Time lag and decrement factor

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Time lag (the time between the max outside face temperature and max inside face temperature) and decrement factor (ratio of max inside face temperature to max outside face temperature) are not standard output variables that EnergyPlus or OpenStudio can generate. You can read more about these thermal mass properties in a blog post here.

These thermal mass properties change each day (or whatever period you want to measure these max temperatures for). Here is the process I would recommend:

1. Add Output:Variable objects to your model that will generate results for Surface Outside Face Temperature and Surface Inside Face Temperature for the surfaces you want to calculate time lag and decrement factors for.
2. Use Microsoft Excel to find the max of each of these variables for each surface for a day (or whatever period you want to use), then calculate the time lag and decrement factor for that period from these max temperatures.

There are alternatives to Microsoft Excel:

• Use a script with Python, Ruby, R, or a similar scripting language to read the CSV generated by EnergyPlus with surface face temperature output variables and calculate time lag and decrement factor
• Use the Energy Management System (EMS) feature of EnergyPlus to have sensors read the face temperature output variables, use a @TrendMax built-in function to find max surface temperature trends for the desired period, then calculate the time lag and decrement factor in a program (and possibly write them to the CSV file as EMS output variables).

These alternatives to Microsoft Excel are more complex, but will make this process MUCH faster if you need to review the time lag and decrement factor across multiple models or iterations of the same model.