Backdraft is when air flows the wrong way through an opening. Typically barns are set up so outside air moves through an inlet, across the animals and the fans pull it back outside. When backdraft occurs, outside air moves into the barn through the fan. The main reason drafts come through the fans is the wind is blowing towards the fan. The winter is particularly susceptible to this phenomenon because most fans are not running to push back at the wind. Backdraft can occur when fans are operating, particularly if the fans are variable speed and are set low. However, it is more likely to occur when the fans are not running. In order to prevent back drafts there are a few things that you can do.

1. Cover the fan openings completely. This can be dove with heavy mil plastic, foam board or vinyl fan covers. This is a good way to ensure air cannot move backwards through the fans. However, if there is any chance of the fan needing to run, this is not a good option. For large fans that only need to run in the summer this is a great option.
2. Reduce the ability of air to move backwards. Louvers are on most fans and they are there to stop air from flowing backwards. Make sure louvers are in good operating condition (not stuck open or broken). Some air can still move through the louvers as they are not an airtight seal, but the should stop most gusts. If gusts typically come from directions, instead of using cones, diverters can be placed on the outside of the fan. The attached image shows a diverter that pushes air down. There are also fabric covers that can be placed over the fans often referred to as fan socks. When the fans run they allow air out, but otherwise the fabric falls flat to provide a barrier to the wind. Both the diversions and socks provide some protection at the cost of reduced air movement when the fan is running. These are good options for fans that run occasionally or continuously and are very susceptible to wind.
3. Provide enough air through the fans that are running. Make sure the fans that are running have the ability to overcome the pressures of the wind. As I mentioned earlier operating fans can experience backdraft, similar to fans that are off. Do not turn your variable speed fans down below 50%. Also, consider if you have some fans, which are better positioned (where winter winds to not hit them directly). If you have some protected fans these may be better situated for minimum ventilation .