Thermal conductivity is an intensive property of a material that measures its capacity to sustain heat flow. The units of thermal conductivity are often provided in metric values of Watts/meter-degree Kelvin or W/m-K. The symbol used to denote thermal conductivity is k (or lambda, λ).

• There are many different unit conversions to Imperial and others:
  
  • 1 BTU/ft hr F = 1.73 W/m-K or 1730 mW/m-K
    
  • 12 BTU-in/ft² hr F = 1 BTU/ft hr F = 1.73 W/m-K
    
  • 1 BTU-in/ft² hr F = 0.144 W/m-K or 144 mW/m-K
  
  
• The reciprocal of thermal conductivity is thermal resistivity; an intensive property of a material to resist one-dimensional heat flow. Thermal resistivity has units of m-K/W.
  

Thermal Resistance (R value)

• R is thermal resistance, which is the capacity of a material to impede heat flow over a given area and at a specific temperature. The greater the value at a given set of conditions, the better the material will perform as a thermal insulator.
  
• The units of thermal resistance are provided in m²-K/W (or ft² hr °F/BTU)
  
• Thickness/k value = R value
  
• Resistances of insulators in series can be added
  
• R-value can be for whole thickness or normalized (e.g. R-value is 12 per in or 24 for 2 inches of R-12 per inch material)
  
• R/in = 144/k (in mW/m-K) -> 12 mW/m-K = 12 per inch R value
  
• The reciprocal of thermal resistance is thermal conductance (also known as the heat transfer coefficient), with units of W/m² K. This value is frequently measured in systems as the overall heat transfer coefficient (OHTC).
  
• Thermal resistance values are often used in the building and construction markets as a relative measure of thermal insulation performance for materials and systems.
  

Heat Transfer Coefficient (U Value)

• U values measure thermal conductance, a measure of a material or system to permit power flow per unit area and temperature change (W/m² K).
  
  • U-value is heat transfer coefficient
    
  • Low U values mean good insulation values (very similar to thermal conductivity k)
    
  • OHTC -> Overall heat transfer coefficient; frequently used to measure the thermal conductance of systems
    
  • In the world of systems thermal engineering – U value and OHTC are often used synonymously.
  
  

Clo Value (I_cl Value)

Clo value is a measure of the thermal resistance value of clothing, especially as it relates to relative human comfort within that clothing at a given set of conditions. Clo value is approximately calculated by multiplying R value by 1.136:

• I_CL = R * 1.136

Clo has the same units as thermal resistance R but has specific magnitude relative to human comfort:

• 1 clo maintains the temperature comfort of a sedentary man indefinitely at 21°C, 50% RH, and a wind speed of 0.01 m/sec.
  
• Wind speed effects clothing insulation
  
• Porosity - water vapor transfer through clothing affects its insulation
  
• 1 clo = 0.155 m² °C/W