Building software tools that we would want to use ourselves

Elements is a free, open-source, cross-platform software tool for creating and editing custom weather files for building energy modeling.

Modelkit (formerly Params) is a cross-platform framework for parametric building energy modeling. It combines reusable templates with scripting to enable flexible, transparent, and highly scalable model generation workflows.

Euclid is a free, open-source SketchUp extension that makes it easy to create, edit, and manage building geometry for energy modeling workflows. Originally based on the Legacy OpenStudio extension developed by NREL, Euclid is now actively maintained and extended by Big Ladder Software.

Kiva is a free, open-source ground heat transfer calculation engine used to model heat loss and gain from building foundations on a timestep basis. Designed for accuracy and performance, Kiva enables multi-dimensional foundation heat transfer to be integrated into whole-building energy simulation workflows.

Unmet Hours is a free, community-driven question-and-answer platform for building energy modelers. Developed by Big Ladder Software in collaboration with IBPSA-USA, it enables practitioners to ask questions, share expertise, and build a living knowledge base covering tools, methods, and standards used in energy modeling.

CSE (formerly the California Simulation Engine) is a general-purpose building energy simulation model developed to perform the core calculations required for California Building Energy Code compliance. It serves as the calculation engine behind CBECC-Res and is focused on accurate, transparent simulation of residential building performance.

Citadel is a web-based computational planning platform for evaluating energy, water, waste, and resiliency scenarios across large building portfolios. Developed by the U.S. Army Corps of Engineers in collaboration with Big Ladder Software, Citadel supports community-, campus-, district-, and installation-scale master planning through integrated modeling, analytics, and visualization.

Penumbra is a free, open-source C++ library for high-performance solar shading calculations using GPU-accelerated pixel counting. It is designed to support accurate and efficient shading analysis within building energy modeling and simulation workflows.

Btwxt is a free, open-source C++ library for performing numerical interpolation on regular (rectilinear) gridded data sets. It provides a flexible, high-performance API for interpolating and extrapolating multi-dimensional data commonly used in scientific and engineering applications.

HPWHsim is a free, open-source simulation model for heat pump water heaters (HPWH). It is designed for whole-house simulation workflows where many year-long simulations are run efficiently, and where HPWHsim operates independently of an overarching engine's timestep and data aggregation.

libtk205 is a C++ library autogenerated from the current version of the ASHRAE Standard 205P data exchange specification. It is generated from template code provided by the ASHRAE 205 toolkit, which supports cross-format translation of ASHRAE 205 schema and examples. While developed in conjunction with the ASHRAE Standard 205 project committee, libtk205 is not an official ASHRAE product and is not part of the standard.

ResDX is a Python package for modeling residential direct expansion (DX) systems. It supports model comparison, new model development, standard efficiency rating calculations (including AHRI 210/240), inverse efficiency calculations, and generation of ASHRAE 205-compliant representations.