UPDATED February 1, 2012

In 2004, researchers at the National Renewable Energy Laboratory (NREL) developed BEopt, a software program that finds the least-cost solution to designing a zero-energy house. Now that the software developers — a team that includes Craig Christensen and Scott Horowitz — have spent seven years improving the program, it has finally been released to the public. The development of BEopt was funded by the U.S. Department of Energy.

BEopt can be downloaded for free from a new BEopt website maintained by NREL. In addition to its original functions for designers of new homes, BEopt now includes new functions that prioritize energy-retrofit work in existing homes.

The BEopt website includes links to “help” files and training videos to get new users oriented. Although the software still has a few flaws and limitations, patient designers will find BEopt to be a useful and intriguing software program.

How good does a home’s thermal envelope need to be?

BEopt performs at least two functions: it is an energy modeling program that calculates the expected annual energy use for any house design, and it is an optimization program that identifies the least expensive way to build the envelope of a net-zero energy house. Users of BEopt can access weather files for 1,000 locations in the U.S.

The program includes a database of user-modifiable construction cost data. This feature allows BEopt to determine (for example) whether a house with 12-inch-thick double-stud walls and double-glazed windows will cost more or less to build than a house with 2-inch-thick foam sheathing, 2×4 walls, and triple-glazed windows. It will also determine which of these options will perform best.

BEopt shows designers of zero-energy homes how to improve envelope specifications, and lets them know the right point to quit making envelope improvements. After all, including energy-efficiency measures in a new…