The indoor thermal climate conditions are primarily depended on the internal heating load from people, equipment, artificial light and the sun. The indoor thermal climate is regulated by ventilation and night cooling of the building by using the internal mass of the library, e. g. concrete floor and books.

For reducing the internal heat load as much as possible and thus reducing the numbers of hours the fans needs to assist the natural ventilation, integrated blinds are used in the facade windows and the artificial lighting is subdued/shut off after the level of daylight.

The indoor thermal climate has been simulated with the thermal simulation program BSim2002 (BuildingSimulation 2002), which is a computer based calculation program for simulation and analysis of the indoor climate and energy consumption in buildings. By constructing a
detailed mathematical model of the structure, it is possible to simulate a large number of indoor climate and energy parameters. This is done taking into consideration the dynamic interaction between the outdoor climate and miscellaneous design of structures, installations and running situations.

4.1 The model

Based upon the construction details derived from among others the daylight simulations and the natural ventilation system the following building model was made in BSim2002 that corresponds close to the actual conditions.

The library is divided into 4 sections, children’s section (south), north section, east section and a west section that is closed and has its own mechanical ventilation system with cooling. The west section is the administrative area for the library, while the rest of the library is public.

The most important elements in this simulation are the ventilation system, the internal mass from all the books and the internal heating load from the artificial lighting system due to high demand of lighting level on the bookshelves and wishes for the artificial light by the architects.

4.2 Natural Ventilation system

The ventilation system is a natural ventilation system with fan assistance. The minimum temperature of the intake air is 18 °C and the amount of fresh air is regulated by the CO2 level during the heating season and by the indoor temperature outside the heating season. There is no cooling coil in the ventilation system. The air change is set to a maximum of 1 h-1 during the heating season and 5 h"1 outside heating season. The high rate of air change, 5 h, during the summer is kept by the assisting fans. The low rate of air change during the heating season and the fact that it is controlled by the level of CO2 decreases the energy demand for space heating and ventilation. During summertime, when needed, night cooling is initiated and with fan assistance if needed. The internal mass from all the books acts as a good buffer and has been taken into account in the simulations.