After you understand how clothes keep you warm, you need to decide exactly how much insulation will be needed based on the anticipated weather. Intuition derived from experience is the most accurate way to make these decisions; however, when faced with unfamiliar conditions, it is possible to calculate what to wear to stay comfortable.
Clothing insulation is measured in clo units. 1 clo (= 0.155 m² K / W) is defined as the amount of clothing insulation that will keep a resting human in thermal equilibrium indoors at 21°C (70°F).
Use this page to determine how much insulation is required for the expected weather, then work out the insulation of your clothes to find an appropriate outfit.
Look at both the minimum and maximum temperatures expected during your trip, as well as the maximum and minimum level of physical activity that you anticipate. Remember that to remain comfortable in the coldest conditions you expect, you need to be able to adjust the amount of insulation so you don’t overheat and sweat, since any moisture reduces the insulating ability of the clothes.
Once you know how much insulation you need, find out how much insulation your clothes have.
How much insulation you need varies primarily with:
- temperature – the colder it is, the more insulation you need; and
- metabolic rate – the more you move and exercise, the less insulation you need.
How It’s Calculated
Using the relationship:
It = A ⋅ ( Tsk − Ta ) ÷ H where
- = total insulation (m² K / W),
- = minimum comfortable skin temperature (°C),
- = operative temperature (°C),
- = heat loss through skin (W / m²)
For a stable body temperature, the total rate of heat loss must equal the rate of body heat production. According to ASHRAE 55, the average metabolic rate of a resting human is 58.2 W/m² which is defined as 1 Met.
Let M = metabolic rate (Met)
Assuming 25% of body heat is dissipated through evaporation leaves 75% to be transmitted through the skin.
It = ( Tsk − Ta ) ÷ ( 75 % of 58.2 M )
Using a conservative estimate of 33°C for the minimum comfortable skin temperature, and converting to clo units yields:
It ≈ 0.148 ⋅ ( 33°C − Ta ) ÷ M clo
Or in Farenheit:
It ≈ 0.082 ⋅ ( 91.4 °F − ta ) ÷ M clo
- Human Thermal Environments (3rd Ed.), Ken Parsons
- The introductory material and Chapter 9, “The Thermal Properties of Clothing” may be helpful to better understand the theory of how the body, clothes and environment interact, as well as the equations that model these interactions. Chapter 15, “Cold Stress”, presents different equations for working out required insulation and the effects of various conditions such as wind.
- ANSI/ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy
Contains tables of metabolic rates for different activities and lists of insulation values for various garments.
Since the standard is mainly concerned with the heating and cooling of buildings, much of it is not applicable to outdoor environments or any significant physical activity.
There is a Thermal Comfort Tool hosted on the UC Berkely website which performs calculations to test compliance with ASHRAE 55.
- ISO 11079, Ergonomics of the thermal environment (2007)
- Clothing Systems for Outdoor Activities, Morrissey & Rossi (2013)
- Fairly accessible overview of the literature on cold-weather clothing. The section on mid-layers discusses the effect of wind on clothing insulation and the boundary air layer.