Discrepancy between needed CLO and theoretically needed CLO?

[Cord Dennig]

Hi folks,

I recently spent a couple of nights sleeping under a low-pitched tarp tent in weather that hovered in the mid-30's (Fahrenheit). I slept on a pad with a CLO of approximately 3, and used a quilt with one layer of 4 oz. Primaloft Sport insulation, which has a CLO of about 3. A 4 oz. Primaloft Sport balaclava completed the ensemble. My base layer consisted of cotton pants, thin polypro socks, and a lightweight mesh long sleeve polyester shirt.

Theoretically, I should have needed a CLO of about 6 to sleep comfortably at ~35 degrees. But I was comfortable, and even occasionally sweaty, with a sleep system of 3-3.5 CLO.

So what gives? Why the great discrepancy between the insulation I SHOULD have needed and the insulation that I ACTUALLY needed? Am I misunderstanding CLO?

Thanks for your help. I'm having trouble understanding this.

[Joe Clement]

What gives? The theories can't account for a warm sleeper, which you must be.

[Bill Fornshell]

Hi,

Read over this article. It lists some Down Loft numbers vs temperature ratings.

This can be used for planning.

Sleeping Bag Temperature Ratings

For an idea of CLO ratings vs temperatures look here:

Thru-Hiker Forum

[Cord Dennig]

Thank you both for your responses. I'll try lower temperatures and different sleep setups to try to establish a baseline for myself.

[Richard Nisley]

Cord,

No discrepancy was demonstrated between “needed CLO and theoretically needed”. What your experiment demonstrated is that your body fat content is lower and your mass is higher than the average woman.

Another away of explaining your error is to point out that a contemporary rating system, such as EN 13537, provides you, at minimum, three numbers to understand a sleeping system. You roughly calculated the LLimit number but didn’t calculate an Extreme number.

Let’s first look at your LLimit approximation. The current version of Primaloft Sport provides .79 clo/oz; 4 oz will theoretically provide 4*.79 = 3.16 clo. A sleep system with an average of 3.16 clo will provide a LLimit temperature of 57F which LOOKS LIKE a glaring discrepancy from your experience.

Let’s next look at the EN13537 Extreme calculation for sleep system with 3.16 clo of average insulation. This temperature is 38F when, after approximately 6 hours, the average female’s body temperature drops enough for shivering to begin. For a male, this temperature is much lower. How much lower is dependent on your body fat percentage (Cg increases with less fat) and your mass.

The amount of heat energy (q) gained or lost by a substance is equal to the mass of the substance (m) multiplied by its specific heat capacity (Cg) multiplied by the change in temperature (final temperature – initial temperature)

q = m x Cg x (Tf – Ti)

Both your mass (m) and your average body tissue heat capacity (Cg) are higher values than for the average female.

No one would ever publish this male Extreme temperature rating without inviting law suits from the families of UL backpackers who perished from hypothermia (smile)… hopefully you understand.

[Cord Dennig]

Thank you for the response. That makes it much easier for me to apply your CLO/Temp/MET system to my circumstances. Thank you!

[David Lutz]

I really enjoy reading Richard's post and have benefited directly, even though mostly I don't understand it!

A couple of questions, first, how do you know all this stuff?

And, second, what is CLO?

[Richard Nisley]

David,

Most materials have been measured to determine how much heat energy moves through them for a STANDARD size of material. This number is typically referred to as the Conductivity or W/m K for the material. (cal/sec)/(cm2 C/cm) and BTU in /hrft2F simple conversions of W/m K to represent Conductivity.

Thermal Resistance is the inverse of Conductivity. 1/C = R. If you want to know how much insulation your jacket or quilt/sleeping bag provides, you look up the Conductivity value, take the inverse, and then adjust the standard thickness for the CUSTOM thickness of the item you want. The scientific community uses m2K/W as the standard thermal resistance number. R-Value, clo, and TOG are simple conversions of the m2K/W number.

Most of the answers, to insulation related questions posted to this forum, require a moderate understanding of human physiology, thermo dynamics, and thermal testing methodology. I studied these disciplines in support of a start-up venture I am dabbling in. Related to that venture, I personally thermal tested many of the products that are discussed on this forum. Roger Caffin is also quite knowledgeable in the above fields and we are fortunate to have him as our moderator.

My summary advice is that you can ignore all of the technical stuff posted by me and others and still make excellent sleep system consumer decisions. Do this by looking at the EN 13537 test results for a particular product or one that is similar in construction to a tested model.

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