Simulation of historic buildings for enhancement of preservation and energy performance – issues and methods

Sammanfattning: Historic buildings are often particularly poorly insulated and difficult to heat, and are also at risk for several reasons. These include potential lack of use due to low thermal comfort leading to low or no profitability, damage risks due to over and under-climatization and risks due to climate change altering the boundary conditions that they were built for.Building simulation can be a valuable tool with which to assess the building performance of historic buildings, it can provide a background of knowledge for the design of potential strategies, and then assess such strategies in order to facilitate the decision process. Thus, simulation can contribute to minimized energy usage and optimized preservation, and prevent the need for potentially risky full-scale experiments that could jeopardize the cultural values. Thus a wide-spread use of simulation of historic buildings would be useful. However, it is difficult to simulate historic buildings with tools and methods developed for modern buildings. This may lead to a level of uncertainty that can greatly limit the usefulness of the simulation results.The work of the thesisThe aim of the work presented in this thesis is to facilitate the choice and design of strategies for enhanced energy efficiency and preventive conservation in buildings of the cultural heritage. The subject of the thesis is the use of building simulation as a tool in that context, and specifically as a tool in the hands of the consultants who should perform such investigations. The work relates to the work of practitioners in the field, as a basis for making decisions on suitable strategies for enhancing energy performance and/or damage risk mitigation, not to academic tools. The focus is on whole-building simulation, in order to be able to obtain a full picture of the building performance, including deviations in conditions between different parts of the buildings.The work contains several parts:Investigation of tools and methods relating to the specific demands of historic buildingsIdentification of particular issues that challenge the usefulness of the simulationsInvestigation of the particular issues and suggestions for how to deal with themSeveral case studies testing the usefulness of the suggestionsSeveral issues have been identified and investigated:Issue 1: Conditions by thermal bridgesMould issues do not occur in mid-air, they take place on surfaces, and often at the coldest, or most extreme temperatures of the room – which is often at the thermal bridges. Hence there is a need to assess the thermal performance of the thermal bridges as soon as possible in the simulation process.In this thesis wall-parts mimicking the performance of geometrical thermal bridges are proposed as a way to solve the problem and to allow evaluation of the potential risk of high levels of RH at places where there are two-dimensional thermal bridges in a calculating environment that can only handle one-dimensional building parts.The suggested method can be implemented in most whole-building simulation environments as long as wall-parts models can be created and their surface temperatures logged in the simulations. The method can be seen as satisfactory for the cases shown in the study. It manages to include the impact of two-dimensional heat conduction and the impact of thermal inertia in one-dimensional simulations. This makes damage risk assessment, specifically mould issues, possible at the points where it is most likely to occur.Issue 2: The impact of the wind pressure on the air exchange rateIn buildings with natural ventilation and irregular shapes the wind pressure coefficients have a considerable impact on the air exchange rate. If the air exchange rate in the simulation does not correspond to that of the actual building, it does not matter how correct the rest is: the energy performance cannot be correctly assessed and neither can the damage risk, since the relative humidity will not be correct either.To investigate the wind pressure coefficients a series of wind tunnel experiments were performed with models of Skokloster Castle: The building with surrounding vegetation and neighboring building, the building without the surrounding, and the building without surrounding and without the characteristic towers. The last one was included to test a building body resembling a simpler shape, corresponding to the models used to derive the generic wind pressure coefficients that are likely to be used if no object-specific ones are available.The results of the wind tunnel experiments were then analyzed and compared, to check the validity of different methods to estimate wind pressure coefficients in this case. The geometric complexity added by the towers of the studied building caused additional turbulence compared to simpler geometry. This turbulence affected the pressure coefficients which in turn influenced the air exchange rate in the rooms profoundly. The value of the investigation is the quantification of the deviations between the different models, and the consequences of simplifications of the wind pressure coefficients for the model reliability.Issue 3: Mould riskIn order to visualize the potential mold risk a display method was also developed to facilitate the overview and understanding of the mold risk. The most important benefit of the method is that the most critical time periods become easy to identify. This is not a new mould risk index or mould growth prediction tool as such, but rather a way of displaying the mould risk over time, so that critical periods can be identified and the pattern of the mould growth risk can be analyzed, to facilitate the design of preventive measures.Issue 4: Combining complexity in the model with moisture calculations without stability issues and long run timesCommercial tools have to specialize on certain aspects of the simulation, which makes it difficult to find a single tool that can fulfill all the requirements for being a suitable tool for simulation of historic buildings.In the work, a new method is suggested, which is built on dividing of the simulation process into several steps in a serial fashion. This solves the issue by adding complexity and function-nality gradually. That way calibration and error-checking is facilitated while all the desired functions are provided, and stability issues are avoided. The method is tested in a case study, it proved fast and stable, and the results displayed good correspondence to measured values.ConclusionThe contribution of this work is the identification and development of simulation tools and methods that are suitable for planning retrofitting strategies in historic buildings. The simulation process of historic buildings has been analyzed and a series of demands on suitable tools formulated. The issues of thermal performance of thermal bridges and the impact of wind pressure on the air exchange rate have been investigated, leading to a series of suggested methods and tools that have been found to fulfill their purposes under the conditions that they were made for and tested in. Hence, the investigations and suggestions should be of value in the simulation of historic buildings.