Draft, unwanted local cooling due to air movement, causes occupant dissatisfaction in indoor spaces. Stratified air distribution systems, such as displacement ventilation (DV) and underfloor air distribution (UFAD), supply conditioned air at the floor and are intended to maintain a thermally stratified environment. The high-speed and low-temperature airflow at the floor level can cause draft at the ankles and lower legs. Take a look at the following photo using infrared camera.

In addition, winter downward airflow along cold inner surfaces due to inadequate insulation of outer walls or windows can induce draft to occupants seated in perimeter zones. In vehicle cabins, air currents near the floor can result in discomfort for the feet.

Many factors influence the intensity of draft discomfort. Draft risk increases with increasing air speed and turbulence intensity, while it decreases with increasing air temperature.

Human subject to develop a model to predict the draft at ankles

We developed a model for predicted percentage dissatisfied with ankle draft (PPD_AD) based on laboratory experiments with 110 college students. We assessed the effect on ankle draft of various combinations of air speed (nominal range: 0.1-0.6 m/s), temperature (nominal range: 16.5-22.5°C), turbulence intensity (at ankles), sex, and clothing insulation (<0.7 clo; lower legs uncovered and covered). Check more details of lab tests here.

Major result

The major delivery of this project is to determine what is the acceptable range of airspeed at ankles under which most occupants feel comfortable.

Our model has been adopted by ASHRAE Standard 55 as an addendum

A few online design tools have incorporated our model

PAYETTE Glazing and Winter Comfort Tool

CBE Thermal Comfort Tool

See more details in our published paper:

Liu, S., Schiavon, S., Kabanshi, A. and Nazaroff, W. (2017). Predicted percentage dissatisfied with ankle draft. Indoor Air. 00:1-11. doi:10.1111/ina.12364. (download)