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.

Btu (th) per Second per Square Foot [Btu(th)/(s·ft²)]

The unit Btu (th) per second per square foot, written as Btu(th)/(s·ft²), measures heat flux density or thermal power per unit area using the thermochemical British Thermal Unit (1 Btu(th) = 1054.35 J). It represents the amount of heat energy transferred through 1 square foot of surface every second.

This unit is commonly used in high-intensity heat transfer applications, industrial processes, solar energy studies, and material testing, especially where energy flux over relatively large surfaces is significant. Applications include:

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  Heat flux from furnaces, radiators, or heaters

  
  
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  Solar radiation received by panels or flat surfaces

  
  
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  Thermal testing of building or industrial materials

  
  

Mathematically, heat flux is expressed as:

where q is heat flux in Btu(th)/(s·ft²), Q is energy in Btu(th), A is area in ft², and t is time in seconds.

Conversion to SI units:

Although W/m² is the SI standard, Btu(th)/(s·ft²) remains useful in U.S. engineering, thermal studies, and historical literature, providing an intuitive measure of high-intensity heat flux per square foot per second.