02 Fakultät Bau- und Umweltingenieurwissenschaften

When a timber building gets disassembled and its elements either are burned or biodegrade, the carbon stored in the timber structure gets released to the atmosphere as CO2. Reusing timber elements prevents this process from happening and thus delays the global warming caused by greenhouse gas emissions. Even if there is a long historic tradition of timber reuse in Switzerland, currently a low fraction of a timber building’s elements is being reused after its disassembly. In this study, strategies that could facilitate circular use of timber elements are analyzed. The focus lies on the design process, which is investigated from two perspectives: strategies at the start-of-life of buildings to enable new timber element cycles to emerge (design for disassembly, or DforD), and strategies at the end-of-life of buildings to keep existing timber elements cycles closed (design from disassembly, or DfromD). Two case studies of recently completed multi-story timber-hybrid buildings in Switzerland were analyzed from both perspectives. Regarding DforD, a scoring system was developed that assesses single elements according to their disassembly and reuse potential. Regarding DfromD, a building design optimization tool was created that takes dimensional design tolerances of a building as an input and proposes a procurement-optimized and structurally safe arrangement of reused elements, which are taken from an inventory that is based on the two case studies. It was found that connections between reinforced concrete and timber parts play a crucial role in terms of DforD and that building layouts with DfromD elements may vary widely according to the chosen optimization variable. In conclusion, both applications have the potential to scale up the competitiveness of reused elements.

A high-performance building must fulfill comfort and energy efficiency requirements. Possible solutions include passive strategies, such as improving the building envelope and taking advantage of natural light and ventilation. Natural ventilation (NV), for instance, can provide both thermal comfort and energy savings. However, its performance relies on building design and interaction with the local environmental characteristics. In this study, Natural Ventilation Potential (NVP) was analyzed under two approaches: a general evaluation using meteorological data and a specific investigation through building simulation, using an experimental house as a reference case located in a temperate climate with warm summer. Although there are many parameters and metrics applied in assessing NVP, predicting building air change rates (ACH) and airflows is a challenge for designers seeking to deal with this passive strategy. Among the methods available for this task, Computational Fluid Dynamics (CFD) appears as the most compelling, in ascending use. However, CFD simulations have high computational costs, besides requiring a range of settings and skills that inhibit its wide application. Therefore, a pragmatic CFD framework to promote wind-driven assessments through 3D parametric modeling platforms was proposed as an attractive alternative to enable the tool application. The approach addresses all simulation steps: geometry and weather definition, model set-up, control, results edition, and visualization. Besides, it explores alternatives to display and compute ACH and parametrically generates horizontal planes across the spaces to calculate surface average air velocities. Usually, network models throughout Building Energy Simulation (BES) are the most employed NV investigations approach, especially in annual analysis. Nevertheless, as the wind is a significant driving force for ventilation, wind pressure coefficients (Cp) represent a critical boundary condition when assessing building airflows, influencing BES models’ results. The Cp values come from either a primary source that includes CFD simulations or a secondary one where the primary is considered the most reliable. In this sense, a performance metric was proposed, namely the Natural Ventilation Effectiveness (NVE). It verifies when outdoor airflows can maintain indoor temperatures within a comfortable range. The metric uses BES results, and within this context, the impact of five different Cp sources on its outputs was investigated. Three secondary sources and surface-averaged Cp values calculated with CFD for both the whole façade and windows were considered. The differences between the CFD Cp values are minor when wind direction is normal to the surface, with more significant discrepancies for the openings close to roof eaves. Although there was considerable variance among the Cp sources, its effect on the NVE was relatively small. Additionally, when designing high-performance buildings for cold climates, efficient insulating systems are encouraged since they help reduce heat losses through the building envelope, thus promoting building energy savings. Still, climate exposure deteriorates material properties, compromising a building’s energy performance over its lifetime. Therefore, this aging impact on the hygrothermal performance of an aerogel-based insulating system was investigated through a large-scale test, U-Value measurements, and heat and moisture transfer (HMT) models, calibrated with the experimental data. A low thermal conductivity degradation was measured after the tests, showing that its effectiveness is not harshly compromised throughout its life-cycle. Finally, this research performed parametric modeling and optimization to minimize annual building energy demand and maximize NVE. The workflow was divided into i) model setting, ii) sensitivity analyses (SA), and iii) multi-objective optimization (MOO), with a straightforward process implemented through a parametric platform. Input variables dimension was firstly reduced with SA, and the last step ran with a model-based optimization algorithm (RBFOpt). MOO results showed a remarkable potential for NV and heating energy savings. The design solutions could be employed in similar typologies and climates, and the adopted framework configures a practical and replicable approach for design approaches aiming to develop high-performance buildings through MOO.

02 Fakultät Bau- und Umweltingenieurwissenschaften

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