Introduction
Reducing and optimizing energy consumption and consequently environmental pollution reductions, besides improving building performance providing occupants comfort conditions, make the building's concept closer to the concept of green building and sustainability. Once the decision to build green has been made, one of the first steps in the green design process is to establish firm environmental goals for the project.
In recent years, much research in the field of energy efficiency in buildings has been carried out. Also indoor air quality issues are on debate these days due to its direct effect on people's health.
Indoor pollution sources that release gases or particles into the air are the primary cause of indoor air quality problems in homes. Inadequate ventilation can increase indoor pollutant levels by not bringing in enough outdoor air to dilute emissions from indoor sources and by not carrying indoor air pollutants out of the home. High temperature and humidity levels can also increase concentrations of some pollutants. Carbon dioxide is often measured in indoor environments to quickly but indirectly assess approximately how much outdoor air is entering a room in relation to the number of occupants. Replaces oxygen in the blood-exposure to concentrations around 40,000 ppm is immediately dangerous to life and health. CO₂ poisoning, however, is very rare.
In this research a building as a case study is selected and in one of the most characteristic energy audit software is modeled to estimate the amount of its energy consumption. It is estimated with calculating building heating and cooling loads in different seasons. The ventilation rate of the building as one of the factors in energy consumption is assessed As well as the concentration of gases in closed environments will be reviewed since all these standards directly and indirectly are trying to improve the human's life condition. Then some optimizing strategies were applied on the building to investigate the relationship between energy consumption and indoor air quality.
Methods
1-Energy Modeling
At first, the educational building was modeled in the DesignBuilder Software and its energy consumption was calculated. Building characteristics such as occupancy, number of persons living in each sector, heating and cooling systems, fuel type, materials used in the construction of the building shell, openings and its type, As input to the model was imported into the software. As seen, the highest rate of heat dissipation through the air in the building is from ventilation.
2-Indoor Air Quality Modeling
Contam software is used for indoor air quality modeling. This program does not draw a geometric model fully, just can draw external walls and partitions as a general plan. Defining airflow and showing the location of doors and windows. As well as environmental factors such as air temperature outside and inside, wind speed and direction, wind sped ratio and outside pollutants can be introduced as an input to the model.
3-General Characteristics of Building
 
The case study is an educational building, various parameters including U values of external walls, window to wall ratios, and orientations are investigated in this parametric study. different U values of external wall U=1.798[W/m².k], U=1.976[W/m².k], and U value of roof 1.256[W/m².k] are employed to analyze heating and cooling loads for the educational building. In the simulation, the heating setpoint is 21 C and cooling setpoint is 25 C. The energy consumption of the house is taken as the criteria for the optimal sustainable facade design. Building heating by the central system is provided and building cooling system is independent. Building's lighting system is provided by fluorescent light bulbs. Geometry is drawn in the model space. Doors and windows and all openings in accordance with their actual proportions in the same location and orientation of the main building were drawn in the software.
Discussion of Results
 
For assessing Efficiency of buildings ventilation, carbon dioxide concentration in the simulated building is evaluated. Carbon dioxide is a byproduct of respiration. When breathing, people inhale oxygen into the lungs and exhaled carbon dioxide. This will cause the carbon dioxide levels rise in buildings over the next several hours to reach a maximum value. In model Carbon dioxide content in fresh air is approximately 400 parts per million and for human inspiration rates equal 0.1m3/h is intended. According to the diagram1, the class supposed to start at 8 Am and 12 noon finished. Time period is considered till 14 Pm, and the number of people present in room, have been considered 20.
 
 
Figure 1. Carbon dioxide concentration changes with time
 
The results clearly indicate that the CO2 concentrations can give an indication of the indoor air quality in these indoor and enclosed environments. At the start of class, Co2 concentration was assessed 400 ppm. It is clear that the CO2 concentration increased immediately at 8am until it reached a steady state condition at about 9 am as shown in Figure 1. The maximum CO2 concentration was constant at 642 ppm for the entire duration until 12 am. The maximum concentration of carbon dioxide is 642 ppm which is consistent with existing standard allowance . But the increasing number of individuals in the model to 30 people, it increases nearly 800ppm. The situation will require increasing ventilation within the class.
Building ventilation rates were 2/4 time in hours and the heat load of the building is estimated at 64,508 kWh. To investigate the effect of air changes in energy consumption, all parameters affecting energy consumption are assumed to be constant at any given stage, only the rate of change of air had changed. The amount of air changes in the building start from 0/2 to 3 times at the time ended. We can see that the building heat load carrier ratio increased linearly .Thus, the amount of air changes is one of the most important factors affecting energy consumption of heating the building. But the behavior of the building cooling load is different with the heating load and the effect of air changes on it, is limited over time. Increasing the ventilation rate reduces the amount of carbon dioxide in the building and it is essential for improving conditions in indoor environment. The amount of air changes in the studied buildings vary. For example, in the corridors and rooms air change rate per hour is high, and this matter will increase energy dissipation.
In order to optimize building energy consumption, a set of optimizing actions, including wall insulation, roof insulation, replacing windows with double-glazed windows pvc building are applied in the model and seen that heating and cooling load decreased. But concentration of Co2 increased due to decrease ventilation rate. To solve these problems, air conditioning systems can be used.
 
Conclusions
Evaluation of carbon dioxide as an indicator of indoor air quality was assessed and it is found that with the increasing number of people in the class with the air change rates equal 2/4 times per hour, the amount of carbon dioxide from respiration, increased to 800ppm. To solve this problem it is necessary to increase the rate of air changes. It was observed that increasing the rate of air changes greatly increase the building heating loads and it will increase energy consumption in the building. Performance of cooling loads are different and  the rate of air changes has very little impact on the building cooling loads.then some optimization strategies applied to the building and 67% of building heating load and 39% of cooling load is reduced . But after these actions, leakage and penetration rate will be reduced. This increases the amount of carbon dioxide in stable condition and breathing through people. Thus reducing the rate of air changes should be considered measures to ensure indoor air quality.