Tallinn University of Technology

In March, Martin Thalfeldt, a PhD student of the Department of Structural Design of TUT, defended his doctoral thesis "Total economy of energy-efficient office building façades in cold climate".

It has become more and more common to use effective large glazed areas on modern office building facades. The reverse of the medal, however, is that without the use of super-windows and proper solar shading, this may increase significantly the heating and cooling costs and cause problems related to thermal comfort and glare.

One of the supervisors of the doctoral thesis "Total economy of energy-efficient office building façades in cold climate" Professor Jarek Kurnitski, Director of the Department of Structural Design, explains: "When designing low energy and nearly zero energy buildings (which is a growing global trend in view of sustainability), designing of the building façade is one of the key issues. "Façade solutions are closely related to technical systems (heating, ventilation, cooling, lighting), indoor climate, i.e. user satisfaction, energy efficiency and building costs. Finding an optimal solution for such a large system with hundreds of variables is a hard nut to crack. Thalfeldt tackled this challenge successfully in his doctoral thesis. As a result of the thesis the best possible compromises were found in terms of energy efficiency, indoor climate and for keeping building costs under control – architects and engineers can implement the solutions immediately."

In the defended doctoral thesis the heating, cooling and electric lighting energy needs of typical open plan offices were modelled, access of daylight to rooms in case of different window sizes, types of glazing units and insulation thickness was calculated. In addition, the external dynamic shading control algorithms and control principles were developed. The building costs of different solutions and their cost-effectiveness in 20 year perspective were calculated.

It was found in the doctoral thesis that in office buildings it would be reasonable to use clear triple pane windows with two spectrally selective coatings with the window-to-wall ratio in the range of 25% and 40%, however, on the north façade larger glazed areas may also be used. For external wall insulation a layer corresponding to a 200 mm mineral wool layer should be used. Larger quadruple windows and increasing insulation thickness can be used to improve energy efficiency, however it increases global cost of the facade. It was also found that external dynamic shading should be controlled in a way that would avoid glare during working hours and outside working hours shading should be controlled according to room temperature. At present, external shading is not economically feasible due to its price, but it allows, however, to use more ambitious architectural solutions with larger glazed areas.

Finally, it was identified that interest, inflation and building costs had the largest influence on the cost-effective façade solutions as a single variable compared to energy prices and simulation model accuracy, however the combination of all variables had the largest impact on the outcome of façade analysis. Since such extensive analysis cannot be carried out upon designing of each single building, there should be standard solutions, which the architects, structural engineers and energy efficiency specialists could use as a starting point and which can be tailored to the specific building.

The supervisors of the doctoral thesis were Professor Jarek Kurnitski and Visiting Professor Teet-Andrus Kõiv (TUT).
Opponents: Professor Per Kvols Heiselberg (Aalborg University) and Associate Professor Laurent Georges (Norwegan University of Science and Technology).

The doctoral thesis has been published in the digital collection of TUT library at http://digi.lib.ttu.ee/i/?4068