Spandrel Thermal Simulation Techniques

Objective: Better evaluate thermal performance of spandrels in buildings to help achieve more accurate building energy and carbon performance.

Problem Statement: Many spandrel assemblies are not accurately evaluated which leads to misleading energy performance and operational carbon results for whole buildings.

Introduction

Glazed wall systems, such as curtain wall and window wall systems, are one of the most used façade systems in modern buildings in North America and throughout the world. Their aesthetic appeal and modularity make these systems popular for many building designers and contractors since many components of a façade system are fully integrated. Glazed wall systems often include both transparent and opaque areas. The transparent or vision areas allow natural light, views, and solar gains into the building. In contrast, the opaque or spandrel areas help conceal building components, such as slab edges, and mechanical equipment. Many spandrel areas are often insulated to help improve thermal performance of the façade.

While glazed wall systems have been commonly used in buildings for decades their thermal performance has not been well understood. Heat flow through glazed wall systems is complicated by:

• Thermal bridging from highly conductive mullions not only bypass more insulating areas of the system such as the Insulated Glazing Unit (IGU) and spandrel insulation.
• Complex lateral heat flow paths between vertical and horizontal mullions.

The industry recognizes this phenomenon in the vision areas and have provided two-dimensional thermal simulation procedures, standards, and tools to evaluate heat flow for glazed wall fenestration such as the NFRC-100 and 200 standards and fenestration performance software like WINDOW and THERM from Lawrence-Berkely National Laboratory (LBNL). However, there is less guidance for evaluating the thermal performance of spandrel assemblies, leading to confusion within the industry regarding the misuse of fenestration thermal simulation procedures on opaque spandrel assemblies as it oversimplifies spandrel thermal performance.

Until recently the thermal performance of spandrel assemblies has not been subjected to the same level of scrutiny as other parts of the building façade since it is often assumed that spandrel thermal performance is correlated with the amount of insulation in the spandrel. However, stringent energy requirements in many building energy codes and standards, such as the inclusion of thermal bridging in façade thermal performance requirements, building envelope back stop requirements, lower energy targets for whole building energy simulations, and penalties for excessive energy use, require more accurate quantification of the thermal performance of spandrel assemblies.

Recent updates to spandrel simulation techniques and procedures such as the new NFRC Spandrel procedure is the first intent to address the need for more accurate thermal simulations within a standard. However, these new techniques are currently limited and may not be appropriate for project specific façade thermal performance calculations or whole building energy performance simulations since:

1. These procedures are meant to rate and compare between different spandrel assemblies for a limited standard size.

1. The ratings only consider spandrel assemblies in isolation and does not consider the impact of adjacent assemblies such as vision glazing and interior framed walls.

As a result, these procedures may not be able to provide accurate thermal performance of spandrels that differ from the reference sizes nor common conditions such as interior insulated spandrels where insulation is added to the framed-wall assembly behind the spandrel to improve thermal performance. This may lead to inaccurate façade thermal performance values, whole building energy performance, additional building greenhouse gas emissions, and higher condensation risks.

This research investigates various thermal simulation techniques for spandrel assemblies and details that may be found in building façades and highlights some of the differences in overall heat flow reported as U-factors and surface temperatures to evaluate condensation risks. These results may help identify shortcomings of various procedures and help determine appropriate thermal simulation techniques to improve the accuracy of evaluating spandrel thermal performance on building façades.

Background

The thermal performance of spandrel assemblies in glazed wall systems is often evaluated with finite element analysis (FEA) computer simulations. While these thermal simulation programs are extensively validated against many laboratory measurements of fenestration and opaque assemblies, the accuracy of the simulated results varies for spandrel assemblies depending on the analysis methodology.

The most common methodology for glazed wall systems currently used in North America is two-dimensional (2D) thermal simulations following NFRC standards, which has been mostly developed through validation of total product U-factors of discrete vision units to within 10% of laboratory measurements. Until 2022 glazed wall systems have mostly been evaluated using the NFRC-100 standard. While this method can provide accurate thermal performance results for fenestration products, it can provide misleading results for spandrel assemblies. Even though NFRC standards are not approved or meant for calculating opaque spandrel performance, the industry has used it to provide assembly U-factors since there is no current standard approved to date besides a hot box test.

Studies such as Norris 2015 and Bettenhausen 2015 have shown thermal transmittance values derived from 2D thermal simulations following ANSI/NFRC 100 and ISO 10599 standards may differ from calculations from guarded hot box measurements by 20% to 30%. The difference in thermal performance depends on the size and configuration of the spandrel assembly as greater differences were found in spandrel assemblies with more spandrel insulation, as noted by Bettenhausen, 2015. Differences between simulated and measured surface temperatures were greater than differences in calculated thermal transmittance values (Bettenhausen, 2015), which could lead to inaccurate condensation risk assessments. As a result, many studies have noted 2D thermal simulation results following NFRC-100 should be compared to physical testing since these simulations may not provide accurate thermal transmittance values and surface temperatures (Dunlap 2018, Jackson et al 2018).

Other thermal simulation methods have been proposed and evaluated such as the new NFRC spandrel procedure and three-dimensional (3D) thermal simulations. The new NFRC spandrel procedure addresses some of the key criteria that require adjustments in the previous ANSI/NFRC-100 standard by extending the edge distance from 2.5 inches to 10 inches to better align with laboratory measurements, which studies such as Norris 2015 have shown to produce simulation results closer to hot box measurements. While this new procedure was developed based on laboratory measurements, studies comparing the new methodology to physical testing or other simulation methods have not been available since it was recently released in 2022.

In comparison, three-dimensional (3D) thermal simulations of spandrel assemblies have provided more accurate results to a physical test result. Studies from Norris 2015 and Boafo 201 have shown 3D thermal simulations can provide thermal transmittance values to within 5% of guarded hot box measurements. Similarly, 3D thermal simulations can also provide surface temperatures that are more closely aligned with hot box measurements than with previous 2D thermal simulations based on NFRC for glazing systems. Since the new procedures could soon become the method for spandrel thermal performance, understanding how the current and new procedures compare to each other is the focus of this paper.

Thermal Simulation Study

For this research, the effectiveness of various spandrel thermal simulation methods was evaluated with a curtain wall system for various spandrel sizes and conditions. The evaluated system is a thermally broken aluminum pressure cap curtain wall system with spandrel glazing and 4 inches of mineral wool insulation (R-16.8) and a galvanized steel back pan as shown in Figure 1.

Figure 1: Evaluated Curtain Wall System

The spandrel assembly was evaluated for three sizes and two façade conditions that represent details commonly found in most modern building façades for 12 scenarios, as listed in Tables 1 and 2.

Table 1: Spandrel Thermal Simulation Sizes

Table 2: Adjacent Spandrel Conditions

The spandrel assemblies were evaluating using the three thermal simulation methods listed below. These methods were chosen based on current industry practices and recent industry guidelines.

1. Two-dimensional (2D) thermal simulations following NFRC-100: This procedure follows NFRC 100 per U-factor calculations meant for vision glazing by dividing the spandrel assembly into three areas: frame, edge of panel, and center of panel. The spandrel assembly U-factor is calculated as an area weighted average of U-factors determined with 2D thermal simulations following the ISO 15009 standard for the frame, edge of panel, and center of panel sections based on a 2-1/2 inch (63.5 mm) edge of panel distance as shown in Figure 2.

Figure 2: NFRC-100 Layout

1. Two-dimensional (2D) thermal simulations following NFRC spandrel procedure: This procedure follows the latest NFRC 100 2023 per new sections for spandrel panel systems. The area-weighted calculation still follows ISO 15009 with the exception of an edge of panel to be 10in (254 mm). The panel size is still the same standard size as a vision unit of 79 inch x 79 inch (2000mm x 2000mm) as shown in figure 3.

Figure 3: NFRC-100 Layout

1. Fenestration Association of BC (FENBC) :

This procedure is based on NFRC 100 for glazing. The main difference is that the edge of the panel is 6 inches instead of 2.5 inches as shown in Figure 3. The area weight is done in a spreadsheet and not on Window software, similar to the new NFRC spandrel method. The Reference Procedure also includes context assemblies such as glazing or a slab. For the purpose of this paper, the 10in have been kept for consistent comparison between the methods used.

Figure 4: NFRC-100 Layout

1. Three-dimensional (3D) thermal simulations following ASHRAE 1365 RP procedure /CSA Z5010 standard: This procedure is based on the methodology presented in ASHRAE 1365 RP and the CSA Z5010 standard for evaluating thermal bridging of opaque building envelope and façade assemblies. This approach is similar to the ISO 10211 standard and uses ISO 10077-2 standard to approximating heat transfer in enclosed air cavities less than 1/2 inch (12.7 mm) in depth. The thermal simulations are completed using the Nx and SimCenter software package from Siemens, which is a general-purpose computer aided design (CAD) and finite element analysis (FEA) package.

Spandrel Assembly Simulation Results

The simulated thermal performance of the three thermal simulation methodologies vary depending on the size of an isolated spandrel assembly as shown in Table 3.

Table 3: Thermal Performance of Isolated Spandrel Assemblies determined with Various Thermal Simulation Methods

Thermal performance following the 2D thermal simulation procedures consistently report lower U-factors and higher effective R-values when compared to 3D thermal simulation methods. Thermal simulations following the 2D NFRC-100 standard is differ from 3D thermal simulation results by 23% to 41% in terms of U-factors and 30% to 41% for effective R-values, while simulations following the 2D NFRC Spandrel procedure differ by 5% to 39% in terms of U-factors, and 5% to 63% in terms of effective R-values for the spandrel sizes simulated for this study.

The difference in simulated thermal performance between the 2D NFRC spandrel method and 3D thermal simulation method varies depending on the size of the spandrel. The difference in simulated spandrel thermal performance is less for larger spandrels and greater for smaller spandrels. This is shown in Figure 5, which plots the difference in simulated spandrel U-factors for the two NFRC simulation methods compared to 3D thermal simulations for various spandrel sizes.

Figure 5: U-factor and Effective R-value Difference Compared to 3D Thermal Simulations for Isolated Spandrel Assemblies of Various Sizes

As previous studies have noted, thermal simulations of spandrel assemblies following the NFRC-100 standard for glazing provide inaccurate U-factors that could be 20-30% different than guarded hot box measurements. This is primarily due to the 2.5 inch (63.5 mm) edge distance specified in the standard. The edge distance in glazed wall assemblies is the region between the center of the vision glazing or spandrel panel and mullions where heat flow transitions from one-dimensional heat flow at the center of the panel to multi-dimensional heat flow at the mullion. It is used as a simplification to approximate three-dimensional heat flow at vertical and horizontal mullions with 2D thermal simulations. While the 2.5 inch (63.5 mm) edge distance provides enough area to capture this transition in heat flow for vision glazing, it is not enough for most spandrel assemblies since glass is much more conductive than insulated spandrels. This is shown in Figure 6 below, where temperature gradients in the edge region are shown for vision glazing and spandrel assemblies.

This is not a limitation for 3D thermal simulations since actual dimensions of the spandrel assemblies are modeled, and heat flow paths and temperature gradients are directly simulated.

Figure 6: Comparison of Temperature Gradients between Vision Glazing (right) and Spandrel Assembly (left)

Another difference of 2D and 3D thermal simulation methods for spandrel assemblies is surface temperature. Since 2D thermal simulations do not consider lateral heat flow between horizontal and vertical mullions directly there may be variations in surface temperatures due to impact of localized geometry. The surface temperatures of the three simulated isolated spandrel assemblies are listed in Table 3. The surface temperatures at presented as a temperature index, which is a ratio of the difference between critical location temperature and the exterior temperature to the overall temperature difference as shown in Equation 1.

Ti = Tlocation− TexteriorTinterior− TexteriorEquation 1:

Figure 7: Critical Locations of Isolated Spandrel Assemblies

Table 3: Simulated Temperature Indices at Critical Locations

Simulated surface temperatures differ between spandrel assemblies evaluated using 2D and 3D thermal simulation methods. Surface temperatures evaluated using 2D thermal simulation methods tend to be lower than equivalent temperatures evaluated from 3D thermal simulations. Since 2D thermal simulations only evaluate slices or sections within the spandrel assembly, it does not consider lateral heat flow paths from perpendicular mullions. For example, the frame and backpan temperature indices listed in Table 3 are consistent across all spandrel sizes for 2D thermal simulations, while these values vary for 3D thermal simulations. The center of frame and edge of backpan temperature indices at the mid-height of the spandrel assembly from 3D thermal simulations decrease with smaller spandrel sizes since the mid-height of the spandrel is closer to the corner of the spandrel, where heat loss is greatest, for smaller spandrels.

Although 2D thermal simulations cannot evaluate surface temperatures near the intersection of horizontal and vertical mullions, the simulated surface temperatures at the center of the vertical mullion appear to be lower than the surface temperatures near the corner of the spandrel assembly for the evaluated spandrel sizes. As a result, these surface temperatures may serve as conservative proxies for evaluating condensation risk to spandrel assemblies in isolation. However, this approximate only applies to the mullions since the edge of the backpan near the corner appear to be colder than the edge of the backpan at the vertical mullion evaluated using 2D thermal simulations. 3D thermal simulations may be required for more precise condensation analysis which not only captures surface temperatures but can also show the extent of areas where surface temperatures are below the interior dewpoint temperature.

Spandrel Assembly Detail Simulation Results

As previously noted, spandrel assemblies are often not installed in isolation from the rest of the façade and building. Adjacent systems, such as vision glazing, interior insulated steel-frame wall assemblies, and intermediate floor slabs are often attached to spandrel assemblies. . The impact of these adjacent systems on spandrel thermal performance was evaluated to determine how similar the simulated spandrel thermal performance is using 2D and 3D thermal simulations methods.

Following the User Guide to Reference Procedure for Simulating Spandrel U-factors from the Fenestration and Glazing Industry Alliance (FGIA) FENBC Region for adjacent systems, the thermal performance of the spandrel with adjacent vision glazing was evaluated. For condensation analysis using 2D thermal simulation methods the specified IGU was simulated. No material substitutions were made for 3D thermal simulations, the thermal performance and condensation analysis was evaluated using the specified IGU. The simulated thermal performance of the spandrel assemblies using 2D and 3D thermal simulation methods are listed in Table 4. The 2D thermal simulations completed with adjacent assemblies followed the FIGA Spandrel U-factors reference procedure with the 10 inch (254 mm) edge distance specified in the NFRC Spandrel procedure.

Table 4: Thermal Performance of Spandrel Assemblies with Adjacent Vision Glazing

Figure 8: U-factor and Effective R-value Difference Compared to 3D Thermal Simulations for Spadnrel Assemblies of Various Sizes adjacent to Vision Glazing

The simulated spandrel thermal performance based on 2D thermal simulations are between 5% to 22% better than 3D thermal simulations in terms of U-factor, and 6% to 28% better than 3D thermal simulations in terms of effective R-values. The thermal performance difference is dependent on the size of the spandrel as larger spandrels show better agreement while smaller spandrels show greater difference in simulated thermal performance.

The spandrel thermal performance determined using 2D thermal simulations appear to be not only better than the same detail evaluated using 3D thermal simulation methods, but also isolated spandrel assemblies without adjacent vison glazing. This is because when spandrel assemblies are simulated with an adjacent assembly that has higher thermal transmittance, such as vision glazing, more heat is drawn through the adjacent assembly due to thermal bridging making the spandrel portion of the model to appear to have less heat flow. Since current 2D thermal simulation procedures rely on U-frame and U-edge values based on heat flow across specific sections of the spandrel in the models to calculate spandrel U-factors and effective R-values, this approach overestimates spandrel thermal performance.

In comparison, a spandrel assembly adjacent to vision glazing detail will have greater heat flow than the sum of the heat flow of the vision and spandrel sections modeled in isolation. This additional heat flow is captured in the spandrel thermal performance values since the spandrel heat flow was calculated by subtracting the overall heat flow with the isolated vision section heat flow. This approach is aligned with most building energy codes and standards where vision section U-factors are considered to be independent of adjacent assemblies. For the current 2D thermal simulation methods to properly account for this additional heat flow, the U-factor of the vision section would need to be adjusted based on the adjacent system. The FGIA Spandrel Reference Procedure has recognized that their approach is meant for rating products rather than simulating the installed system performance.

Surface temperatures also differ for spandrel assemblies with adjacent vision glazing as shown in Table 5 which lists temperature indices at critical locations of the spandrel assemblies as shown in Figure 9.

Figure 9: Critical Location for Temperature Indices for Spandrel Assemblies with Adjacent Vision Glazing

Table 5: Temperature Indices at Critical Locations of Spandrel Assemblies with Adjacent Vision Glazing

The temperature indices for spandrel assemblies with adjacent vision glazing show similar trends as the isolated spandrel scenarios. Temperature indices evaluated using 3D thermal simulations are higher at center of frame, edge of backpan at mid-height of the spandrel, while other locations show agreement between the two simulation methods. While 2D simulation methods are unable to evaluate surface temperatures at corners and intersections between vertical and horizontal mullions, the temperature indices at the center of the frame and edge of glass at the middle of the horizontal mullion may be good approximations for condensation analysis. Surface temperatures and temperature indices may vary for other glazed wall systems where the configuration of the vision and spandrel sections could influence local temperatures. Analysis that requires precise surface temperatures should use 3D thermal simulations that directly model the built system geometry.

The addition of insulation inboard of the back pan as part of the interior steel stud wall, referred to as interior insulation, is a common strategy for improving spandrel thermal performance. The additional interior insulation reduces heat flow through the spandrel which reduces the thermal bridging impacts of the mullions. The impact of adding 2 inches of (51 mm) of mineral wool insulation (R-8.4) inboard of the spandrel backpan for spandrel assemblies with adjacent vision glazing is shown in Table 6.

Table 6: Thermal Performance of Spandrel Assemblies with Interior Insulation (R-8.4) Adjacent to Vision Glazing

Figure 10: U-factor and Effective R-values Difference Compared to 3D Thermal Simulations of Spandrel Assemblies with R-8.4 Interior Insulation Adjacent to Vision Glazing

The simulated spandrel thermal performance based on 2D thermal simulations are between 45% to 67% is better than 3D thermal simulations in terms of U-factor, and 82% to 206% better than 3D thermal simulations in terms of effective R-values for the NFRC height and safety height spandrel panels. The thermal performance difference is dependent on the size of the spandrel as larger spandrels show better agreement while smaller spandrels show greater difference in simulated thermal performance.

The simulated spandrel thermal performance is much closer for the slab cover height spandrel, which includes the 2inch (51 mm) fire safing insulation and 8 inch (203 mm) thick concrete slab floor. However, the overall spandrel U-value and effective R-value calculated using 2D thermal simulation methods only considered the overall values with the mullions and slab as a total. Referencing the FGIA Spandrel U-factor Reference Procedure, the slab edge results in the 2D thermal simulation did not account for the vertical mullions at this condition which were subsequently omitted from the analysis. The similar thermal performance values may be a coincidence of the system and slab edge configuration and may differ for other systems or detail configurations.

The simulated spandrel thermal performance results between 2D and 3D thermal simulation methods shows similar trend the other scenarios with better performance values from 2D results relative to 3D thermal simulations. This difference in thermal performance is also due to the approach of U-frame and U-edge factors to calculate spandrel thermal performance used in 2D thermal simulations. The addition of interior insulation in these models directs more heat flow through the less insulating vision glazing due to thermal bridging, making the spandrel section appear to perform better.

Temperature indices based on simulated surface temperatures of spandrel assemblies adjacent to vision glazing with interior insulation are listed in Table 7 and are based on critical locations shown in Figure 11.

Figure 11: Critical Location for Temperature Indices for Spandrel Assemblies with Adjacent Vision Glazing and Interior Insulation

Table 7: Temperature Indices at Critical Locations of Spandrel Assemblies with Interior Insulation (R-8.4) Adjacent to Vision Glazing

The temperature indices determined by 2D and 3D thermal simulation methods for locations directly exposed to the interior, such as the center of the horizontal mullion and edge of vision glazing at the mid-span of the detail are very similar for the simulated NFRC spandrel size and safety height spandrel size details. The temperature indices deviate for surface temperatures of the spandrel assembly outboard of the interior insulation with 2D thermal simulation methods providing conservative values. However, for the slab height spandrel assembly with the floor slab and fire safing insulation, the 2D thermal simulations provide less conservative temperature indices compared to 3D thermal simulations. This would underestimate the condensation risks which could lead to more frequent conditions where condensation on the system would occur.

Impact of these results on whole building energy performance

The variance in spandrel thermal performance determined using 2D and 3D thermal simulation methods can have an impact on the façade thermal performance and building energy performance. The impact of these differences were not evaluated but acknowledged how critical having project size values be calculated for the energy modeling input criteria.

Conclusion

The results of this study highlights some of the potential differences between simulated thermal performance among 2D and 3D thermal simulation methods for spandrel assemblies. Current 2D thermal simulation methods, such as the NFRC spandrel procedure and the FIGA Spandrel U-factor Reference procedure, have reduced the variance in simulated thermal performance with 3D thermal simulations for larger spandrel assemblies that are are at least close to or larger than 79 inch x 79 inch (2000 mm x 2000 mm). However, other 2D methods show greater differences in thermal performance compared to 3D methods for smaller spandrel sizes, spandrel assemblies adjacent to vision glazing, and spandrel assemblies with interior insulation inboard of the back pan, all of which are common conditions found in most building façades. These findings are from “snapshot” of a specific system and most common variables for a spandrel system.

Some findings from this snapshot of an analysis include:

• 2D thermal simulations following the NFRC spandrel procedure for large spandrel assemblies (79 inch x 79 inch, 2000 mm x 2000 mm) can provide similar results to 3D thermal simulations for isolated spandrel assemblies and spandrel assemblies adjacent to vision glazing with less than 7% difference in U-factors and effective R-values.
• 2D thermal simulations, in general, over predict spandrel thermal performance compared to 3D thermal simulations. This difference is greater for spandrel assemblies adjacent to vision glazing, where more heat is transferred to the less insulating vision section due to thermal bridging.
• The difference between 2D and 3D simulated spandrel thermal performance increases with decreasing spandrel size. This difference can vary from less than 17% to 39% in terms of U-factors for isolated spandrel assemblies with 24 inch and 10 inch height, respectively.
• The difference between 2D and 3D simulated spandrel thermal performance increases significantly for smaller spandrel sizes when simulated with adjacent systems and interior insulation. Simulated spandrel U-factors can vary from 24% for 24 inch and 10 inch tall spandrels adjacent to vision glazing, to 62% and 72% for 24 inch tall and 10 inch tall spandrel assemblies with interior insulation and intermediate floor slab edge, respectively.
• 3D thermal simulations provide more precise surface temperatures and temperature indices for condensation risk analysis compared to 2D thermal simulations. However, 2D thermal simulations can provide surface temperatures that are similar to 3D thermal simulations near the mid-height and mid-span of the spandrel assemblies.

The findings from this study is only specific to the curtain wall system and spandrel details that were simulated. The differences in the simulated U-factor and effective R-values may change depending on the system. This study presents a “snapshot” of the potential differences in simulated spandrel thermal performance based on different thermal simulation methods. Further investigation is required to determine if similar trends are found for other spandrel system types using similar simulation techniques.

These differences in simulated spandrel thermal performance can be significant, especially for spandrel conditions that are commonly found in most glazed wall façade systems. An overprediction in spandrel thermal performance can lead to inaccurate façade thermal performance and whole building energy performance. As building energy codes and standards introduce more stringent requirements and jurisdictions introduce policies that penalize excessive building energy use, the need for accurate thermal simulation techniques is essential. Current 2D thermal simulation methods are able to provide accurate thermal performance values for large isolated spandrels, however, 3D thermal simulations may be required to evaluate thermal performance of other spandrel details that may be found in many façade systems such as smaller spandrels, spandrels adjacent to vision glazing, and interior insulated spandrel assemblies.

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