Loading... Please wait...

Subscribe to Wiggy's Newsletter

clo value?

Posted by


In 1941 A.P. Gagge, A.C. Burton, and H.C. Brazett published a paper in Science Publication explaining a Clo Unit.

Clo Units

The insulation of a clothing or sleeping bag system is defined in terms of a clo unit. Gagge et a. (1941) derived the value for 1 clo by first considering that the resting metabolic heat production of an average man is about 50 kcal/ m2.h. Approximately 25% of this heat is lost via the respiratory system and by diffusion of moisture through the skin. Therefore, 38 kcal/m2.h remains to be lost through clothing via radiation, conduction and convection (Hollies and Goldman, 1977) {I have a copy of this report which I will show later}. The temperature difference across the clothing is equal to the difference between the mean skin temperature (Ts) and the ambient air temperature (Ta), assuming the mean radiant temperature of the surroundings is equal to the air temperature. Consequently, a clothed person with a comfortable skin temperature of (92 F) in a comfortable environment at (70 F) has temperature gradient across which 38 kcal/m2.h is transferred. A heat transfer coefficient of 32.576 Fm2.h/kcal is calculated by dividing the temperature difference by the heat flow (i.e. Hollies and Goldman 1977). Thus, 1 clo of insulation is equal to 32.324 Fm2.h/kcal. The reciprocal of this value, 41.99 kcal/m2.h or 43.61 w/m2 F, is often used in calculations for convenience.

REFERENCES Gagge, Burton, Brazen Science 1941, Pages 94, 428 – 430.

Whoever transcribed this information obviously did it after 1977 when Hollies and Goldman were doing their work.

The second explanation of Clo comes to me from Natick Army Testing Labs. I enquired about the temperature capability of boots being requested via a sources notice in F. The response I received is as follows and I quote; “Clo does not refer directly to a temperature range, but instead is a measure of thermal insulation. In general, the higher the Clo value of a boot, the greater will be the warmth provided by the boot in a cold environment.”

The first explanation suggests that one Clo unit is about 42 degrees F while the second explanation suggests that the thicker the insulation the greater the Clo value. I do not believe a temperature can be applied to any Clo number, however, I do agree the thicker the insulation is the greater the heat retention is going to be.

The first explanation as I say suggests a temperature of 42 degrees F but when you read the Hollies and Goldman study they show one Clo with clothing is 80 degrees F and in a sleeping bag on the ground (no tent or any shelter is noted) 4 Clo is 50 degrees so one Clo in a sleeping bag is also probably 80 degrees; both temperatures are noted in ideal conditions. They further reference if the climatic conditions are wet or dry which increases the Clo which decreases the temperature ending with a 12 Clo equaling -55 degrees F which is what my Ultima Thule reaches. Goldman may have been in the employ of Natick at one time. He left and opened his own business with Hollies in the mid 1970’s in Alexandria , VA. Comfort Technologies.

My first involvement with Clo testing came about in 1978 when I submitted two of my Olam bags to Natick to the tester Frank Calibrese. I gave him what I called the Zero Bag and the Sub Zero Bag (-20 degree rating), these were bags that I made at Olam my first company using laminated Polar Guard the early Lamilite. His results were 5.18 for the Zero bag and 5.81 for the Sub-zero bag. Each bag increased slightly after 3 laundering. What I find remarkable is how close the Clo value is even though the top half of the Sub-zero bag was double the Zero bag. More recently Natick purchased an Ultima Thule (UT) bag from me (2011) and their testing showed the UT at about 10 Clo. According to HG that means the UT would have a Clo value of 10 or a temperature capability of -35 F, which I know is true of the temperature capability from reports from customers and with even colder conditions.

I have several graphs showing the Clo value versus temperature rating, however, every one of these charts in their title state either theoretical, predicted, or extrapolated ratings for sleeping bags. When I questioned Frank Calibrese about an actual temperature rating he told me he could not give me one. And the conversation went no further. Now you understand the reason they refer to Clo in theoretical terms.

What I have noted about the various charts is the lack of consistency. One chart shows the temperature of 40 degrees and a Clo of 3.9. Chart number two shows a temperature of 40 degrees and a Clo of 6. The third chart (which happens to be the Natick chart) shows a temperature of 40 degrees and a Clo of 3.9 degrees. The 4th chart shows a temperature of 56.5 degrees and a Clo of 4.

In the 1980’s the manufacturers that were still in the USA started sending their bags to Kansas State University environmental labs for testing on their manikin. So in 1986 I sent one of my prototype Wiggy’s bags to them in May of 1986. The report I received stated that my bag showed after 3 tests an average Clo reading of 4.42. I called the tester B.J. Rueschoff (probably not there any longer) and asked what the temperature was for 4.42 Clo and he told me he could not give me that number but for the weight of the bag it was the highest they ever recorded. Basically it was my 3.25 pound Ultra Light.

In 1990 the Celanese Company who marketed the Polar Guard at the time unbeknownst to me purchased my Ultima Thule and sent it to KSU to be tested against the North Face Igloo model and I was sent the report. My bag being 2 ½ times heavier and three times thicker than the original bag I sent to them in 1986 had an increase in Clo from 4.42 to 4.9, while the NF bag was 6.5. Today and for many years the Igloo bag has not been made because it never performed, while the Ultima Thule has been made for 30 years, since the inception of Wiggy’s and it actually performs lower that I advertise it to perform. The only change has been the Lamilite/Climashield is a better material today than it was when I first started, and that goes for all of the weights.

When I reviewed the reports I realized that they are very inconsistent because the testers were getting sleeping bags with various forms of insulations and constructions. Some were chopped staple polyester, feathers and down mixture, all down and eventually continuous filament Polar Guard. All were quilted construction until I came along.

It is my understanding that REI will not carry any bags that do not comply with the Clo testing or the European testing known as EN. In Europe retail stores cannot sell bags that do not comply as well. So Wiggy’s will just have to stay mail order, which suits me.

Now you have available to you all of the facts surrounding Clo so you know that Clo has no value, never did and never will. When buying a sleeping bag always ask for the temperature rating in F or C depending upon where you live. in addition ask if you can return it if it does not perform as advertised!

As I have said in the past; to buy any sleeping bag other than a Wiggy’s bag you have wasted your money.

Wiggy's Signature

Our Locations  +  Contact

Corporate Office & Factory

To place an order, please contact our corporate office & factory at:

Wiggy’s Inc.
PO Box 2124
Grand Junction, CO 81502
+1 (866) 411-6465 f:  (970) 241•5921 e:  

Retail Stores


2482 Industrial Blvd  •  Grand Junction, CO
(970) 241-6465


8225 Old Seward Hwy, Ste A  •  Anchorage, AK
(907) 336-1330

When it comes to extreme cold weather gear, Wiggy's has you covered.

Check out all our products from sleeping bags & shelters to footwear & clothing. Our uniquely developed continuous filament fiber called Lamilite insulation is what sets Wiggy brand insulated products apart. What is Lamilite and why does it perform better than all other forms of insulation? Click here to keep reading & find out more »

© Wiggy’s Inc. All Rights Reserved. Website design by Monument Graphics & Communications // Mobile Site