Btu (IT) per Hour per Square Foot [Btu(IT)/(h·ft²)]

The unit Btu (IT) per hour per square foot, written as Btu(IT)/(h·ft²), measures heat flux density or thermal power per unit area using the International Table British Thermal Unit (1 Btu(IT) = 1055.06 J). It represents the amount of heat energy transferred through 1 square foot of surface every hour.

This unit is widely used in U.S. building construction, insulation, HVAC, and solar energy studies to quantify energy transfer through walls, roofs, or floors. Typical applications include:

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  Heat loss or gain through building surfaces

  
  
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  Solar radiation incident on horizontal surfaces

  
  
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  Design and analysis of heating and cooling systems

  
  

Heat flux is mathematically expressed as:

where q is heat flux in Btu(IT)/(h·ft²), Q is energy in Btu(IT), A is area in ft², and t is time in hours.

Conversion to SI units:

Although modern practice uses W/m², Btu(IT)/(h·ft²) remains prevalent in U.S. building codes, insulation tables, and historical thermal data, offering a convenient measure of energy transfer per square foot per hour.

Calorie (th) per Minute per Square Centimeter [cal(th)/(min·cm²)]

The unit calorie (th) per minute per square centimeter, written as cal(th)/(min·cm²), measures heat flux density or thermal power per unit area using the thermochemical calorie (1 cal(th) = 4.184 J). It represents the amount of heat energy transferred through 1 cm² of surface every minute.

This unit is often used in laboratory experiments, solar energy studies, and material testing, particularly when small surfaces and longer exposure times are involved. Applications include:

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  Solar radiation measurements on small panels or surfaces

  
  
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  Thermal testing of materials over extended periods

  
  
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  Calorimetry experiments on small samples

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in cal(th)/(min·cm²), Q is energy in calories, A is area in cm², and t is time in minutes.

Conversion to SI units:

Although SI units such as W/m² are standard, cal(th)/(min·cm²) remains useful in historical calorimetry, laboratory experiments, and solar radiation studies, providing an intuitive measure of energy transfer per minute per small surface area.