1. DRAUGHT (British English) Draft (American English)
Indoor Climate -course
Marianna Luoma

2. Local thermal discomfort
The maintaining of the thermal balance of the body does not guarantee the feeling of thermal comfort
Local thermal discomfort can be caused by :
Draught
Vertical air temperature difference
Too warm or too cool floor surface temperature
Radiation asymmetry
… thus the thermal conditions in an occupied space have to be uniform

3. Definition
Sensation of draught is caused by the local cooling of skin, which happens too suddenly
Thus, it is caused by heat transfer – not by the irritation of air movement on the skin

4. Sensibility to draught
Different parts of the body react differently to similar conditions
Neck is the most sensible part of the body for draught
Usually the air flow coming towards human body is tolerated better than the air flow coming from the back of the human body
Things like what we are doing, how we are dressed and how we like the thermal conditions, affect our feeling of draught

5. Thermal conditions
Operative temperature for normal office building is ºC (THE FOLLOWING SLIDE)
When the temperature is above this temperature or close to 24 ºC, our body tolerates higher air velocities compared to the situation when the temperature is closer to 21 ºC
Sensibility to draught increases when the temperature is below the optimal operative temperature
It is typical for Finland that indoor air temperature is close to 21 ºC which means that there is a high possibility for draught ….

6. Optimal thermal conditions for office work
Metabolic rate in office work is 1,2 met
Winter:
Insulation of clothing is 1 clo
Optimal operative temperature is 21,5 °C
Summer:
Insulation of clothing is 0,5 clo
Optimal operative temperature is 24,5 °C

7. Essential environmental factors
Air temperature
Air velocity
Fluctuation of air velocity
In the simpliest draught criteria the percentage of dissatisfied persons depends on air temperature and air velocity

8. Fluctuation of air velocity
It is expressed by the turbulence intensity
… which is the standard deviation of air velocity devided by the mean air velocity
The greater the turbulence intensity, the greater the possibility for draught

9. Air velocity in a room, measured by direction-independent velocity sensor
Measured by Hannu Koskela, Occupational Health Institute, Finland

10. Percentage of persons dissatisfied
People are different and the sensibility for draught varies from person to person
Can be expressed as the percentage of dissatisfied to certain air flow conditions in the occupied zone
The amount of dissatisfied persons depends also on the air temperature
The air movement towards the neck is felt more uncomfortable than the air movement towards the face or the air movement coming from upwards
Feet are not as sensitive to draught

11. Draft rate DR=(0,37 v Tu+3,14)(34-Ti)(v-0,05) 0,62 DR = Draft rate, %
Ti = Air temperature, ºC
Tu = Turbulence rate of air flow = the standard deviation of the air movement divided by the mean air velocity, %
V = Air velocity, m/s
The formula is based on a set of experiments, in which the air jet is blown towards the necks of persons that are in optimal operative temperature. (150 persons, 3 minutes averages used in calculation of air velocities ISO 7730)

12. The effect of thermal radiation when sitting by a cool window
Besides the air temperature and air movement, heat transfer is affected also by thermal radiation
If the heat transfer caused by the air movement and by the cool window are appearing at the same side of the body, the likelihood to draught is obvious
Especially harmful are the air movements through the window frames and leakages through the walls, that are appearing on the same side of the body

13. Local draught risk in an office room, cooling load 100 W/m2
Simulation by Hannu Koskela, Occupational Health Institute, Finland

14. DIFFERENT DRAUGHT CRITERIA
Finnish recommendation:
For the draught free conditions the maximum air velocity is 0,2 m/s, when the temperature is +20 C.
International criteria

15. An international draught criteria that takes into account temperature and velocity, but no turbulence intensity

16. ISO 7730 DR=(0,37 v Tu+3,14)(34-Ti)(v-0,05) 0.62
Category A: DR not more that 10 %
Category B: DR not more than 20 %
Category C: Dr not more than 30 %