If you have been following cold exposure enthusiasts like Dr. Andrew Huberman, Dr. Rhonda Patrick, and Dr. Susanna Soeberg, then you have likely heard them discussing thermal barrier. Specifically, they discuss why it is important to break your thermal layer while ice bathing / cold exposure therapy.

If you go into cold water and stay perfectly still, you build a thermal layer that keeps you warm. Think of it as a thin insulative layer right above your skin that's formed from your body heat emitting in the water.

As Dr. Andrew Huberman explains on his podcasts, the thermal layer reduces the effectiveness of deliberate cold exposure therapy—your body is challenged less per second in the ice bath. This is especially true in still water, low flow ice baths and cold plunges.

Filling tubs, barrels, horse troughs with ice water provides immediate cold dose in the beginning of your plunge.

The water temperature can approach mid to low 30s with enough ice.

However, your body heat begins to immediately heat up surrounding water as soon as you enter (98.6° degree body temperature).

Rising water temperature combined with your thermal layer means a less effective cold therapy dose, as your body is not forced to continue adapting to stay warm.

And if you are planning on running multiple people through the ice bath, that means you have to buy lots of ice!

By the time the second, third, fourth person cold plunges, the water temperature is no where near what was prior to the first plunge.

Any level of flow—water that is continuously chilled and
circulated—is better than still water for cold exposure therapy.

However, not all flow rates are created equal. Low flow rates are not powerful enough to completely break your thermal barrier.

How can you tell if your cold plunge flow rate is high enough? Move your arms and legs while in the ice bath.

If you feel a drastic change in intensity from doing so, then the flow rate is not strong enough to break your thermal barrier.

If you try cold plunges with low flow rates, you will feel the cold more aggressively if you move. This is because the thermal layer is broken / reduced with movement; your body is exposed to more cold water per second.

Although significantly better than still water, low flow rates are not powerful to prevent your body heat from affecting the water temperature. In addition, they cannot handle back-to-back, high volume cold plunging.
  

The main benefit of high flow ice baths and cold plunges is
that they rob you of your thermal barrier. They are anti-thermal barrier. This means your body is forced to work harder per second to stay warm.

There are no breaks; there is no give. This is akin to swimming in a winter stream or cold ocean water off the coast of California. You can tell if you are experiencing high flow if moving your arms and legs versus staying still does not increase intensity significantly / at all.

With high flow, your body is working significantly harder than in low flow / still water. This means more acute hermetic stress, more adaption, more cascading health benefits in less time. In other words, higher quality cold exposure therapy dose.   

This is akin to lifting free weights quickly, leveraging momentum, versus using an ARX fitness machine. The ARX machine uses Adaptive Resistance, meaning your muscles are constantly stressed. There is no give.

There is no cheating through using momentum / swinging the weights. Your
muscles are stressed to the maximum every time because there is a machine
fighting you aggressively every muscle contraction and extension.

 This same analogy applies to high flow ice baths. Every second your body is forced to continuously adapt because there is no give, no breaks. Staying still doesn’t save you. You are getting true water temperature and your body is forced to adapt as if it is in natural, flowing body of water.