Today we're making core parts of Vulcan open source: the functionality to import, create, edit and validate a Home Energy Model input file using a 2D / 3D canvas, together with all of our documentation. The code is public on GitHub, and it is free forever to use on the Vulcan platform.
Alongside the open-source release we're introducing a free forever tier, so anyone can use a hosted version of the editor without having to run it themselves.
We'll retain a Pro tier, which all current paid subscribers will move to. Pro is perfect for power users of the Home Energy Model who need technical support and model in bulk, with:
- in-app technical support, so users can check their modelling and answer questions;
- building regulations functionality (access to the official ECaaS HEM calculator);
- AI functionality, including toolkits for local LLMs and modelling workflow harnesses;
- organisation of dwellings into 'projects', viewable on canvas and editable in bulk;
- scenario modelling and analysis functionality; and
- cloud backups of models.
Vulcan's goal is to make good predictions on how homes use energy widely available
Open access to good predictions about how homes use energy is critical to homeowners' confidence to make improvements.
Vulcan's purpose is to make homes low cost and high comfort. This purpose is critical to national energy security, our climate, health, and personal wellbeing.
A key requirement for better homes is widespread information about the impact of different design decisions. Today, that information is unevenly distributed. Our homes are enormously inefficient, and fabric and system upgrades can cut the cost of running them. But these upgrades are interdependent, and identifying the optimal combination of improvements is very challenging. A small number of professionals, armed with information on the structure and thermal properties of a home, can use building physics to predict the energy use of homes and impact of different upgrades. Most people cannot access this information themselves.
Too often, a homeowner who wants to improve their home must pay a high price for an independent expert opinion, or rely on the recommendation of a solution supplier, which may not be objective.
If homeowners were armed with the right information, they could hold developers and contractors to better expectations, and build an independent perspective on the best solutions for them. Collectively, we could move with greater speed to demand and deliver homes that are low cost and high comfort.
Today's home energy information is insufficient
The methodology to understand home energy use in widespread use today is both inaccessible and inaccurate.
Inaccessible. SAP, the methodology currently used to model domestic energy use for building regulations and EPCs, is only accessible in a validated form through approved software. The methodology document itself is public, but it is implemented as separate closed-source programs by government-approved software vendors, several of which are also energy assessor accreditation schemes. This slows progress: every change must be centrally agreed as a methodology update, separately implemented in each vendor's software, and then validated. It also reduces auditability and understanding of how the methodology works, and closes off contribution to improvements, because the calculations are black boxes. This stops users from identifying the cause of unexpected results, in a way that is possible with transparent methodologies like PHPP, where every intermediate calculation is inspectable.
Inaccurate. SAP provides an approximate monthly estimate of building energy use, using standardised assumptions for the utilisation of systems and occupant behaviour rather than modelling them explicitly. It is not capable of fine-grained predictions about the interactions of weather, occupants, a dwelling's thermal mass, and systems like heat pumps, solar and batteries. Studies have demonstrated SAP's inaccuracy time and again. Most recently, DESNZ's EPC accuracy research (May 2026) compared EPC-modelled energy use against smart-meter data and found that EPCs overestimate energy use in gas-heated homes by around 16% on average, with the largest errors in older, traditionally built homes. Earlier work reached similar conclusions: a 2019 study of 1.6 million repeat assessments found EPC ratings carry enough measurement error that roughly a quarter of band D homes should have been band C.
The Home Energy Model partly solves the information gap
The Home Energy Model (HEM), the planned replacement for SAP, is designed to solve many of SAP's issues. It is a half-hourly model that explicitly simulates the dynamic interactions of occupants, weather, fabric and systems.
HEM is also open source by default. The methodology is available as clear, deterministic Python code rather than only a potentially ambiguous methodology document, and MHCLG maintains a single open-source Rust implementation of the calculation. This makes HEM easier to audit, faster to evolve, and easier to build on. The Future Homes Standard, in force from March 2027, will bring HEM into building regulations.
However, the calculation engine is only part of the system. HEM will run as a single central government calculation service, which assessors and the wider industry will reach through software providers, including assessor schemes, that build their own front ends. And because HEM requires much more detailed inputs than SAP, front ends are critical: they are where the inputs are created. A front end can make inputs simpler to produce, for example through visual data entry, but in doing so it risks introducing its own black boxes. The input file determines the result. If the tools that author it are closed, the audit trail stops where most of the modelling judgement is applied.
Open source is the natural next step for Vulcan
Vulcan is an assessor scheme built for HEM, with a visual workflow to import, author and validate HEM inputs.
Vulcan has been openly developing for the Home Energy Model since its consultation - so being open is part of the company's DNA. Initially this was to give professionals interested in preparing for HEM a way to use and understand it. It also provided valuable feedback that made the Vulcan software better.
That made the next step natural: open source the editor itself. Alongside it, the free forever tier means anyone can use a hosted version of the editor. The goal is to make HEM modelling software easier to access, easier to understand, and easier to improve. Anyone can now inspect how the front end works and contribute to it. The advent of AI makes inspecting the Vulcan open source code and making contributions even more accessible. This step is a natural part of Vulcan's mission: making the information to model homes widely available.
How Vulcan Open Source works
We have kept the licensing as simple as possible, and the obligations minimal.
The licence. Vulcan Community is licensed AGPL-3.0-only, supplemented by the Vulcan Origin Terms, with separately identified licences for interoperability code, data and upstream components. In practice: you can use, study, modify, self-host and fork the editor freely. If you distribute it, or run a modified version as a network service, you must share your source under the same licence and preserve a short factual attribution.
Attribution. The attribution requirement is deliberately small: preserve the standard notices, reachable from a plain-text link such as Powered by Vulcan. No logo, fixed badge, data gathering, or any obligation to submit changes upstream.
Closed-source applications. There are two routes for closed-source software:
- The interoperability layer is Apache-2.0.
vulcan-csv-codec, the parser for Vulcan's sectioned Geometry CSV format, is licensed Apache-2.0 so that any application, including closed-source commercial software, can read and write Vulcan-compatible files with no copyleft obligations. Using the codec does not by itself require application-wide Vulcan attribution, and truthful statements like "compatible with Vulcan CSV" are expressly permitted. The self-hosting deployment configurations are Apache-2.0 too, and the documentation is CC BY 4.0. - Commercial terms. For closed-source use of the AGPL components, including white-label terms without visible attribution, Home Energy Foundry offers separate commercial licences. Contact info@usevulcan.app.
Contributions and the CLA. Contributions are welcome. The contributing guide explains how. Contributors sign a contributor licence agreement based on the standard Harmony agreements; for individuals it is a single click-through comment on your first pull request. You keep ownership of your contribution. You grant Home Energy Foundry the right to license it, including commercially, which is what lets us offer commercial licences and fund the free hosted editor through the Pro tier. In return, the CLA binds us too: your contribution must always remain available under the open licence you submitted it to. It can never be taken proprietary-only.
Recognition both ways. Applications built on Vulcan Community carry a factual origin notice pointing back to usevulcan.app/open-source. In return, we will name compatible applications and forks on that page. Attribution flows in both directions: your users can see what your tool is built on, and our users can see what has been built on Vulcan.
Where to find Vulcan Open Source
- Vulcan Community: the open-source browser editor for the Home Energy Model. It includes 2D / 3D geometry authoring, an assembly U-value calculator, automatic thermal-bridge topology, CSV-to-HEM conversion, Future Homes Standard preflight validation, and optional IFC4 format CAD file import. It is offline-first: it runs against an ordinary local folder, with no account required, and your files stay on your machine.
- H3 (HEM HTC Harness): our open-source harness for comparing HEM heat-loss predictions with real-world measurements, which we introduced earlier this year, developed in collaboration with Knauf Energy Solutions.
Vulcan Community authors and validates HEM inputs; it does not run simulations. The calculation itself is the government's own open-source HEM engine (MIT-licensed), which the repository pins as an upstream at an exact revision. usevulcan.app/open-source is the permanent home for all of this.
Start using Vulcan Open Source today
Whether you want to understand your own home, prepare for HEM professionally, build a tool that reads and writes Vulcan files, or contribute to the editor: explore the code, run it locally, or start using hosted Vulcan today.