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)/Hour/Square Foot

The unit Btu (th)/hour/square foot measures the rate of heat transfer through a surface over time. Here, Btu (th) refers to British thermal units of thermal energy, and the measurement quantifies how much heat passes through one square foot of a material in one hour. This unit is widely used in building design, insulation analysis, and HVAC system planning to evaluate thermal performance. A higher value indicates more heat loss or gain, suggesting lower insulation efficiency, while a lower value reflects better thermal resistance. Engineers and architects rely on Btu (th)/hour/square foot to calculate heating or cooling loads, optimize energy consumption, and improve indoor comfort. It is essential for energy audits, material selection, and sustainable building practices. By accurately assessing heat transfer in this unit, professionals can design energy-efficient structures, minimize utility costs, and reduce environmental impact. Understanding Btu (th)/hour/square foot is therefore critical for both practical construction decisions and long-term energy management.