Outdoor Gear Insulation Ratings Explained

In this article we explain the mystery behind the various ways clothing insulation is rated to help you choose the best for those cold weather activities.

Choosing the Proper Insulation is Important

We already know that in the wintertime we want more insulation to help protect us against the cold. However, if we go out to buy just any old winter coat or sleeping bag in the store or online there are hundreds of items to choose from and without knowing a little bit about types and ratings of insulation we could buy something that we later regret.

It’s important we choose the proper type of insulation for both the temperature and activity the outdoor gear will be used for. Heat will always flow from a hotter region to a colder region and the greater the temperature difference is between your body and the outside temperature, the faster our bodies will loose heat.  

If we aren’t wearing the appropriate insulating layers to protect ourselves from the surrounding cold temperatures, there will come a point where our body will not be able to generate sufficient heat to keep us warm and the effects of hypothermia will begin.

How Does Insulation Work?

Based on the physics of heat transfer, heat will flow from a hotter region to a colder region to try and achieve thermal equilibrium. The amount of heat lost by our bodies depends on several factors including body temperature or body heat generation, outside air temperature, wind speed, thickness of insulation, the distance between the insulation fibers and the insulation fiber size.

Insulation works to prevent heat from naturally leaving your body by trapping air within the insulating layer thus increasing the effectiveness of the insulation. It also works by reflecting the radiant heat that you lose through your skin back to your body. Note that adding windproof outer materials to the piece of clothing will increase its ability as an insulator by additionally reducing heat loss. 

Our bodies will loose heat via several different mechanisms. First, the heat your body generates will be conducted through the materials in your clothing therefore it is important to have clothing with a low thermal conductivity. Many types of insulation boast about loft or airspace in their materials and this is because air is an excellent insulator with low thermal conductivity. Secondly, our bodies will radiate heat towards the colder ambient environment in a similar fashion to the radiant heaters in your home. Lastly, depending on the wind speed outside, our bodies will loose heat convectively if our outer clothing layers are not sufficiently windproof.

The Types of Insulation


Down comes from the underbelly of waterfowl and is the best insulator on the market as long as it is kept dry. Down is not the feather but rather the covering beneath the feathers on the waterfowl. It is highly compressible and has an excellent warmth to weight ratio. There are two primary sources of down, from duck and from geese. Down from a mature goose lofts better than immature goose down and goose down generally lofts better than duck down at the same down cluster %. 

An investigation by Swedish News Program Kalla Fakta found that 50-80% of down is live-plucked, making it important to be aware of sourcing.

Down is one of my favorite types of insulation for winter gear. I would highly recommend the latest Down-tek products which utilize a natural down fill that is treated with a water resistant coating. Be wary of the down to feather % ratio from manufacturers as lower quality and less costly products will have higher percentages of feathers which do not posess the same insulative properties.


There are a large variety of synthetic insulations on the market. PrimaLoft One has the highest CLO value (warmth value) of any synthetic insulation available. The warmth of PrimaLoft One or Gold when dry, is equivalent to 500-550 fill-power down.

In addition to Primaloft, some other types of synthetic insulation available on the market today include 3M Thinsulate, Coreloft (Arc’teryx), Exceloft (Mont-bell), Climashield Apex, and Polarguard.

Outside of the textile industry many other different types of high tech insulation are utilized that may one day make it into our clothing and outdoor gear. Some notable ones include: 

Aspen Aerogels: Having a thermal conductivity of 14500 to 21000 W/m-K, this amazing insulator is used in industry, defense and aerospace applications as well as some clothing applications (Aerotherm). http://aerotherminsulation.com/

How Insulation is Rated: The CLO and R-Values

In 1941 the American Scientists Gagge, Burton and Bazett created the “Clo” which is the insulation rating value used by some outerwear manufacturers. It is analogous to the R-value used in house insulation and sleeping pads where 1 R-value (US) = 1.136 Clo and both units are a measure of thermal resistance or the reciprocal of thermal conductance (otherwise known as the heat transfer coefficient). The units of thermal resistance are generally provided in m2-C/W (or ft2 hr °F/BTU).

The unit of the Clo is slightly subjective but a Clo value of 1.0 is defined as the amount of clothing needed by an inactive person to feel comfortable at a room temperature of 71F (21C) in a light breeze ( having a 20 ft/min or 10 cm/sec air flow rate) with a relative humidity <50%. A value of 0 Clo would be equivalent to the amount of insulative clothing on the naked body.

Clo values are additive for each piece of Clothing that someone is wearing thus summing the Clo values of individual components will give an approximate total Clo value for the person.

Down Clo

Many times manufacturers and researchers use the “Intrinsic Clo”, ICL, to describe the insulation rating of a material. This is a normalized value so various sizes and thicknesses of clothing of the same material can be compared. In the US the units for intrinsic Clo values are typically clo/oz/yd2.

For example, 10 oz of insulation having a reported Clo value of 4.2. What is the standard Clo value? We need to know the weight of the insulation to get the standardized Clo value in US units. Thus, let’s assume this is 10 oz of material per 1 square yard. Now, we use the following equation: 

Total Clo= 4.2

4.2 / 10 oz = 0.42 Clo (Intrinsic per oz/yd2)

R-values are typically calculated per inch of material. From the thermal conductivity value (k) the R-value can be determined by the following equation: 

R-value = Thickness / k

In Europe, The Shirley Institute developed the unit of the “Tog”. The British standard BS4745, otherwise known as the Tog Test is used to measure the thermal resistance of textiles.

One Clo is equivalent to a thermal resistance (r) of 0.155 m² C/W or 0.88 R-value or 1.550 tog.

ICL= R-value x 1.136 = r (m² C/W) x 6.451

Below is a basic summary of the required Clo for various ambient temperatures. Note that other factors such as wind, a persons metabolism and  the physical makeup of a person must also be considered.

     0 Clo to 1.0 Clo ~ Summer Clothing (68F to 86F)

     2 Clo ~ Spring and Fall Clothing (50F to 68F)

     3 Clo ~ Spring and Fall Clothing (32F to 50F)

     3.5 Clo ~ Winter Clothing  (32F to 14F)

     4 Clo ~ Winter Clothing (14F to -4F)

     4+ Clo ~ Winter Clothing (-4F to -40F)

Here are the Clo Values for different types of insulation (note that 1 oz =28.34952 grams and 1 sq m=1.19599 sq yd.): 

Type of Insulation CLO-value (clo/oz/yd2)
550 fill Down 0.7
650 fill Down 1.0
800+ fill Down 1.68
Climashield APEX  0.82
Coreloft by Arctyrex 140 grams/sq m clo = 4.01, noted to be 5% less than Primaloft One
Down-Tek (Water resistant Down) Similar to Down
DriDown: 600-fill  down by Sierra Designs Similar to Down
Exceloft by Mont-bell 0.68
Marmot Thermal R Eco 0.8
Polarguard by Invista 0.488
Primaloft Eco Dry: 0.68(0.020 clo/g/m2)
Wet: 0.60 (0.017 clo/g/m2) 
Primaloft Eco Footwear Dry: 0.250 m2 C/W/IN (0.100 m2 C/W/CM)
Wet: 0.115 m2 C/W/IN (0.046 m2 C/W/CM)
Primaloft Infinity Dry: 0.57 (0.017 clo/g/m2)
Wet: 0.47 (0.014 clo/g/m2) 
Primaloft One Dry: 0.92 (0.027 clo/g/m2)
Wet: 0.90 (0.026 clo/g/m2) 
Primaloft Sport Dry: 0.79 (0.023 clo/g/m2)
Wet: 0.72 (0.021 clo/g/m2) 
Primaloft Synergy Dry: 0.73 (0.022 clo/g/m2)
Wet: 0.61 (0.018 clo/g/m2) 
Primaloft Black 0.65
Primaloft Silver 0.79
Primaloft Gold 0.92
Thermolite Dry: 2.9 CLO/cm
Wet:1.55 CLO/cm
type C, CS and CDS 100
type C, CS and CDS 150
type C, CS and CDS 200
type C, CS and CDS 40
type C, CS and CDS 70


How Insulation is Rated: The Gram Weight Value

In lieu of providing the Clo value for products, some manufacturers such as 3M who makes Thinsulate insulation include the gram weight on their product tags. This value is the weight in grams of one square meter of the insulation (g/m2).  In general, thicker, warmer insulation will weigh more per square meter and hence have a higher number.  

Down gm

Low Warmth, <80 grams of insulation: This type of lightweight outerwear is meant for high activity or mild temperatures.

Med Warmth, 80g to 150g of insulation: This type of medium weight outerwear is meant for mild activity or cold temperatures.

High Warmth, >150 grams of insulation: This type of heavier weight outerwear is meant for low activity or very cold temperatures.

Here is a table of typical gram weights for various outerwear:

Insulating Material Weight (g/m2) Weather Conditions for various Clothing
40 gm Hats and Gloves: Cool condition or high activity level. Jackets: Warm or slightly cool conditions
50 gm Hats and Gloves: Cold conditions or moderate activity level. Jackets: Cool conditions
100 gm Hats and Gloves: Very cold condition or light activity level. Jackets: Between cool and cold conditions
150 gm Hats and Gloves: Extremely cold condition or very light activity level. Jackets: Cold conditions
200 gm Jackets: Very cold conditions. Boots: Cool condition or high activity level
400 gm Jackets: Extreme cold conditions. Boots: Cold condition or moderate activity level
600 gm Boots: Very cold condition
800 gm Boots: Extremely cold condition and light activity level
1000 gm+ Boots: Extremely cold condition minimal activity level


How Insulation is Rated: Fill Power

However it is important to note that the type of insulation and how effective it is at preventing heat loss, not just the weight, plays a factor in the warmth as well as the types of shell/lining materials. For example, you could purchase a very heavy winter jacket however if the insulation and fabric does not trap your body heat in very effectively, then the jacket is not suitable. 

When it comes to down insulation, it is primarily rated by fill power on a scale of 400 to 900, with the higher numbers providing the greatest warmth-per-weight ratio. The fill power is measured by taking a standard weight of the down, prepping it then placing it in a graduated cylinder and measuring the volume displaced.

Down Loft

For example, one ounce of 650 fill-power down will displace 650 cubic inches and 800 fill-power down will be able to displace 800 cubic inches. The higher the fill power the greater loft the down has.  A greater loft means that there is more airspace between the down plumules and a thicker layer of trapped air results in better thermal insulation.

In general the higher the fill power the larger the down clusters:

  • The larger the down cluster:
    1. The higher the quality it is as it will retain its loft and firmness best. 
    2. The more mature the bird from which it came.
    3. The more air it traps.
    4. The higher it will loft.
    5. The better the insulating power.
    6. The lighter the down garment or sleeping bag will be.


Comparing Insulation Ratings

Fill Power Weight of Down Displaced Volume Clo (clo/oz/yd2) Tog
550 1 oz 550 in 3 0.7  1.085
650 1 oz 650 in 3 1.0 1.55
800+ 1 oz 800 in 3 1.68 2.604

When buying different types of down garments it is important to note the weight of the down used in addition to the fill power. For example, let’s calculate and compare a different weights and down fill values to see which will perform as the better insulator.  

Fill Power Weight of Down Weight of Down Displaced Volume Clo Value Tog Value
(Fill) (g) (oz) (yd2) (Total Clo) (tog)
550 50 1.764 0.021 1.23 1.91
550 100 3.527 0.042 2.47 3.83
550 150 5.291 0.062 3.70 5.74
550 200 7.055 0.083 4.94 7.65
550 300 10.582 0.125 7.41 11.48
550 400 14.110 0.166 9.88 15.31
550 600 21.164 0.249 14.82 22.96
550 800 28.219 0.333 19.75 30.62
650 50 1.764 0.025 1.76 2.73
650 100 3.527 0.049 3.53 5.47
650 150 5.291 0.074 5.29 5.47
650 200 7.055 0.098 7.06 8.20
650 300 10.582 0.147 10.58 16.40
650 400 14.110 0.197 14.11 21.87
650 600 21.164 0.295 21.17 32.80
650 800 28.219 0.393 28.219 43.74
800 50 1.764 0.030 2.963 4.59
800 100 3.527 0.060 5.926 9.19
800 150 5.291 0.091 8.889 13.78
800 200 7.055 0.121 11.852 18.37
800 300 10.582 0.181 17.78 27.56
800 400 14.110 0.242 23.704 36.74
800 600 21.164 0.363 35.556 55.11
800 800 28.219 0.484 47.408 73.48

Total Clo versus Down Fill Weight

Sleeping Bags

The first recorded use of down sleeping bags was by Alfred Mummery in 1892. Ajungilak, an insulation manufacturer, began producing sleeping bags by 1890.

Important Standards for Sleeping Bags
– BS4745-1984 Thermal Resistance of Textiles
– ISO 5085 Thermal Resistance of Textiles
– EN 31092 Thermal Resistance of Textiles
– ASTM F 1720 – 96, Standard test method for Measuring Thermal Insulation of Sleeping Bags Using a Heated Manikin
– Norme Francais G08-013 Requirements for Sleeping Bag
– EN 13537 Requirements for Sleeping Bags

Aside, note that in the European standard EN 13537 which is used in Europe to standardize the temperature ratings for sleeping bags, the comfort value for women is typically 5C higher than that for men.


~Note: This article is updated annually. If any information is missing or incorrect please contact the webmaster. Thanks!



  1. IDFL Laboratory and Institute http://idfl.com/info/articles/
  2. DEA 3500 Notes by Professor Alan Hedge, Cornell University. http://ergo.human.cornell.edu/studentdownloads/DEA3500pdfs/Thermcomfort.pdf
  3. Mammut, Sleep Well, A review of Temperature Standards for Sleeping Bags. https://d1qxh2iwg385ci.Cloudfront.net/images/Mammut_Sleep_well_pt1_E.pdf
  4. http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA222291
  5. Thinsulate Data
  6. A Comprehensive Data Base for Estimating Clothing Insulation http://rp.ashrae.biz/page/RP411.pdf
  7. US Airforce Survival Handbook http://www.dtic.mil/dtic/tr/fulltext/u2/a032336.pdf
  8. Polarguard Website http://www.polarguard.com/
  9. Montbell Sleeping Bag Warmth
  10. Aspen Aerogel Data http://www.aerogel.com/products-and-solutions/all-insulation-products/default.aspx , http://www.aspenaerogels.com.tr/features/values.html
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One Comment on “Outdoor Gear Insulation Ratings Explained

  1. great article. Thanks for including references and accurately explaining clo, fill, and weight. Really appreciate disclosing the name brands.

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