Wall Assemblies Research Articles

Evaluation of thermal bridging mitigation techniques and impact of calculation methods for lightweight steel frame external wall systems

Thermal bridging is known to affect the thermal performance of lightweight steel-framed external walls. Various manual calculation methods have been developed to estimate this impact, including the parallel path, NZS 4214 and modified zone methods, with the NZS 4214 method recently adopted for non-residential buildings by the Australian National Construction Code 2019 via its explicit reference in AS/NZS 4859.2. These methods have been validated against different wall assemblies from those currently used in the Australian context, therefore this paper aims to examine the validity of thermal bridging calculation methods in this context and evaluate the impact of mitigation methods on the overall thermal performance. We developed a set of 34 realistic external cold-formed steel-framed wall configurations based on available market products and construction practices and compared the R-values from manual calculation methods with outputs from the 2D simulation THERM. Results have shown that the NZS 4214 method as recently adopted by the Australian National Construction Code 2019 for non-residential buildings has good applicability for wall assemblies with lower R-values (R = 1.8–2.3 m 2 K/W). However, it was shown to have larger differences for calculations of walls with higher R-values (R > 2.5 m 2 K/W). The isothermal planes method can produce various results for the same building assembly, depending on which planes are assumed to be isothermal. This study has demonstrated that superior accuracy may be obtained by varying the definition of such planes depending on the R-value of the assembly. When considering designs to mitigate thermal bridging effects and improve overall wall performances, outside frame insulation and high resistance claddings have been shown to provide the best results (decreasing variation in temperature across the surface by 75%). • R-values for suite of Australian cold formed steel wall configurations assessed. • NZS4214 calculation method shown to have good applicability for low R-values. • Isothermal plane boundary location for NZS4214 requires investigation. • External insulation and high resistance claddings provide thermal bridge mitigation.

Hygrothermal Monitoring of Two Pilot Prefabricated Exterior Energy Retrofit Panel Designs

NRCan undertook a proof-of-concept project to retrofit a small building with prefabricated wall panels in 2017 in Ottawa, Canada. The retrofit used two wall panel designs: nailbase and woodframe. The Nailbase panel consisted of fiberglass batt, an expanded polystyrene (EPS) core, oriented strand board (OSB) sheathing, a rainscreen, and cladding. The Woodframe panel also featured OSB sheathing and included a 90 mm stand-off gap filled with dense-packed, fibrous insulation. A side-by-side comparison of cost, constructability, and performance was performed. The wall assemblies were instrumented to monitor the temperature, relative humidity, and moisture content of sensitive layers. The data was used to evaluate the hygrothermal performance, moisture accumulation, and risk of associated problems such as mould growth. This paper presents the monitored hygrothermal data from 2017 to 2021, compares the two approaches and assesses their feasibility. During construction, some of the fibrous insulation may have been wetted by wind-driven snow before completion. The data showed that this moisture was able to dissipate without significant risk. The sheathing of the Woodframe panel experienced a higher peak moisture content during the dry-out period. Otherwise, both panel designs showed limited potential for mould growth on monitored surfaces over the monitored period.

Determining the impact of sensor orientation on moisture content measurements in eastern white pine

As buildings become more airtight and insulated, the movement and accumulation of moisture within building envelopes become paramount in determining its resiliency. Current methods for quantifying the moisture content (MC) of wood species involve the measurement of electrical resistance between two installed electrodes and the use of existing empirical correlations to evaluate the MC. However, these correlations do not adequately consider the impact of sensor orientation within wall assemblies. The objective of this paper is to determine the impact of MC readings within a wood sample due to sensor orientation. A total of 126 eastern white pine samples were tested with electrodes placed along the grain of the wood (longitudinal), across the grain of the wood (tangential), and in a diamond pattern, using six different fasteners as electrodes. The samples were placed in a controlled environmental chamber until steady state was achieved at approximately 18% MC. Electrical resistances of the samples were measured in both directions at temperatures ranging from -10°C to 40°C. It was found that the tangential-to-longitudinal resistance ratio is 1.1-1.35 depending on the electrode type.

Transient thermal response of opaque building envelope elements: EPFL campus case study

The building envelope acts as a shield against varying weather conditions and modulates thermal energy flow between outdoors and indoors. The choice of layers used in the assembly impacts the heat loss and gain through the wall structure and potentially can affect the comfort indoors. Thus, the building envelope plays an essential role in the thermal performance of the building. Optimizing the cladding design in the envelope has recently become increasingly important to reach sustainable development strategies for reducing greenhouse gas emissions by 2050. This paper aims to analyze several cladding types used on the EPFL campus in Lausanne and compare their impact on the energy performance of the building envelopes. The building assemblies constructed on the EPFL campus in different years vary in composition and thermo-physical properties of the layers used. The impact of these parameters on the thermal performance of the wall assembly is evaluated by comparing the variation of heat flux and temperature fluctuations within the wall structure. The results obtained highlight the importance of the building envelope layers and materials used in the wall structure. Due to the variations in the thermal inertia of different wall assemblies, a time shift of more than 3 hours in the transient response of the building envelope to the fluctuation of the outdoor weather conditions is observed.

Component sequence and thermal mass effects on the transient thermal performance of concrete walls

The increased requirements of buildings to reduce energy use have highlighted the importance of accounting for all factors that influence energy use in buildings. One consideration that requires further study in the envelope design of concrete-based wall assemblies is the placement of the thermal mass layer. In this study, two thermally massive walls, Insulated Concrete Form (ICF) and tilt-up walls, with the same thermal resistance but different sequencing of layers are investigated. In addition, a wall made of a homogeneous insulation layer with an identical thermal resistance was considered to further investigate the thermal mass effect on the potential for energy savings. Results of the numerical simulations performed using COMSOL Multiphysics® software indicate that, for the transient scenarios investigated, thermal mass can contribute to shifting and dampening peak heating and cooling loads, as well as saving energy. Also, less intense fluctuations were observed in the heat fluxes when considering the ICF wall. Energy savings during the primary seasons (i.e. winters in Montreal and summers in Miami) are found to be marginal but the existence of a thermally massive layer considerably reduced the demands during secondary seasons i.e. summers in Montreal and winters in Miami.

Adapting residential envelope assemblies for full circularity

Residential external wall assemblies are among the key contributors to embodied carbon emissions in the building industry. Their design, however, is still largely oriented towards linear consumption trajectories of extraction-use-waste. Within this context, this paper investigates how established material recovery potential assessment metrics could be used to inform design decisions aimed at improving circularity in buildings. A redesign of a typical timber frame assembly is presented and its material recovery performance is compared to standard systems. Results show a 35%-47% improvement in material recovery potential.

Mass Timber Envelopes in Passivhaus Buildings: Designing for Moisture Safety in Hot and Humid Australian Climates

The uptake of buildings employing cross-laminated timber (CLT) assemblies and designed to Passivhaus standard has accelerated internationally over the past two decades due to several factors including responses to the climate crisis by decarbonising the building stock. Structural CLT technology and the Passivhaus certification both show measurable benefits in reducing energy consumption, while contributing to durability and indoor comfort. However, there is a general lack of evidence to support a fast uptake of these technologies in Australia. This paper responds to the compelling need of providing quantitative data and adoption strategies; it explores their combined application as a potential pathway for climate-appropriate design of energy-efficient and durable mass timber envelope solutions for subtropical and tropical Australian climates. Hygrothermal risk assessments of interstitial condensation and mould growth of CLT wall assemblies inform best-practice design of mass timber buildings in hot and humid climates. This research found that the durability of mass timber buildings located in hot and humid climates may benefit from implementing the Passivhaus standard to manage interior conditions. The findings also suggested that climate-specific design of the wall assembly is critical for mass timber buildings, in conjunction with excellent stormwater management practices during construction and corrosion protection for metallic fasteners.

In-Situ and Predicted Performance of a Certified Industrial Passive House Building under Future Climate Scenarios

The Wood Innovation Research Lab was designed as a low energy-use building to facilitate the construction and testing of engineered wood products by the faculty and staff of the Master of Engineering in Integrated Wood Design Program at the University of Northern British Columbia in Prince George, BC, Canada. Constructed using a 533 mm thick-wall and 659 mm flat roof assembly, it received certification as Canada’s first industrial facility built to the International Passive House standard. Temperature and humidity sensors were installed in the north and south exterior wall assemblies to measure long-term hygrothermal performance. Data collected between 2018–2020 shows no record of long-term moisture accumulation within the exterior assemblies. Data collected during this time period was used to validate hygrothermal performance models for the building created using the WUFI® Plus software. Long-term performance models created using future climate data for five cities across Canada under two global warming scenarios shows favorable results, with an increase in average annual temperatures resulting in lower average relative humidity values at the interior face of the exterior sheathing board in the exterior wall assemblies.

A decision-making framework for life-cycle energy and seismic loss assessment of buildings

The integration of building seismic losses and environmental impacts is a key step toward the improvement of building design approaches accounting for resilience and sustainability in parallel. A decision-making framework including the life-cycle energy assessment and seismic loss estimation is proposed in this study in order to investigate the trade-offs between buildings’ environmental sustainability and reducing seismic loss. The framework includes a Pareto front analysis to reach a balance among the criteria. Building operational energy, embodied energy, seismic repair cost, and injuries were the performance metrics considered. The framework was applied to three reinforced concrete building designs including low- to high-rise buildings along with six exterior wall assemblies and four Window-to-Wall-Ratios. The contradictory trend among the performance metrics underlines the importance of merging these concepts for the development of the current building design methodologies. The hypervolume indicator was computed for each building’s Pareto front set to evaluate the quality of Pareto front design solutions. The results showed that compared to the 20-story building, the 2- and 12-story buildings have higher number of Pareto solution points and the solutions are of higher qualities in terms of diversity and proximity to the true Pareto front based on the hypervolume indicator solutions.

Toward understanding ignition vulnerabilities to firebrand showers using reduced-scale experiments.

Over the past few years, the large outdoor fire problem has been a growing concern throughout the world. It is recommended to clear the combustibles around homes and within communities to avoid potential loss of properties, as firebrand shower ignition is a dangerous threat. One of the common combustibles around homes is mulching materials. A reduced-scale experimental protocol was developed to study ignition of mulching materials by firebrands and resulting impact to adjunct wall assemblies. Reduced-scale experimental results were compared with full-scale experimental results. Specifically, two trends were of interest in the comparisons. First, the ranking of the ease of ignition for various mulch types from exposure to firebrand showers. Second, if a given mulch type ignited from exposure to firebrand showers, was the resulting mulch bed fire able to ignite the adjacent wall assembly. The reduced-scale experimental results captured some of these trends observed from full-scale experiments but not completely. The findings still suggest that the reduced-scale experiments may give insights into how easily different mulch beds may be ignited by firebrands, as compared to much more costly and time-consuming full-scale experiments. While it is interesting to conduct full-scale experiments, this is very expensive and not always practical, so the authors are devising far cheaper reduced-scale experiments to provide more in-depth scientific understanding of firebrand shower ignition of construction components.

Temperature Control to Improve Performance of Hempcrete-Phase Change Material Wall Assemblies in a Cold Climate

Phase change material (PCM)-enhanced building envelopes can control indoor temperatures and save energy. However, PCM needs to undergo a phase change transition from solid to liquid and back to be fully effective. Furthermore, most previous research integrated PCM with high embodied energy materials. This study aims to advance the existing research on integrating PCM into carbon-negative wall assemblies composed of hempcrete and applying temperature control strategies to improve wall systems’ performance while considering the hysteresis phenomenon. Four hempcrete and hempcrete-PCM (HPCM) wall design configurations were simulated and compared under different control strategies designed to reduce energy demand while enhancing the phase change transition of the microencapsulated PCM. The HPCM wall types outperformed the hempcrete wall assembly through heating (~3–7%) and cooling (~7.8–20.7%) energy savings. HPCM walls also maintained higher wall surface temperatures during the coldest days, lower during the warmest days, and within a tighter range than hempcrete assembly, thus improving the thermal comfort. However, the results also show that the optimal performance of thermal energy storage materials requires temperature controls that facilitate their charge and discharge. Hence, applied control strategies reduced heating and cooling energy demand in the range of ~4.4–21.5% and ~14.5–55%, respectively.

Acoustic design tools for estimation of sound insulation performance of wood wall and floor assemblies

The National Building Code of Canada 2015 stipulates the minimum requirements of the airborne sound insulation transmission through common interior walls and ceiling/floor assemblies. The required minimum Apparent Sound Transmission Class (ASTC) is 47 in Canada, whereas the Impact Insulation Class (IIC) for floors is recommended to be higher than 55. For many years, significant efforts were made to develop sound insulation prediction models or tools to predict the sound insulation performance of wall and floor/ceiling assemblies at the design phase in order to meet the requirements and the recommendations made by codes. However, today few models can provide a reliable acoustics design tool. In this document, three prediction tools thought to be practically useful are presented and evaluated. Between these three prediction tools, one is an analytical model of the Insul software while the other two are empirical models developed by the National Research Council of Canada and the American Wood Council. This paper compared the STC and IIC ratings of wood wall and floor assemblies estimated by these three models and verified them by the measured STC and IIC ratings. This work aims at providing an idea for readers to choose a suitable design tool to proceed with their acoustic designs.

Gauge repeatability and reproducibility study of airborne sound isolation measurements

A gauge repeatability and reproducibility study (GRR) uses analysis of variations (ANOVA) on an appropriately designed experiment to separate and quantify the components of the overall uncertainty. The authors have previously presented results of GRR studies of the measurement of airborne and impact insulation of floor-ceiling and demising wall assemblies in several apartment buildings, in which the uncertainty in the measurement method and the variability of the nominally-identical assemblies were compared. The results of two additional GRR studies on measurements of airborne noise isolation of wood stud demising walls are presented. The first study, like previous studies, evaluates the components of variance attributable to operator, repeatability, and part. The second study uses a fixed operator and part, and evaluates the variance due to loudspeaker type, position, and level on the measured noise reduction. The measurement standard (ASTM E336) gives limited guidance on the type and location of the loudspeaker used on the source side, and this study can inform whether changes in the standard with regards to the loudspeakers could reduce the uncertainty in measurement.

Hygrothermal Analysis of CLT-Based Retrofit Strategy of Existing Wall Assemblies According to EN 13788 Standard

In the framework of the ongoing EU-funded innovation project called e-SAFE (energy and Seismic Affordable rEnovation solutions), several solutions for the energy and seismic deep renovation of reinforced-concrete (RC) framed buildings in the EU countries are going to be developed and demonstrated. One of these solutions makes use of cross laminated timber (CLT) panels connected to the existing RC frame through specifically designed dampers to increase the seismic and energy performances of the existing envelope. This paper aims to preliminary assess the hygrothermal performance of such CLT panels when applied to various typical wall structures under different climate conditions in Italy through numerical simulations carried out according to the EN 13788 Standard and considering various indoor vapor production classes. Results show that the most problematic existing wall structures are uninsulated concrete walls, for which a risk of surface condensation and mold growth is predicted in all climate zones because of their low thermal resistance (U-value of 3.55 W·m-2·K-1), followed by uninsulated solid brick walls (U-value of 1.81 W·m-2·K-1). The application of CLT panels is found to not only significantly improve the thermal behaviour of the walls, but also to eliminate any surface and interstitial condensation issues in all climate zones.

Hygrothermal investigation of lightweight steel-framed wall assemblies in hot-humid climates: Measurement and simulation validation

In recent years, lightweight steel-framed (LSF) constructions are considered sustainable and widely used worldwide. However, the LSF wall assemblies have the risk of durability failure in hot-humid climates. This study conducted fundamental research on the hygrothermal performance of LSF wall assemblies by characterizing the hygrothermal responses, modelling and validating the simulation models under hot-humid climatic conditions. Through measuring the relative humidity and temperature of various positions inside, four typical and full-scale LSF wall assemblies were tested thoroughly and comparatively on two room-like test cubes in Guangzhou, a hot-humid city in China. It was found that the ventilated rainscreen provided better hygrothermal responses of the wall assemblies than the face-sealed cladding; the closed-cell external insulation could significantly improve the hygrothermal responses of the wall assemblies with stucco cladding; generally, wall assemblies with ventilated rainscreen and external insulation exhibited the best hygrothermal response in a hot-humid climate, followed by wall assemblies with ventilated rainscreen, wall assemblies with face-sealed cladding and external insulation and wall assemblies with face-sealed cladding. Furthermore, the measured data were used to model and validate the simulation models of the four typical wall assemblies in software. It was shown that hygrothermal simulation is sufficient for evaluating the long-term hygrothermal performance of LSF wall assemblies in hot-humid climates, and relatively conservative settings are necessary for evaluating cases with great uncertainties. Overall, this study could lay a foundation for better assessing and improving the hygrothermal performance, durability and sustainability of LSF buildings in hot-humid climate regions worldwide.

Experimental and numerical investigation of fixed-height cold-formed steel wall assemblies bearing on concrete slabs

Cold-formed steel bearing walls are frequently installed on concrete slabs, which do not provide perfectly rigid and uniform conditions. However, existing design guidance assumes bearing condition has no impact on axial capacity, though cold-formed steel compressive members are particularly susceptible to these end conditions. Additionally, studs are typically installed at the edge of the concrete slab, which is prone to spalling and geometric irregularities, and stud walls are occasionally installed inadvertently overhanging from them. In this paper, the impact of non-uniform and partial bearing conditions on the axial strength of cold-formed steel (CFS) wall assemblies is identified and characterized experimentally and numerically. Studs of various cross-sections and bearing conditions were considered. Bearing conditions include: full bearing (edge distance ≥ 20.32 cm (8 inches)), close to the edge, at the edge, and partially overhanging from the slab. In addition to the experiments, high-fidelity finite element modeling of all the systems was conducted to validate the experimental results and elucidate the impact of parameters not captured during the tests. The results provide a means of evaluating existing design guidelines presented in the North American Specification of the American Iron and Steel Institute (AISI S100-16). Twenty-seven tests were conducted on CFS wall assemblies. They reveal that there is a potential need for improvement of current AISI specifications, where all bearing conditions are assumed rigid and uniform. Design recommendations for improving the current specification are discussed.

Edgewise Compressive Behavior of Composite Structural Insulated Panels with Magnesium Oxide Board Facings.

Edgewise compression response of a composite structural insulated panel (CSIP) with magnesium oxide board facings was investigated. The discussed CSIP is a novel multifunctional sandwich panel introduced to the housing industry as a part of the wall, floor, and roof assemblies. The study aims to propose a computational tool for reliable prediction of failure modes of CSIPs subjected to concentric and eccentric axial loads. An advanced numerical model was proposed that includes geometrical and material nonlinearity as well as incorporates the material bimodularity effect to achieve accurate and versatile failure mode prediction capability. Laboratory tests on small-scale CSIP samples of three different slenderness ratios and full-scale panels loaded with three different eccentricity values were carried out, and the test data were compared with numerical results for validation. The finite element (FE) model successfully captured CSIP’s inelastic response in uniaxial compression and when flexural action was introduced by eccentric loads or buckling and predicted all failure modes correctly. The comprehensive validation showed that the proposed approach could be considered a robust and versatile aid in CSIP design.

Airtightness of cross-laminated timber envelopes: Influence of moisture content, indoor humidity, orientation, and assembly

Decrease in the relative humidity (RH) of the indoor air causes cracks in the cross-laminated timber (CLT) surface resulting in air leakages in the CLT panel. The effect of construction moisture on CLT air tightness properties is not clearly defined, which is important for moisture safe construction. The aim of this study was to investigate the effect of high MC on CLT air tightness properties compared to conventional factors such as indoor RH change, insulation type and façade orientation. An air permeability test was carried out on 12 different test walls with CLT as the airtight layer exposed to indoor environmental conditions. Test walls were constructed from CLT panels with two different initial moisture content (MC) values, ≈13%, and ≈26%, and in addition, different insulation materials were used in the wall assemblies: mineral wool, cellulose wool and polyisocyanurate (PIR) plates. Air leakage measurements were carried out at two different times, first in mid-autumn and second at the end of winter. Based on the results of this research it was concluded that high initial moisture content in the CLT panels significantly weakens the airtightness of the external wall. CLT exposure to water and wetting the CLT panels during construction carries more than a risk of moisture damage, and the planning and implementation of moisture safety in the construction of CLT buildings is therefore essential if the CLT is to be considered as an airtight layer. The effect of a change in indoor RH is small on the air permeability properties of the CLT wall. Therefore the 5-layer CLT panel can be considered and used as an air-tight layer in external wall construction as long as its initial low moisture content (about 13%) is maintained during both construction and service life.

Determining overall heat transfer coefficient (U-Value) of wood-framed wall assemblies in Canada using external infrared thermography

Quantitative thermography is considered as a reliable method to measure the thermal transmittance U-values of opaque building envelopes. Previously developed external infrared thermography (IRT) methodologies mainly focused on comparison of measured U-values with nominal U-values of wall assemblies in European construction. This study attempted to develop an external IRT method to determine clear wall U-values, where the impacts of repeating members (studs) were considered. The proposed method was compared with two established practices in Canada, namely the parallel path method and 3D thermal simulations. The IRT measurements were conducted on a conditioned at-scale insulated wood-framed wall structure. Besides the importance of environmental conditions on the thermal images, two thermal imaging artefacts were assessed and discussed, including nonlinear characteristics of infrared (IR) camera focal array, a.k.a. non-uniformity corrections (NUC) and vignetting. The results demonstrated that the location of the region of interest (ROI) plays a key role in U-value measurement due to the vignetting effect and colder thermal bridges at corners. It was also found that NUC should be considered during the survey. Furthermore, U-value measurement with IRT in the best-case scenario (depending on the location of ROI) deviated from nominal U-values by 6.25%–25.00%. The clear wall U-value results with IRT were validated with three-dimensional (3D) finite element analysis software, Siemens NX, which differed by −11.53%–10.00% (in the best case scenario). Additionally, the clear wall U-values obtained with parallel path method were comparable with simulation values for walls without highly conductive materials such as metal.

Future projected changes in moisture index over Canada

As a consequence of global warming, buildings in Canada and around the globe are expected to face unprecedented climate over their design lives. The Moisture Index (MI) is a climate-based indicator currently used in the National Building Code of Canada (NBCC) to guide the design of wall assemblies for acceptable durability performance. In this study, future changes in MI are calculated across Canada under 2 and 3.5 °C of future global warming. Results indicate that the coastal and great lakes regions of Canada will have increased MI, whereas prairies and northern regions will in the future have decreased values of MI. Furthermore, it is demonstrated that drastically different projected future MI changes can be obtained with different values of drying potential normalization factor, and it is suggested that this value is chosen by carefully analyzing the historical and future projected drying potential values. Finally, our analysis shows that wall assemblies at an increased number of reference locations given in the NBCC, importantly those falling in the densely populated south-western Ontario region, will need to be designed with additional safety measures such as a capillary break in the future for them to achieve satisfactory moisture performance over their design lives.