The Clo: Quantifying Clothing Insulation and Thermal Comfort

The clo unit quantifies the thermal insulation provided by clothing ensembles. Originally developed to support aircraft crew survivability research, clo has become a cornerstone of ASHRAE Standard 55, ISO 7730, and ergonomic design. Understanding clo values enables designers to balance HVAC system loads, occupant comfort, and personal protective equipment requirements without sacrificing SI coherence.

Definition and Units

One clo represents the insulation necessary to maintain a sedentary person in thermal equilibrium at 21 °C, 50% relative humidity, and air velocity of 0.1 m·s⁻¹ with a metabolic rate of 1 met (58 W·m⁻²). In SI terms, clothing insulation Icl is expressed in square metre kelvin per watt. The conversion is:

1 clo = 0.155 m²·K·W⁻¹.

Values below 0.5 clo describe lightweight attire (e.g., summer outfits), while values above 2.0 clo correspond to multilayer cold-weather gear. Reporting both clo and SI units supports interoperability with building simulation tools that expect Icl in m²·K·W⁻¹.

Historical Background

Origins in aviation physiology

The US Army Air Forces’ Environmental Protection Section pioneered the clo concept during World War II. Researchers sought a quantitative method to specify flight suits that would keep aircrew warm at high altitude. They measured heat loss from heated mannequins and human subjects in climate chambers, developing tables of insulation for layered garments.

Standardisation in ASHRAE and ISO

ASHRAE adopted clo in its 1966 comfort standard, and ISO 7730 later incorporated the unit into the Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD) framework. Today, ASHRAE Standard 55 provides detailed clo tables for business attire, hospital garments, and protective clothing, ensuring designers can document assumptions when performing comfort calculations.

Digital-era refinements

Infrared thermography, sweating mannequins, and computational fluid dynamics have refined clo estimates by capturing local variations in heat transfer. These tools support adaptive comfort models and wearable technology development, where garments may include phase-change materials or active heating elements.

Concepts and Equations

Heat balance for the human body

The human energy balance equates metabolic heat production with heat losses via convection, radiation, evaporation, and conduction. The PMV equation from ISO 7730 incorporates clothing insulation Icl to model skin temperature and heat transfer. Adjusting clo modifies the surface area factor fcl, influencing convective and radiative heat exchange terms. Accurate Icl inputs therefore directly affect predicted comfort indices.

Layering and ensemble calculations

Clo values are additive for garments separated by air layers. For example, a long-sleeve shirt (0.25 clo) plus trousers (0.25 clo) and a blazer (0.35 clo) yields approximately 0.85 clo. However, compression from tight clothing or wind infiltration reduces effective insulation. Field assessments should adjust tabulated values to reflect actual fit and activity.

Coupling with ventilation and humidity

Clo interacts with environmental parameters such as air velocity, humidity, and mean radiant temperature. Increasing air speed enhances convective heat loss, effectively lowering insulation. Designers evaluate these interactions using the PMV/PPD calculator to ensure building automation strategies maintain comfort under varying clothing levels.

Measurement Techniques

Thermal manikins

Modern thermal manikins feature segmented heating zones controlled to maintain constant skin temperature. By measuring power input and surface heat flux, researchers derive clothing insulation across different body regions. Manikin results are then averaged and normalised to produce ensemble clo values.

Sweating manikins and evaporation

For protective garments and athletic apparel, latent heat transfer is critical. Sweating manikins simulate perspiration by releasing water through porous skin surfaces. Data from these tests inform effective clo values under high humidity or high-activity scenarios, complementing convective measurements.

Field observations

In practice, facility managers often estimate clo by observing occupant attire. Surveys may include seasonal wardrobes, uniform policies, or personal protective equipment requirements. Documenting observed clo levels informs HVAC scheduling and adaptive comfort policies aligned with ASHRAE’s extended comfort zones.

Applications

HVAC system optimisation

Building energy models account for occupant clo levels when determining sensible and latent loads. Lower clo assumptions may require cooler supply air temperatures, while higher clo allows relaxed setpoints that save energy. Integrating clo schedules with demand-controlled ventilation aligns comfort goals with energy efficiency strategies.

Occupational safety and ergonomics

Industries ranging from food processing to foundry operations mandate specific protective clothing. Quantifying the clo contribution of aprons, gloves, or thermal liners helps safety officers evaluate heat stress risk. The wind chill calculator is valuable for outdoor crews, translating environmental conditions into required clothing insulation.

Outdoor recreation and apparel design

Apparel brands design layering systems with target clo values to guide consumers. By stating insulation ratings, brands help hikers and climbers tailor outfits to anticipated weather using resources like the heat index and cold-chain hold-time calculators that highlight thermal risks.

Importance for Sustainability

Accurate clo assumptions enable HVAC setpoint adjustments that reduce energy consumption without compromising comfort. Adaptive comfort models supported by clothing policies can deliver measurable carbon savings, especially in mixed-mode buildings. Document clo values alongside met levels when reporting to sustainability frameworks such as LEED or WELL to demonstrate compliance with occupant comfort credits.

Maintain SI notation when documenting clothing insulation: write 0.8 clo (0.124 m²·K·W⁻¹) to provide both familiar and standard units. Consistency in notation streamlines data exchange between ergonomists, mechanical engineers, and energy analysts.

Where to Go Next

Continue refining thermal comfort strategies with these references: