Evaluating building envelopes for energy efficient buildings : Energy- and moisture performance considering future climate change

Sammanfattning: Buildings account for a signifi cant proportion of the energy use thatgenerates greenhouse gases and consequently drives the ongoing climatechange. As the population of the world increases, the need for buildingsincreases, while the energy use needs to decrease. A reduction in energyuse and increased use of renewable energy are important measures forclimate change mitigation. A fi rst step in decreasing the energy use of abuilding in a Nordic climate is to increase the thermal resistance of thebuilding envelope.The combination of climate change and more insulation in buildingelements will result in a different microclimate within the building elements. External parts in well-insulated building envelopes will have amicroclimate more similar to the exterior climate as the thermal resistanceincreases and moisture may take a longer time to dry out.Today, there are several established ways to calculate and quantify theenergy performance of buildings and building components. As regardscalculation of transmission heat transfer through building envelopes, thereis a lack of knowledge among Swedish engineers and architects. Thereare ambiguities regarding the defi nition of a thermal bridge and the waybuilding elements are quantifi ed in energy calculations.There are models to quantify the moisture performance of a buildingelement, where the focus is to evaluate the risk of mould growth. Regarding other moisture-related problems, such as corrosion, deformations etc.,critical moisture levels are established. However, these levels are not validfor short-term loads. Only models of mould growth take into accountfl uctuations in hygrothermal conditions.Traditionally, durability and robustness of building elements are basedon experience and are not specifi ed in quantitative terms. However, increasing the thermal resistance in combination with climate change will result indifferent hygrothermal conditions within the building envelope. Buildingelements needs to be designed with reference to these aspects.To enable evaluation of energy- and moisture performance, whichare presented in dissimilar units, a model has been developed in order topresent a weighted value which includes both aspects. The model includes Evaluating building envelopes for energy effi cient buildings4a performance failure indicator that ensures that the weighted value ofan evaluation is unacceptable if any of the evaluated indicators are belowacceptable level. This means that it is not possible to compensate for thepoor performance of one indicator by achieving a very high value foranother indicator.Initial tests have been conducted by using the model to evaluate a limited part of a building envelope, but also for a whole building. The testsof the model showed that it is possible to handle a large set of criteria andto weight them into one value.Future work in this project will investigate building envelopes in orderto indicate measures that could have a large effect on the transmissionheat transfer losses. Furthermore, additional work to investigate futureclimate scenarios and improvement of the usability of the model will beperformed.