Understanding Thermal Performance is Key to Ensuring Glazing’s Place on the Façade

By Ellen Rogers

Does it seem like you’re hearing the terms “thermal bridging” and “thermal break” more frequently? You are, and there’s a good reason for it. Simply put, when it comes to façades, thermal bridging is the movement of heat from the inside to the outside or from the outside in. Helen Sanders with Technoform North America in Beachwood, Ohio, describes it as the thermal equivalent of an electrical short circuit, or the thermal equivalent of water flowing through holes in a dam.

“Heat will find the path of least resistance and will flow through the pathways that offer the best conduction path,” she says. “The leakage of heat can cause serious issues relating to condensation and mold, as well as occupant thermal discomfort and degraded energy performance,” she says, adding that this can be especially problematic when it occurs in the middle of the wall where condensation and mold cannot be seen.

Today, building and energy performance requirements are becoming increasingly stringent, and when it comes to the building envelope, much of that focus is on improving thermal performance. Many companies in the glass and metal industry are working to improve performance with thermal breaks. These breaks are used to stop the transfer of heat in and around fenestration. Sanders explains that without a barrier to heat flow in the frame or edge of glass—no matter how good the glass—the heat will flow around the edges.

“If we want to get a seat at the design table with the HVAC contractors, our industry needs to understand thermal performance of façade systems inside and out and be able to offer practical and aesthetically pleasing solutions to the design team,” says Sanders.

Bridging the Gap

Thermal bridging can be a concern on several different areas of the façade. These areas can include the window frame and edge of glass connections between the perimeter of the fenestration and the adjacent wall; within spandrel panels and between the spandrel areas of curtainwall and the transparent areas; connections of opaque panels to the building structure; floor slab edges/balcony edges; parapets; and at attachments, such as sunshades.

“The thermal performance of the façade is the linchpin of building performance. While it can be traded off with higher efficiency HVAC and lighting systems to achieve the same overall energy performance, this trade off negatively impacts building resilience and thermal comfort, especially next to the envelope,” says Sanders. “If the glazing industry wants to ensure investments are made in the façade and that glazed areas continue to be maintained, we need to understand thermal performance of every aspect of the façade, from the individual components, to the thermal impact of how they interface to each other and the building structure, and strive to improve them all.”

Michael Smalley, director of business development at IWR North America in St. Louis, adds that with increased efforts of sustainability within the built environment, there is a heightened awareness of thermal bridging and how to mitigate it.

“The increasingly popular use of façade features and elements, such as sunshades, fins and decorative elements, has led to more opportunities for thermal bridging as these features can increase susceptibility for thermal transfer.”

Sanders also notes that spandrel is a key area to focus on.

“We need to deliver high performance solutions here in order to improve curtainwall performance and keep up with building performance requirements.”

Overcoming Obstacles

Improving thermal performance, however, does not come without challenges.

“Within the framing system, there’s not a lot more to do other than the pour and debridged or structural strut methodology many systems utilize,” says Chuck Knickerbocker, curtainwall manager with Technical Glass Products, which is based in Snoqualmie, Wash. “The next challenge is going to be the break in the thermal line from the glazing system to the surrounding construction. As much attention as we pay to keeping continuity in air and water barriers through these transition details at head jamb and sill, there’s going to be a lot more attention paid to ensure there’s a thermal continuity between surrounds and window systems.”

He continues, “With all the focus that energy is getting, every drop of lost performance costs dwindling energy supplies. We, the glazing industry, have to do all we can to plug the proverbial holes in the dykes to help reduce any future energy demands. So every facet of any/all systems is going to be reviewed to cut down on any needless energy loss. The designers, and especially the HVAC folks, are going to challenge all aspects of the glazing system to maximize performance to drive down the energy requirements of the buildings  they are installed in.”

What Can We Do?

According to Sanders, the key issues with thermal bridging are ones that can occur from not addressing the problem, such as the building’s energy performance, condensation risk and occupant thermal comfort.

“Thermal bridging is not easily assessed using typical simulation tools. For example, THERM does not adequately calculate the thermal bridging within spandrel areas, and between spandrel and transparent areas of the curtainwall,” she says. “This leads to an over-estimate of thermal performance and buildings that don’t perform as anticipated. 3-D thermal modeling typically is needed, which is specialized and expensive. However, the Thermal Bridging Guide from Morrison Hershfield has been developed using 3-D modeling for many different wall constructions, and can act as a library to help specifiers and contractors understand how to assess performance and choose appropriate wall and spandrel constructions.”

Stéphane Hoffman, principal with Morrison Hershfield, agrees that tools and resources, such as 3-D modeling, can help companies improve the thermal performance of their products.

“The industry can look at the actual performance of existing systems using thermal modeling to understand how the current system performs and to start to understand the challenges ahead of them,” he says. “For example, can they make small change to impact the [system] or do they need to redesign it?”

He says that given concerns around condensation, with 3-D modeling they are better able to predict condensation of the window and adjacent assemblies.

Speaking of the installation, Knickerbocker says one question to address is whether the work of closing the transition gaps at head jambs and sills will fall to the glazing subcontractors or not.

“The architects eventually will detail exactly what is required at the transition details. The glazing subs will have to be aware of different detailing as these changes work their way into construction documents in both the drawings and specifications,” he says. “If the general contractors assign that work to the glazing subs, the subs will have to include the necessary materials and labor to fill in or close off those thermal gaps.” Smalley points out that mitigating thermal bridging is a shared responsibility among all project stakeholders from the façade system designers and manufacturers to mechanical designers, architects and glazing contractors.

“Through simulated modeling, such as thermal dew point analysis, understanding the location of the dew point within a given wall assembly is critical to understanding what efforts should be taken to minimize thermal transfer.”

Sanders adds, “They [installers] should understand all the areas where thermal bridging could impact the performance of their scope of work. If they are moving into cladding installation, they should understand the risks of not thermally breaking the attachment mechanisms, and become experts in delivering high-performance solutions.”

She also notes that in specifications, installers should beware of ‘by others’ when it comes to transitions between scopes of work.

“The widespread use of the ‘by others’ clause can cause the responsibility for managing the important details of the transitions to be missed. The person who  has responsibility for tying into the adjacent systems must be specified. If not by the architect, the contractor should ensure it’s clear before work is started and that sequencing of work around the transitions is agreed. In particular, ensure that sufficient tolerances are provided to make the transition between curtainwall and adjacent systems possible and that the contractor sequenced first leaves sufficient length of transition membranes to tie into the second assembly.”

She adds, “Be proactive in collaborating with other subcontractors who have adjacent scopes of work. You will be appreciated by the design team and the façade consultant.”

Ellen Rogers is the editor of USGlass magazine. Follow her on Twitter @USGlass and like USGlass on Facebook to receive updates.

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