## 2 Methods
### Computational Simulations  
The energy performance of buildings is studied by appropriate software tools, simulating the behaviour of a building or its part using the methods of numerical and computational analysis. An appropriate model of energy flows allows us to understand the behaviour of a building as a thermal system even before its construction, including its energy demand under various climatic conditions, thus, it is able to answer crucial questions related to building orientation, optimal size and location of rooms, particular bearing structures and insulation layers, as well as about the design and control of technological devices. Software packages like Design Builder, EQuest, or SW Stabil are typically used for such global modelling tasks, accompanied by specialised software tools for detailed considerations regarding heating, ventilation, and air conditioning in buildings and their parts. These software tools, working with certain engineering simplifications, express the principles of classical thermodynamics, namely that on energy conservation, supplied by appropriate constitutive relations, using certain partial differential equations with various types of boundary, contact, and initial conditions [52]. The SW Stabil software [53], developed and implemented at the Brno University of Technology, Faculty of Civil Engineering, based on the combined finite volume approach with the method of discretisation in time (coming from the theory of Rothe sequences), is for its simplicity preferred in this case study. Alternative methods try to apply some (frequently incomplete) knowledge of fundamental solutions related to special expected solution decomposition, such as multiplicative Fourier one (method of lines) [54, 55]. Yet other approaches, which are not discussed in detail here, have been developed, e.g., for the analysis of special types of buildings or their parts [56], analysis of potential thermal bridges [57], implementation of phase change materials in insulation layers [58], and proper incorporation of further significant physical processes, as moisture or (more general) mass transfer [49, 60, 61]. A strong motivation for the development of SW Stabil was the need to evaluate the dynamic response (i) of particular rooms in buildings to thermal loads under both winter and summer climatic conditions, as required by the European technical standard EN ISO 52016-1 [62], and (ii) of the whole building as a closed structural object, as required by the Czech technical standard CSN 730540-2 [63] (valid from 2011). The considered model applies the system approach, taking into account the inner and outer thermal relations between structural parts (as elements and subsystems of such thermal system), using certain 1D simplifications. Such relations express particular types of heat transfer by conduction, convection, and radiation. Consequently, relying on the time decomposition and backward Euler modifications of nonlinear terms, due to an appropriate initial condition, in any positive time \(\tau\) (assuming sufficiently short, typically equidistant, time steps) we have to solve only a sparse system of linear algebraic equations with a finite number of unknown values of absolute (Kelvin) temperature \(t(\tau)\). The evaluation of the effect of both direct and diffuse solar radiation is rather delicate. Due to the building's location and orientation, including all slopes of exterior building surfaces, the continuous movement of the sun in daily and annual (quasi) cycles is considered, together with the reference climatic year database for appropriate conditions (or sufficiently close and comparable ones, e.g., those recorded at the nearest airport which are processed statistically, if available). More specifically, the air temperature of the outer environment during a typical year is needed, as well as the subsoil temperature for the estimate of heat losses through the building's foundations.