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:

• 
  

  Heat loss or gain through building surfaces

  
  
• 
  

  Solar radiation incident on horizontal surfaces

  
  
• 
  

  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.

CHU/Hour/Square Foot

The unit CHU/hour/square foot is used to measure Cooling Heat Units per hour per square foot, commonly applied in air conditioning and refrigeration calculations. It indicates the amount of cooling energy, in CHUs, that passes through or is required for one square foot of a surface area over an hour. This measurement helps engineers and designers assess cooling loads, select appropriate air conditioning systems, and ensure thermal comfort in buildings. A higher CHU/hour/square foot value shows greater heat gain, meaning more cooling is needed, while a lower value indicates better insulation or lower cooling requirements. It is essential in designing energy-efficient HVAC systems and optimizing equipment sizing, helping reduce operational costs and maintain desired indoor temperatures. By using CHU/hour/square foot calculations, building designers can balance comfort, energy efficiency, and sustainability effectively.