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Structural DESIGN Structural Thermal Breaks Preventing Heat Loss Through Concrete Parapets

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Structural DESIGN Structural Thermal Breaks Preventing Heat Loss Through Concrete Parapets structural DESIGN Structural Thermal Breaks Preventing Heat Loss Through Concrete Parapets Conventional concrete parapets can form a thermal bridge that dissipates heat energy outside like large cooling fins, aided by cold winds whipping over the rooftop. One solution is to cast a structural thermal break between the parapet and the heated interior structure supporting it. By Dritan Topuzi, Ph.D., P.Eng, PMP, LEED AP Figure 1. Examples of thermal bridging; a) Material thermal bridge; b) Geometric thermal bridge. arapets protect the edge of roof assemblies from uplift forces does the insulation, a material thermal bridge is created. Examples created by winds blowing against and over the building. include balconies, as conductive elements that penetrate the verti- However,P solving one problem creates another: a thermal bridge cal building envelope, and parapets, as elements that penetrate the the length of the building perimeter that penetrates the insulated roof insulation. building envelope, conducting heat energy from the building's Geometric thermal bridging occurs when a heat-emitting surface is interior support structure and dissipating it into the environ- larger than the heat-absorbing surface. Building corners are a typical ment. In addition to energy waste, uninsulated parapets chill the example, but geometric thermal bridging can also affect wall/roof and heated interior space adjacent to the building envelope, creating wall/floor junctions, junctions between windows, walls, and doors, uncomfortable living spaces because of the non-uniform tempera- and, of course, parapets. Most of the time, material and geometric ture distribution. thermal bridging occur in concert. The traditional approach to mitigating thermal bridging at parapets Heat transmittance through thermal bridges can be idealized as is to wrap its exposed surfaces with insulation, making it a part of linear or point: the heated building mass. • Linear transmittance through thermal bridges occurs with A newer method addresses thermal bridging more effectively: ther- disturbances in the continuity of the thermal envelope along mally separating or "breaking" the parapet from the heated interior a certain length. Typical examples include concrete balcony structure by casting structural thermal break modules between the two connections with the floor slab penetrating the wall, outer wall masses. These thermal breaks are optimized for thermal performance edges, floor supports, and window-to-wall junctions. without affecting structural integrity. • Point transmittance through thermal bridges occurs with disturbances in one spot that penetrate an insulating layer. Examples include steel balconies, canopies, and roof Thermal Bridging – a Primer extensions. “Thermal bridging in building construction occurs when thermally While heat energy loss is an obvious consequence of thermal bridg- conductive materials penetrate the insulation, creating areas of ing, developers are faced with a newer and potentially more significant reduced resistance to heat transfer. These thermal bridges are most outcome: condensation and mold growth. often caused by structural elements that transfer loads from the build- Since older buildings often leaked air profusely, interior humid- ing envelope back to the building ity levels equalized with low exterior superstructure” (Payette Architects humidity levels, typically between 2015). The results of thermal bridg- 18% to 25% during winter months. ing include higher heat transfer Forced hot air blowing at or near cold resulting in colder internal surface penetrations, such as balconies and temperatures, higher energy use for parapets, further ensured that the heating and cooling, noncompliance local interior humidity never rose to with building regulations, building the dew point, thus preventing con- occupant discomfort, and potential densation and mold growth. for condensation and mold. Because modern buildings are air- The two most common types of tight, humidity levels can reach 30% thermal bridging are material and to 40% during winter months. While geometric (Figure 1). When an ele- comfortable for occupants, high inte- ment made from a material of high rior humidity near chilled building thermal conductivity penetrates an Figure 2. Condensation on the chilled interior side of uninsulated penetrations can reach the dew point, insulating layer, and the protrusion penetrations supports the growth of mold, which can become airborne years forming condensation that supports conducts heat at a higher rate than before spreading to visible wall and ceiling surfaces. mold growth on adjacent surfaces such 12 STRUCTURE magazine installation of the insulation. Since there are many variables in the detailing of a parapet, we started with a sensitivity analysis of parapet height normalized to one construction type. Because the degree of impact of the assembly depends on how much of the building we are looking at in conjunction with the parapet, we also normalized on an extracted detail that includes 24 inches (61 cm) in height of inside wall surface and 48 inches (122 cm) in length of inside roof surface.” The Payette findings were compelling but not surprising. Naturally, the higher the wrapped parapet, the greater the potential for energy loss. Payette also discovered that insulating beneath the parapet – thermally breaking it from the roof – negated the height factor and Figure 3. Left: Continuous parapet connections without thermal break. Right: provided the most effective insulation, even though there was no Parapet connection was thermally broken with a structural thermal break. Courtesy thermal break specifically designed for parapets at the time. “We of Schock Isokorb Design Guide [5752].pdf. tested a commercially available structural thermal break designed for concrete slabs (balconies) and installed this in a vertical ori- as sheetrock, insulation, and any cellulose-based material, particularly entation. The improvement decreased the heat flow through the in stagnant air cavities (Figure 2). Mold can become airborne years assembly by 27%.” before it migrates to visible surfaces, exposing the developer to potential liability and remediation costs. New Parapet Paradigm Today, manufactured structural thermal breaks designed explicitly A Closer Look at Parapets for parapets are commercially available. They thermally isolate the Parapets play an important architectural and structural role in build- parapet from the heated interior, while preserving the structural ings. However, being at a sensitive location – at the connection of integrity of the connection. the roof and wall envelope – they are subject to various potential A typical manufactured structural thermal break for parapets (Figure 4) performance issues. In his paper, Parapets: Where Roofs Meet Walls, is a fabricated assembly consisting of enhanced insulation and stainless steel Joseph W. Lstiburek, Ph.D., mused, “Historically, so many problems reinforcing bars, creating a module capable of transferring the loads from have occurred with parapets that we have a name for it: parapetitus. the parapet to the concrete roof slab that supports it, while minimizing Thermal bridging is everywhere.” thermal conductivity between the two concrete As a high-mass structural element that masses (Figure 5). The insulating block, made penetrates the building at its windiest point, of graphite-enhanced expanded polystyrene, is parapets are especially susceptible to thermal roughly 98% less conductive than concrete, and bridging and its consequences. the stainless steel reinforcing bars are approxi- mately 70% less conductive than carbon steel reinforcing bars, effectively reducing heat loss Wrapping Exterior Parapet at the penetration by up to 90%. Surfaces The vertical reinforcing bars resist tension/ compression and bending moments on the To prevent thermal bridging, architects his- parapet. The interaction between applied torically wrapped the perimeter of the wall bending moments and tension/compres- with an insulation barrier and then wrapped sion is taken into consideration. The crossed the parapet as well, making it part of the bars provide the necessary shear strength, in heated building mass (Figure 3). In addition either direction, through the tensile strength to being costly and minimally effective, this of these bars. method presented long-term risks. When a parapet is wrapped, it functions Figure 4. Structural thermal break for parapets. similarly to an insulated flat roof and has Other Concrete-to-Concrete many of the same problems. Both roofs and parapets are prone to damage and need repair Construction and maintenance, particularly if railings or In addition to parapets, thermal bridging other elements breach the insulating layer. can occur at balconies when constructed Waterproofing of wrapped parapets rarely conventionally as structural extensions of provides effective, long-term moisture pro- interior floor slabs. In cold weather, the bal- tection, and leakage can incur significant cony conducts heat energy from the heated recurring maintenance costs. interior floor slab, dissipating it into the Designers at Payette researched this subject environment while chilling the interior floor in their article (Payette 2015): “One question opposite the balcony. we found intriguing was whether it was better Structural thermal breaks designed for bal- to insulate around the parapet – covering all conies are comprised of insulating blocks structural interfaces – or underneath it by the
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