Within the construction sector buildings are required to be more energy efficient than ever before. Tougher building regulations require ever more accurate energy demand calculations to ensure a building’s operational performance. Transmission heat transfer losses through a building’s envelope play a key role in the overall energy performance of the building. Underestimating these losses may lead to non-fulfilled energy requirements. Currently used standard methods for calculating the transmission heat transfer of buildings are for the most part sufficient but lack in certain areas where current knowledge is inadequate and requires further investigation. One of these areas is wall constructions using slotted steel stud framing, where there currently is no simplified standard method of calculation. The thermal transmittance through a slotted steel stud creates a complex three-dimensional problem which has proven difficult to transfer into a more manageable model. This study analyzes some of the suggested methods for calculating the thermal performance of slotted steel stud walls, where the results showed that the suggested methods have a rather large margin of error in relation to 3D simulations or do not include all the parts of the stud. To solve this issue, this study presents a new calculation method for the thermal performance of slotted steel stud walls. The new method of calculation is based on 3D simulations of the heat flow through 117 different wall structures together with non-linear regression analyses. It suits a large variety of slotted steel stud wall structures and has a smaller margin of error than the current simplified methods of calculation.