What is the Clo Value of FR Clothing?

As we celebrate the New Year and continue on into winter, one question many workers are asking is how to stay warm in cold outdoor weather. Environmental conditions including outside temperature, wind speed and humidity level play a large part in how comfortable a worker will be. We’ve taken a look at heat stress, when heat needs to be released from the body in summer, and cold stress, when heat needs to be retained by the body in winter. In this post, we will take a look at the different terms related to clothing insulation, and specifically, what a Clo Value on your FR clothing is referring to.

Clo Value is used to measure clothing’s thermal properties – how warm a garment will keep you. If you look up “clo,” you will find it defined as “the amount of insulation that allows a person at rest to maintain thermal equilibrium (1).” A clo value indicates how resistant a garment is to thermal loss. For example, a clo value of zero would mean a person is not wearing anything. Therefore, the higher the clo value, the warmer a person will be and the colder the environment, the higher clo value needed.

In technical terms, 1 clo is the amount of thermal insulation required for clothing to be comfortable in a normally ventilated room at 70 degrees F with less than 50% humidity in a sitting/resting position (1). While clo value can be used to compare two different insulative garments, this rating will not tell workers the coldest outdoor temperature they can work in and still stay warm.

This is where a temperature rating comes in. This rating is based on the insulation capability of the clothing system and the level of activity of the employee. Temperature rating indicates the lowest temperature at which a worker can wear a garment and stay warm.

ASTM F2732 outlines Standard Practice for Determining the Temperature Ratings for Cold Weather Protective Clothing. This practice is used to measure the insulation provided by different cold weather clothing systems using a heated manikin and a heat-loss model to predict the lowest environmental temperature for worker comfort. The main goal of such temperature ratings estimated by this standard practice is to help outdoor workers avoid hypothermia when wearing cold weather protective garments (2).

Finally, the term MET, Metabolic Equivalent of Task (or metabolic equivalent) describes the variable amount of internal body heat that is generated by a worker’s activity level. 1 MET is defined by the amount of heat produced by an average person at rest. A person who is walking slowly is generally producing 2 MET, while a person walking at a fast pace can produce about 4 MET. As a worker’s physical activity level increases, the amount of metabolic heat produced by the body increases which will help the person stay warm. This means that less insulation is needed to provide the person thermal comfort[3]. More activity means a higher MET and temperature rating, as described above, is directly tied to MET.

Which industries might need to know clo value and temperature ratings? Cold weather protective apparel is made for workers in the utility, oil and gas and a variety of other industries that may face arc flash or flash fire hazards.

It’s important to point out that the clo value and temperature ratings for garments are specific to a combination of materials (outer shell, insulative material, inner lining) and garment configuration used in testing. It is simply not practical to test every configuration and combination, which is why not all FR garments list a clo value on their label.

3 things to remember about FR apparel and cold weather protection:

  1. As required by OSHA in standard 1910.269, the outer-most layer of PPE must be arc-rated FR clothing.
  2. Air is a good insulator. Some apparel insulation can be up to 99.5% air. Insulation is simply impeding the flow of energy out of or into something – so the goal during winter is to trap in more of a worker’s own body heat and prevent it from being released.
  3. Lightweight warmth is better than bulky layers. Look for garments that provide adequate protection, yet enough warmth and breathability for the activity level of you or your workers. Today, advancements in cold weather FR apparel are making compliance and comfort easier than ever.

For more information on the best practices of layering in cold weather conditions, read our previous blog series:


References for this post were accessed January 2015:

(1) http://en.wikipedia.org/wiki/Clothing_insulation

(2) http://www.astm.org/Standards/F2732.htm

(3) http://www.wellservicingmagazine.com/featured-articles/2013/11/dont-get-caught-out-in-the-cold-with-the-wrong-outerwear/

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