The Clo and Thermal Comfort

What is a clo?

The clo is a unit which measures how much insulation clothes provide. It includes the insulation provided by trapped air between layers of clothing and the cooling effect of loose clothing which through movement create ventilation around the body.

Click here to play “What The Clo?” a game about clothing combinations and their clo values.

How is the clo related to the comfort zone in the psychrometric chart?

The clo is related to the comfort zone in the psychrometric chart because it is assumed people wear clothing while in buildings; only temperatures, relative humidities, and specific volumes which would allow people to continue wearing clothing are accepted as part of the comfort zone.

The comfort zone (light green) and acceptable comfort zone (dark green) in the psychrometric chart have been set by keeping air movement, activity type, and clo at predetermined constants. These constants and a variety of other affecting factors like personal weight, health, and age have been reduced down to one very specific situation: working-age office workers, of average weight, in good health, doing sedentary work, in a three piece business suit and cotton undergarments, with air movement at very low speeds so as to avoid paper being blown off desks.

This is what it looks like in my head, and it makes me ask:

Who works like this any more!!??

The above scenario has remained the basis for AC and mechanical heating design for all building types since the inception of the comfort zone itself. There is scope here for broadening temperature and relative humidity ranges contained within the thermal comfort zones based on building type, activity, higher air speeds, and occupant selection of alternative clothing combinations.

One of the many specialist skills of adaptable building designers, is in understanding the science of thermal comfort not just as a set of facts and figures never to be manipulated, but in context with the real world and social aspects of building use.

Following occupant selection of clothing combinations, two comfort zones have been defined to guide AC and heating design; one for summer when occupants are likely to wear clothing combinations equal to 0.5 clo, and one for winter when occupants are likely to wear clothing combinations equal to 1.0 clo.

An experienced adaptable building designer or passive solar architect will be able to provide comfortable interiors at even broader temperature and relative humidity ranges. They do this by arranging building elements and features in a variety of ways, for example to extend access to direct sunshine in winter and harness faster air speeds in summer more diligently than a standard building.

Want to know more?

Read my post on Stretching the Thermal Comfort Zone.

References accessed and correct at time of publishing:

1950’s thermal comfort temperature, relative humidity, clo, and air speeds are from:

Ashrae, A. N. S. I. Standard 55-2017. Thermal Environmental Conditions for Human Occupancy 744 (2017).

What The Clo values for various clothing are from:

Auliciems A., Steven V. Szokolay “Note 3: Thermal Comfort.” Passive and Low Energy Architecture International Notes. Department of Architecture University of Queensland, 2007.

Alshamasi, S. M. R. Socio-cultural and climatic architectural strategies for a sustainable domestic and neighbourhood environment in Qatif, Eastern Province, Saudi Arabia. University of Western Australia PhD Thesis, (Unpublished) 2015.

What The Clo music is from:

Composer: Benjamin Tissot  at  https://www.bensound.com/royalty-free-music/track/funday

You may also be interested in the following resources:

Szokolay, Steven V. Introduction to architectural science: the basis of sustainable design. Routledge, 2014.

Bhatia, A. HVAC Thermal Comfort – Concepts & Fundamentals. PDHonline Course M321 (6PDH), 2012.