Thermal mass is effective in improving building comfort in any place that experiences these types of daily temperature fluctuations—both in winter as well as in summer. When used well and combined with passive solar design, thermal mass can play an important role in major reductions to energy use in active heating and cooling systems. The use of materials with thermal mass is most advantageous where there is a big difference in outdoor temperatures from day to night (or, where nighttime temperatures are at least 10 degrees cooler than the thermostat set point). The terms heavy-weight and light-weight are often used to describe buildings with different thermal mass strategies, and affects the choice of numerical factors used in subsequent calculations to describe their thermal response to heating and cooling. In building services engineering, the use of dynamic simulation computational modelling software has allowed for the accurate calculation of the environmental performance within buildings with different constructions and for different annual climate data sets. This allows the architect or engineer to explore in detail the relationship between heavy-weight and light-weight constructions, as well as insulation levels, in reducing energy consumption for mechanical heating or cooling systems, or even removing the need for such systems altogether.