If you perform building diagnostics testing, energy audits, or work in the Weatherization Assistance Program (WAP), chances are you’ve heard of RED Calc. The Residential Energy Dynamics (RED) Calculator is a software program that simplifies many of the calculations needed to perform those jobs. I use several of the tools on the RED Calc website; zonal pressure diagnostics (ZPD), air leakage metrics, and weather station data are all handy for my work.

RED Calc was first launched in 1995 as a calculator program (TI-85, TI-86, and TI-89); it included 26 building science tools, which have slowly been expanded over the years. In 2012, Rick Karg and Charlie Holly created the web-based software to help building performance professionals more readily run metrics on residential projects.

According to Rick, the most-used tool on the platform is the ASHRAE 62.2 calculator, which is used for getting minimum-required ventilation airflow rates. In addition to the 62.2 CA standard, RED Calc allows you to choose from the 2010, 2013, or 2016 standards. (Version 62.2–2022 is slated to be released “in the near future.”) Other tools include airflow measurements, insulation and R-value calculations, moisture metrics, domestic hot water calculations, electrical use, and weather station data.

With the free version of RED Calc, printed results take the form of a screen shot of the completed calculations. Subscribing to the paid version, RED Calc Pro, enables users to print a more formal report. Other benefits of the pro version include the ability to save work in the cloud, customize reports, add notes while collecting data, and function when offline. The current charge is around $20 per month.

In November 2023, it was announced that the U.S. Department of Energy is buying RED Calc. The software will be hosted on the Building America Solution Center site; the move date is estimated for “late spring,” this year. Charlie will continue to support the software’s ongoing development. Once it reaches its new home, RED Calc should look and function much the same; the biggest change is that the pro version will be free.

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Randy Williams is a builder and energy rater based in Grand Rapids, Minnesota. Images courtesy of the author.