This project aims to compare prefabricated cross-laminated timber (CLT) structures with concretesandwich structural frames with regard to thermal bridges and thermal mass, in order to analyse theirimpact on the energy use of buildings. By maintaining the same U-value for both structures, the studyinstead investigates how these factors are affected by external climate conditions. The study is relevantbecause energy efficiency is a central issue in the construction industry, where the choice of structuralmaterials can have long-term consequences for both energy use and indoor climate.The study investigates how linear thermal bridges in residential buildings affect the U-value, andwhether Boverkets claim that they normally increase the buildings overall U-value by 15-20% iscorrect. A comparison is made between timber and concrete constructions with the same U-value, toanalyse how the thermal bridge proportion differs, and its impact on the building’s energy use.Furthermore, the interaction between thermal bridges and thermal mass is examined to assess theircombined effect on the total energy performance, providing a deeper understanding of how the choiceof structural material affects energy efficiency.

This project is based on previous research in building physics and energy performance, where fiverelevant articles have been analysed to provide a theoretical basis. Data is collected through acombination of literature reviews, numerical calculations and simulations. Thermal bridges areanalysed in Comsol Multiphysics, while their impact, together with the thermal mass, is evaluatedusing calculations and simulations in IDA ICE.

The project was carried out from February to the end of May 2025, and is limited to linear thermalbridges at the junctions of exterior wall - floor slab, exterior wall - balcony, and exterior wall -window in residential buildings. The study is expected to provide deeper insight into how thestructural frame influences energy performance, potentially contributing to more energy-efficientbuilding solutions.

The results obtained in this study highlight measurable differences between CLT and concretesandwich elements, depending on structural material and climate. These differences are small butsignificant. The findings indicate, among other things, that CLT generally has slightly lower thermalbridge factors and slightly better energy performance in colder climates, but lacks the thermal masseffect of concrete, which can store and release heat during cold snaps.