Indoor environment, energy and mass flows within buildings result from complex dynamic interactions between outdoor environment, building structures, environmental control systems and occupants. The thermal behaviour of highly insulated buildings with extensive glazing cannot be calculated with the necessary precision using simplified methods.

The computer simulation has been used to determine some aspects of an indoor climate in a glazed atrium space in office building in Banska Bystrica, Slovakia. The objective of the simulation was to investigate (in the early stages of design process) how different glazing types and ventilation rates will affect indoor environment in the atrium and in attached offices throughout the year.

Using a dynamic simulation computer program, various configurations were simulated that resulted in suggestions for selection of the atrium roof shape, type of glazing, system of an office lighting, and a ventilation of the atrium space.

 The glazed atria have become popular in office buildings in Slovakia in the recent years. An atrium provides an environmentally controlled space which is shelter from rain, snow and wind, and which has marketing value. The sketch project of an office building with glazed courtyards in Banska Bystrica was chosen to study thermal behaviour of glazed spaces in typical Slovakian climate conditions (1). The purpose of this study was (1) selection of atria glazing, (2) give the guidelines for ventilation of the atria, (3) obtain quantitative estimates of the effects of glazed space on attached offices.

The computer simulation was used to investigate these problems quickly and with relative accuracy. The study consists of four interrelated activities: (1) assembly and analysis of databases, (2) collection of new data relating to building materials and constructions used in Slovakia, (3) modelling and simulation of studied building, (4) analyse of simulation results.

The studied office building is to be built in the historic surroundings in the centre of Banska Bystrica, the town in the middle Slovakia. The object of the project is partly the renovation of the existing building and partly new construction of linked office pavilions - Figure 1. Historical part of building has a massive wall construction and for new parts of the building complex it is suggested to use a concrete frame for the main structure and masonry walls. The building has high thermal capacity. Two glazed courtyards will be inserted between the office pavilions and the glazed courtyards (atria) will act as thermal buffer to reduce heating load. The offices linked to glazed atria are naturally ventilated with opening windows.

The construction, heating and cooling system, levels of insulation, and glazing type are of types widely used in the Slovakian construction industry.

The simulations were carried out for Sliac, the spa near Banska Bystrica, using TRY (test reference year). Geographical latitude of Sliac is 48° 39´, longitude is 19° 08´ and altitude is 313 m.

The annual average number of days where the mean temperature stays below freezing is 67, absolute minimum is – 30 ° C. In summer the temperature does not often rise much above + 30 ° C, absolute air temperature maximum is + 37,2 ° C. Mean total global irradiation on horizontal surface is 1148 kWh.m-2.year--1. Mean annual ambient air temperature is 7,9 ° C, mean annual relative air humidity is 78 %, relative sunshine duration is 36 % (1 757 hours per year). Precipitation is in the region of 715 mm per year falling throughout the year. Number of days with snow cover is 67, maximum of height of snow cover s 0,67 m (all parameters according (2)). The ambient dry-bulb temperatures in Sliac according TRY are shown in Figure 2.

The computer program ESP-r (5) was used in this study. ESP-r is a transient energy simulation system, which is capable of modelling the energy and fluid flows within combined building and plant systems. The package comprises a number of interrelating program modules addressing project management, simulation, results recovery and display, database management and report writing. ESP-r was developed at the University of Strathclyde, Glasgow, U K and is known as the state-of-the-art building energy and environmental simulation program.

 For accurate results, the building being modelled by ESP-r must be dissected so that it can be described in detail. Each component of the building that is important in terms of its thermal behaviour is included in the building model. Building components that do not influence the results are not included. For example, interior walls between rooms that are part of one thermal zone do not need to be described in the energy model (if they represent no thermal capacity).

For purposes of this study, 8 thermal zones model, was used which is shown in Figure 3. These zones are influenced by the heat transfer to a large garage, which is situated at the basement level. The garage was modelled as an unconditioned space with constant indoor air temperature of + 10 ° C.

The air temperature in office rooms was keep at 20 ° C during winter and max. 25,5 ° C during summer (set point temperature for cooling). Various air change levels were assumed in glazed atria and offices. These air exchanges were scheduled on weekdays, Saturdays, Sundays and holidays.

The study of offices daylighting has been performed to find out whether daylighting of offices adjacent to the glazed atria satisfy the Slovakian building codes. The state Hygienic service in Slovakia strongly requires providing a good daylighting practice in offices.

At present, one must weigh the solar and thermal impact of glazed atria against daylighting separately. According Slovakian code (3) the minimum value of Daylight Factor (DF) on working plane of an office must be 1.5 %. Daylighting calculations were done by computer program OSV1 (4).

The new offices in the designed building have relative shallow plan and large windows, but proposed pavilions obstruct each other. The offices are in the case without glazed atria lit enough. If the atria are glazed with single clear glazing then there is practically no area on a working plane of the offices linked to atria with DF higher then 1.5 % (see Figure 4). This is mainly due to maintenance factors of slope glazing according (3). So, during overcast days these offices must be supplementary lit by artificial light.

Other types of glazing (such coated and tinted glazing) are not advisable from point of view of daylighting and switchable glazing is too expensive at present.

 The presented results summarise the influence of various systems of atria glazing and ventilation rates on the air temperature in atria. The following glazing types were analysed and compared to each other: (1) single clear glazing, (2) double clear glazing, and (3) double insulation glazing (76 % transmittance of light, 0.52 shading coefficient , U-value 1.8 W m-2 K-1).

The ventilation rates of glazed atria were applied starting from 0.5 h-1 to 7.0 h-1. Ventilation of the offices was kept constant at 0.8 h-1. Figure 5 shows outdoor and indoor air temperate in the glazed atria with a double clear glazing from January 15 to January. Figure 6 shows the summer temperatures in zone 3 (offices) and in zone 6 (atrium). Constant ventilation rate of 0.8 h-1 was assumed in zone 3 and 3.0 h-1 in

Figure 4 Distribution of Daylight Factor on working place of typical office linked to glazed atrium

 zone 6. It can be seen that internal temperature (ti) is much higher then external temperature (te). Figures 7 and 8 show relationship between air temperature and air change in atrium obtained for ventilation rates from 0.5 h-1 to 1.5 h-1 during winter and from 0.5 h-1 to 7.0 h-1 during summer. Impact of ventilation of atria on the thermal performance is significant and the energy benefits of glazed atria depend strongly on ventilation rates.

Indoor air quality in atria and in linked offices is questionable if their ventilation rates are less then 0.8 h-1. There is no enormous potential of energy saving during winter if glazed atria will be well ventilated to keep indoor air quality in adjoined offices (Fig. 7) at the sufficient level. But differences between external and atria air temperatures in range of about 6 degree of Celsius and large extent of the atria can enhance energy efficiency of analysed building considerably. On the other hand problems with overheating of glazed space and consequently of offices may occur during whole warm half of the year. The heating and cooling loads of zone 3 (block of offices) are illustrated in Figure 9.

 A parametric study was carried out using the dynamic thermal simulation computer program and program for prediction of daylight levels in interior spaces that resulted in strategic guidelines for next steps of design process.

In the presented solution of building, problems with daylight levels in offices linked to glazed atria during overcast days, will occur. From this point of view, the application of thermal insulating glazing or fixed blinds in atria is not applicable. In the most offices attached to glazed atria the daylight-linked artificial lighting control should be provided. The windows of offices will be equipped with movable shielding devices.

The first design sketch of the glazed atria does not deal with the system of ventilation. Natural ventilation of atria and its control are inevitable for well-being interior environment in

 atria and attached offices. Any other solutions of this problem are not economical. It was suggested to "pick up" fresh air supply via continuous inlets in the bottom of glazing, i.e. to arrange vertical windows and openings with louvers. The openings with louvers will regulate extraction of air, during hot summer days together with ventilators. The glazed atria will use passive solar energy, wind and solar-driven ventilation, night time free cooling, mechanical ventilation for peak conditions in mid-summer and sophisticated control system.

 (1) Somora, B. – Sovcik, M.: Prestavba radnice na Namesti SNP c. 1 v Banskej Bystrici. Studia. Januar 1997. (Reconstruction of town hall on Square of Slovak National Revolt No 1 in Banska Bystrica. Study. January 1997. (In Slovak)