building-science-concepts. Accessed August 6, 2024.
²Richard D. Rush, ed. The Building Systems Integration Handbook (New York: Wiley, 1986).
³Lstiburek, Joseph. “BSI-001: The Perfect Wall.” July 15, 2010. https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall. Accessed August 6, 2024.
⁴National Roofing Contractors Association. The NRCA Roofing Manual: Architectural Metal Flashing and Condensation and Air Leakage Control. Page 259. National Roofing Contractors Association. Rosemont, IL. 2022.
⁵Kirby, James R., Thomas J. Taylor, Ph.D., James Willits. “Condensation, Dew Point, and Roofing.” GAF Roof Views Blog. March 22, 2018. https://www.gaf.com/en-us/blog/building-science/condensation-dew-point-and-roofing-281474980065075. Accessed August 9, 2024.
⁶“The Hidden Science of High Performing Building Assemblies,” Environmental Building News November 2012.
⁷Grant, Elizabeth. “Thermal Bridging Through Roof Fasteners: Why the Industry Should Take Note.” GAF Roof Views Blog. November 17, 2023. https://www.gaf.com/en-us/blog/building-science/thermal-bridging-through-roof-fasteners-why-the-industry-should-take-note-281474980286177. Accessed August 9, 2024.
⁸Fastener Correction Thermal Bridge Calculator. https://www.phius.org/fastener-correction-thermal-bridge-calculator. Accessed August 12, 2024.
⁹Meyer, Benjamin. “Designing for Moisture Durability & Energy Efficiency.” GAF Roof Views Blog. May 6, 2020.” https://www.gaf.com/en-us/blog/building-science/designing-for-moisture-durability--energy-efficiency-281474980028035. Accessed August 9, 2024.
¹⁰Grant, Elizabeth J. Integrating Building Performance with Design: An Architecture Student’s Guidebook. Page 30. Routledge Taylor & Francis Group. New York. 2017.

Andrea Wagner Watts is the Building Science Education Manager for GAF, engaging with industry professionals to provide guidance, technical support and education for roof and wall assemblies. With more than 15 years of experience in the industry, Andrea strives to improve the overall performance of the building enclosures through application innovation, product development and building science research. Andrea has published on building science, assembly interfaces, durability and resilience and holds multiple patents. She serves as an executive board member of ABAA, is the co-chair of their Technical Committee and chairs the ASTM E06 Task Group on air barriers.

Elizabeth Grant, PhD, AIA, is Building & Roofing Science Research Lead at GAF. In this role, she supports architects and specifiers through technical advice in the design process. Before joining GAF, she was an associate professor at Virginia Tech’s School of Architecture + Design, publishing papers, conducting studies, and offering courses. Her architectural experience includes designing healthcare, civic, and educational buildings. Her work focuses on building enclosures and finding sustainable solutions to pressing architectural and environmental problems.