Sunday, September 22, 2019
RED Calc Free Help Surface Heat Transfer
Surface Heat Transfer Tool User Guide

Surface Heat Transfer
Red Calc Tool User Guide

What this tool can do for you

This tool is most often used to find heat transfer through a surface, but because it is a solve-all tool, you may solve for any one of the five values.

Surface Heat Transfer

Calculated values

  • Heat transfer; or
  • Temperature difference; or
  • R-value; or
  • Time interval; or
  • Area.


  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs). Read more.
  • Because this is a solve-all tool, you may find a result for any one of the five values. Click the radio button to the left of the value label to solve for that value. When you check a radio button to solve for a value, the color of the label changes from black to blue.
  • This tool is most often used to solve for "Heat transfer", the last of the five values.
  • With this tool you can calculate annual heat loss/gain through a surface by entering "1 day" for the time and then entering the number of heating degree days for the temperature difference. You will receive a warning message indicating that your temperature difference input is outside the normal range, but you may proceed with the calculation.

Inputs and field measurements

  • Surface area - measured in field.
  • R-value (or 1/U-factor) - determined in field.
  • Time interval - usually one hour to correspond with heat transfer period used for sizing heating or cooling systems.
  • Temperature difference - the temperature difference from one side of a surface to the other, often the temperature difference between indoors and outdoors on a design temperature day.


It is probably easiest to think of heat transfer through a three dimensional object, like a house window, as being characterized by conduction only. However heat transfer through building parts is usually a combination of conduction, convection, and radiation. The part that each of these plays is represented by the rated R-value of the building part, for example, the complex heat transfer through fiberglass insulation is represented by a R-value of 3.2 per inch (RSI of 0.564).

This RED Calc tool is based on one of the two basic equations used for sizing heating and cooling systems for dwellings. The other equation calculates hourly heat loss or gain attributed to air leakage.

Basic heat transfer equation

Heat transfer = Area × Time interval × Temperature difference R-value


  • 2009 ASHRAE Handbook: Fundamentals: (2009). American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE), Atlanta, GA.
    Comment: The best source for R-values and measuring heat loss and gain. See Chapter 26 for R-value tables.
  • Manual J: Residential Load Calculation, 7th ed. (1986). Air Conditioning Contractors of America. Washington, DC.
    Comment: The "bible" for calculating heat loss/gain in dwellings. Includes R-value tables and other useful data. Much simpler than the 8th edition. If you only want the basics without the bushels of details, purchase the 7th edition.
  • Manual J: Residential Load Calculation, 8th ed. (2003). Air Conditioning Contractors of America. Washington, DC.
    Comment: The "bible" for calculating heat loss/gain in dwellings. Includes R-value tables and other useful data. Much more complex, but more accurate than the 7th edition. If you want all the details, purchase the 8th edition.
  • Mott-Smith, Morton. 1962. The Concept of Heat and its Workings Simply Explained. Dover Publications, Inc. New York.
    Comment: Great classic text on the subject of heat and heat transfer.

Related tools

  • Parallel-Path Equivalent R-Value: Determine the overall equivalent R-value of an area made up of smaller areas, each with a different R-value.
  • Dense-Pack Insulation: Determine the installed density or required bags of insulation when dense packing enclosed cavities.
  • Loose-Fill Insulation: Determine the required bags of insulation or the installed depth when blowing loose-fill insulation in attics.
  • Infrared R-Value: Determine the R-value of a wall section with your infrared measuring device.

Version 2016-07-06_01:30
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