Clothes should provide the correct amount of insulation to maintain a stable body temperature and achieve thermal comfort.
In addition to the clothing insulation, some additional insulation is gained from the air just outside the outermost garment – the boundary air layer – which contributes to the total insulation.
The precise measurement of garment insulation is done on a thermal mannikin in a lab. Lacking this equipment, we can estimate the insulation from either: calculating the insulation based on the type and density of insulation materials used to make the garment; or for common types of everyday clothes, we can look up values that are published by standards bodies.
Calculation for Outdoor Clothes
Richard Nisley has been kind enough to measure and publish the insulation of various fabrics and materials used in cold-weather clothing. Using Mr. Nisley’s figures, we can estimate the intrinsic insulation of any garment made with similar materials.
Values for Common Garments
|Undershirt with long sleeves||0.12|
|Light-weight, long sleeves||0.20|
|Normal, long sleeves||0.25|
|Flannel shirt, long sleeves||0.30|
|Light-weight blouse, long sleeves||0.15|
|Light skirts (summer)||0.15|
|Heavy skirt (winter)||0.25|
|Light dress, short sleeves||0.20|
|Winter dress, long sleeves||0.40|
|Light, summer jacket||0.25|
|Thick, ankle socks||0.05|
|Thick, long socks||0.10|
|Shoes (thin soled)||0.02|
|Shoes (thick soled)||0.04|
Boundary Air Insulation
The thin layer of air held on the outside of the clothes by frictional drag adds some insulation. Indoors, with slight air movement of around 0.1 m/s, this is fairly significant; outdoors, with increasing air movement due to wind, this effect is much less pronounced.
The wind-adjusted boundary air insulation formula from Fourt and Hollies Clothing: Comfort and Function (1970) is:
Ia = 1 / (0.61 + 1.9 × √w) where:
Ia = insulation of boundary air layer (clo)
w = wind speed (m/s)
Insulation of boundary air layer, Ia, for different wind speeds
|Calm||< 1||< 2||< 0.5||> 0.5|
|Violent||> 47||> 75||> 20.8||< 0.1|
Check that the insulation of your clothes matches what you need.
- A practical system of units for the description of the heat exchange of man with his environment, Gagge, Burton & Bazzett (1941)
- Defines the clo unit for thermal insulation. A short (two page) and accessible paper.
- ANSI/ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy
- ISO 9920, Ergonomics of the thermal environment—Estimation of the thermal insulation and evaporative resistance of a clothing ensemble (2003)
- Contain tables of metabolic rates for different activities and lists of insulation values for various garments.